Sigma IV 1.01 Jan 2015(click to open PDF file)
Contents
1. T pot y S NS S o AL Frequency MH z oo i al m 2 aa This plot is opposite to the free space plot Don t get the wrong impression from the gain figure it is slightly higher as ground reflections will provide some additional ground gain This is also the case with any other antenna We know that in free space a long radiator will provide more gain But if we measure at a low angle we notice the gain provided under a low angle isn t going up From that we can draw the conclusion A longer radiator will produce more gain but that gain will be pointed upwards And we all know It is wise to focus on maximum gain at low angle instead Perhaps there are other interesting results if we start to have a closer look at a real situation Looking at the antenna and compare it to real other antennas we can discover quite a few amazing aspects If we ask anyone what could be the advantage of the Sigma IV most will provide the answer 32 www cb antennas com Sigma 4 1 01 Jan 2015 Well its long isn t it It is one of the longest antennas available and as we will find out That is an important aspect Another one would be the performance we expect from other commonly used vertical antennas 6 REAL AND IDEAL What if we measure the strength of the signal in real situation at a distant receiving location and compare those Not only looking at a free space gain figure not only looking at a2. As mentioned some conclude the CST plot provided is proof that the Sigma IV is a collinear antenna That finding is based on the interpretation that the above plot shows antenna current and phase and that we can see that the current within the cone is confined and remains in the cone where current on the outside of the cone can add to the gain due to its being in phase with the top 1 2 wave section And in all honesty understand the confusion It is certainly not something that the average CB or HAM operator will interpret correctly What is it To understand what the CST plot is actually providing we need to find out what it is indicating We have a couple of indications e Wecan draw a conclusion from the size of the antenna in combination with the shape of the pattern For us to notice That because we can see differences between the inside and the outside of the cone it indicates that we are looking at close range to the antenna In antenna terms that would be the near field The CST plot is within the calculated 5 meters distance away from the antenna It is the reactive near field region we are looking at In the reactive near field there are two major fields active The Electric field and the Magnetic field e The magnitude and the colour are expressed in A m top right corner The unit A m is being used to express the density of the magnetic H field e The confirmation that it is The circular magnetic H field in the
3. average user or their judgment Although it s not all scientific they need to be mentioned as they could have an impact Just imagine a person Who just bought the antenna new and is going home with it Of course excited to find out how the antenna will perform That user as often indicated on the forum could have the impression A half wave vertical will produce the same gain as a dipole and the 5 8 wave antenna has more gain Some provide an indication that there is a magical 64 A that provides maximum gain And perhaps we are in a situation where the new antenna will replace the old antenna on the same mast The above doesn t sound so strange we need to realise however that just by being that person we already have mentioned several pitfalls that could influence judgement Including some social aspects 1 Wewere happy in the car driving home with the antenna We have obtained a new antenna we spent money in order to expect improvement The mind is set for better 2 The antenna is long probably bigger than any antenna they have had in the past That mechanical aspect it is bigger enforces the mind to think it is it must be And we are told very often bigger is better And we have some mechanical aspects 1 Itis most likely to be replacing an old antenna Perhaps the old one was not performing to its abilities anymore For example On
4. I I I I I I I I I L Above are Two examples of parallel wires and segment lengths Because the Sigma IV has wires at an acute angle it is important to maintain equal segmentation length in the main radiator as well as the radials The green dots represent segment connections In the left picture wire leg 2 has 5 segments and wire 1 has 11 The manual says those segments need to be equal in order to provide accurate results as done in the picture on the right www cb antennas com Sigma 4 1 01 Jan 2015 2 2 Wires attached to each other at an acute angle quote Eznec manual NEC has some difficulty in accurately modelling multiple wires joining at a very acute angle such as witha fan antenna the difficulty being greater with NEC 2 than with NEC 4 And When modelling very acutely intersecting wires evaluate the results carefully particularly if a source or load is at or near the junction end quote Eznec manual As we can read using NEC4 will already solve most of this possible challenge Other options would be to add a small horizontal wire between the bottom radiator and the vertical radial E NEC Prov EYNEC Prov Above is an acute angle between two wires and a possible way to provide more accuracy 2 3 Source placement at segment junctions quote Eznec manual Conventional EZNEC sources have to be placed on a segment so simulating a source at a wire junction requires special techniques end quote Ezn
5. are actually not out of proportion The interpretation of them on the other hand sometime is The antenna is capable of producing slightly more gain compared to a dipole The advanced software methods and real live testing have confirmed this From a personal perspective was hoping we could find some additional gain and who knows perhaps challenge theory But after all work done it has convinced me there is not Each time the provided claims could be explained nor could find any proof there is a collinear effect as we know imagine collinear to be It is a 1 2 wave radiator with a 1 4 wave matching stub with a minimum amount of radiation from the cone contributing to the far field With that said the performance of the antenna is most likely to outperform others in an average situation From a CB user point of view the antenna could be recommend It is most likely to outperform a half wave end fed antenna or a 5 8 wave antenna With the primary reason not being it has more gain But other factors it seems are responsible for the possible advantage factors like The length of the antenna or the performance of the antenna in a real situation in combination with a mast and coax attached to it And the expectation we have from our reference antenna or the performance of it is often not what is expected That s an important point We need to realise we must be careful what to expect from the traditional vertical antenna
6. density is so bright red left blue right between the main radiator and the elements of the cone is that all the arrows in that region in the FEKO plot are pointing in one direction This actually provides us insight into how the antenna is working cae H 49 4KB It can only indicate The current on the main radiator is travelling in the opposite direction to the current in the radials www cb antennas com Sigma 4 1 01 Jan 2015 22 4 WE ARE GOING TO START DRAWING FINALLY GETTING SOMEWHERE Below a simplified drawing of the currents and the magnetic H field direction Just the 3 4 wave radiator alone with the current in red and the direction of the current displayed by the arrow The 3 4 wave radiator including 2 elements in a vertical position next to it The Sigma 4 with current arrows indication The 3 4 wave radiator with current arrow indication and the direction of the magnetic H field displayed by the green arrows The 3 4 wave radiator with 2 vertical radials Providing current direction and magnetic H field direction PLEASE NOTICE THE DIRECTION OF THE MAGNETIC H FIELD ESTABLISHED BETWEEN THE RADIATOR AND VERTICAL RADIALS ARE IN THE SAME DIRECTION The Sigma IV now please draw the direction of the magnetic H field 23 www cb antennas com Sigma 4 1 01 Jan 2015 And that situation is actually not that strange we notice the same thing happening with a transmission line
7. far field plot But what will happen if we start analysing the total field including the ground wave The first diagram is showing the signal strength in dB s This is measured at a single point 30 Km away from the transmitting antenna At that location it is measured at a height of 10 meters By doing so we are trying to duplicate a real scenario Imagine a person who is testing an antenna with a receiving stations at 30 Km That receiving station has his antenna situated at 10 meters height All antennas were ideal no losses and no other influence The soil type between the antennas was medium The variable is the antenna tip height of the transmitting station where each time the bottom of the antenna was kept equal top height of antennas not equal The height is 5 5 meter TOP HEIGHT DIPOLE 7 m ALL BASE 1 5 Height 7 0 80 90 100 11 0 12 0 13 0 140 150 160 17 0 180 190 20 0 21 0 22 0 23 0 240 25 0 260 27 0 280 29 0 30 0 Signal strength received at 30 Km distance at 10 meters height From transmitter Variable antennas bottom www cb antennas com height equal for all S M 0 005 Diel const 13 antennas soil type medium We notice that for an average user the Sigma IV is perfectly capable of showing gain up to several dB s Again we have a fixed mast length between 1 5 and 24 5 meters and are watching the strength of the signal change at a distant location for
8. fifteen inches This embodiment of the antenna exhibits a qain when compared to a half wave dipole of about 2 2 dB At somewhat greater angles where the radial dimensions are of course larger e g an angle of approximately thirty degrees where the radial dimension is about four and one half feet the gain over a half wave dipole antenna is about 2 7 dB end of quote The gain figure claimed by several other manufacturers varies To give a short indication 6 14 dB 5 15 dBl 4 15 dBI 2dBd 6 5 dB And one speaks of a CSF model gain figure Typically 3db over a dipole It is clear they are not consistent with each other and there are terms free for interpretation As it is free to imagine what a CSF gain figure is Neither do the terms dB without an indication to its reference like dBi or dBd provide an indication about what the gain figure actually is There are those who claim not all software can model the antenna They specifically mention software like EZNEC 4 NEC2 etc would lack at this This based on two points see 3 and 4 Online there is a CST plot film available of the antenna where people claim they see a collinear effect and they mention NEC for some reason is not able to provide the same insight One of the antenna world s best known antenna Elmer s L B CEBIK has said in the past two things about the antenna There is a non apparent collinear effect and the a
9. ground conditions J 27 185 MHz However the gain and plots mentioned are based on an ideal theoretical world If we look outside the earth is not a perfect conductor far from it If we model a 5 8 wave in free space without a perfect ground we will find out the gain is not 3dBi 37 www cb antennas com Sigma 4 1 01 Jan 2015 And yes There is our wake up call Total Field a EFNEC Www cb antennas com 6 8 wave vertical Freespace pattern 27 185 MHz Elevation Plot Cursor Eley 8 0 deg Azimuth Angle 0 0 deg Gain 1 07 dBi Outer Ring 1 07 dBi 0 0 dBmax 0 0 dBmax30 3D Max Gain 1 87 dBi Slice Max Gain 1 387 dBi Elev Angle 3 0 deg Front Back 0 44 dB Beamwidth 61 8 deg 3015 3254 47 2 deg Sielobe Gain 1 67 dBi Elev Angle 172 0 deg FronvSidelobe 0 0 dB The gain of a 5 8 wave vertical and other antennas will have a large influence because of earth And that gain will vary with the distance from the earth At close range to the earth the 5 8 wave will outperform a dipole at same tip height Near the earth surface a 5 8 wave antenna is a known performer and that is where things go wrong Most theory books are based on either a perfect world or are orientated for the low band broadcasting world Something we don t realise nor read in theory books The fact is The average CB user does not use the 5 8 wave close to the ground They will strive for the highest mounting loca
10. happening with the Sigma 4 antenna There is one difference The lines are not parallel they are situated in a cone configuration That is a reason that it is not a perfect balanced cancelling system Secondly the top of the cone sees air where the bottom 1 4 wave radiator sees the end of a 1 2 wave radiator Those two have different impedances also providing some unbalance in the transmission line effect To imagine this try to think you re with two people and they are both pulling on a rope at the same time First we have two people pulling in opposite direction with equal force the net result is O and they are not moving Secondly we have two people pulling in the same direction The combined result is plus and they are moving In the third we have the forces in opposite direction but at an angle The combined result will be that they are moving but very slowly The above is analogous to the sum of active currents As itis not only the transmission line mode currents that are active If a conductor radiates and there is metal near that conductor a current will be induced in that other metal part If you imagine a Yagi Uda beam this becomes a bit clearer Although the parasitic elements are not directly fed the radiation from the radiator will induce a current in the other elements and if element length amp distance are optimised we have gain or front to back etc MoM
11. is the case with modelling software Although there are many who model an antenna and publish results as facts the results often lack accuracy Not in first place due to the limitations of the software but more often the knowledge of the user is the key factor This is a problem not only limited to the average user it is seen with commercial manufacturers as well It is true Within the limitations provided that computers don t make mistakes people do That brings us back to L B Cebik and his words Why would he have said that it would be difficult to model the antenna Sadly L B Cebik passed away in April 2008 leaving behind his work for which we are grateful to still be able to use For us antenna enthusiasts it is a wealth of information His work can be found on http www antennex com 9 After investigating his work we notice He used the terms difficult to model in other antenna examples as well For instance In his work on the coaxial monopole And there are other examples where he warns about possible accuracy issues with NEC that may be of influence What is consistent in his work is When he used the terms difficult to model it was always in combination with a solution on how it should be done to prevent possible errors In situations where NEC was a limitation he has used words similar to it can t be analysed with NEC 2 LIMITATIONS OF MoM NEC If we summarise the possible di
12. near field region comes from the magnitude and the distance provided As the magnitude of the magnetic H field will vary with distance and will lose strength with the cube of distance from the antenna 1 r as can be indicated from the plot This is different from far field radiation where field strength decreases with distance from the antenna ata slower rate 1 r 14 www cb antennas com Sigma 4 1 01 Jan 2015 3 THE COULOR INDICATION To understand what the colours are indicating we need to know some Electromagnetic basics Imagine an antenna with a RF wave signal going into it keeping in mind we are still looking at the near field We will discover there will be two fields established There is an electrical field E and for us the important magnetic H field Those two are at a 90 degree angle to each other and both can be imagined like this The above image represents a half wave dipole and the near fields surrounding it A common mistake made is that the E field combined with the H field are the far field realise it would be easy to see it like that It is just a bit more complicated We can t just add them together For those interested it is really worth investigating books like antenna theory from C A Balanis LOOKING AT THE RED B The magnetic H field has a magnetic flux direction Looking at the picture it looks like the RED magnetic H field is rotating but o
13. software is able to provide the sum of the combined active currents We can divide the Sigma IV in two different sections The top 1 2 wave and secondly the cone We know that because the 1 2 wave on top of the cone will present a high impedance at that point We are able to remove the wave radiator and insert a load If that load represents the impedance of the end fed 1 2 wave we can find out 25 www cb antennas com Sigma 4 1 01 Jan 2015 How the cone behaves on its own without the radiation of the 1 2 wave on top A situation we can visualize as adding a dummy load on top of the cone 27 185 MHz F NEC Pro fa Le F ae m 1 50 ohms Cursor Az 304 0 deg lt F Source F 28 27 dBi Az Angle 129 0 deg 0 72 dB 20 24 dBi 26 24 dBi 100 dBi gt 100 L I p gt 58 j 0 634 ohms Refl Coeff 0 02588 at 13 46 deg 0 02517 j 0 008025 Ret Loss 31 7 dB 52 58 at 0 69 deg 27 2 MHz 1 053 Jd a 5 a c E 2 W a m Slice Max Gain Front Back Beam idth Front Sidelobe 3D Max Gain Sidelobe Gain Outer Ring i i Freq SWR Z www ch antennas com Above is the cone with a load inserted instead of the 1 2 wave radiator 26 www cb antennas com Sigma 4 1 01 Jan 2015 And the provided SWR plot of the cone with the loads inserted instead of the 1 2 wave radiator The analysis of the cone wi
14. 01 Jan 2015 3 PLOT H FIELD CONE SIGMA IV If we look at the magnetic H field in the sigma IV cone by using FEKO we can provide the image below www cb antennas com www cb antennas com Sigma 4 1 01 Jan 2015 18 3 PLOT SIGMA IV H FIELD and FAR FIELD And finally a combination of the far field pattern and the magnetic H field near field including arrows for direction www cb antennas com In the above I have situated the far field plot at the bottom of the antenna to provide direction indication in the cone To determine the direction of the of the magnetic near H field we can apply the so called right hand rule The current direction is provided by the direction of the thumb The magnetic field by the direction of the fingers 90 degree from thumb conductor And the direction of that field is in the direction where the fingers are pointing 19 www cb antennas com Sigma 4 1 01 Jan 2015 4 Perhaps we are now able to understand what the CST film plot is providing Conclusion 1 In the right top corner we see the magnitude expressed in A m 2 The value has two maximums they reach 2 37 and 2 37 3 The magnitude shape looking at the plot on both sides are equal 4 Watching the film you will notice the distance or magnitude is always equal on both sides during the different phases in the RF cycle 5 The strength will decrease according to near H field radiation Th
15. 7 Feko 8 IE3D 9 http www antennex com 10 Eznec manual 11 LLNL G J Burke 12 Roy LeWallen 13 Arie Voors 14 Brian Cake 15 C A Balanis Appendix 1 See website Appendix 2 See website www cb antennas com Sigma 4 1 01 Jan 2015 43
16. In fact it is often used to explain the basics of how an antenna radiates Pag aa ag Na In the above drawing we see a transmission line two black horizontal lines the current and magnitude are displayed by the arrows and the red curved line Although a transmission line open line will have a magnetic and electric field around it in an ideal situation it will not radiate This because the currents are opposite in phase and equal in magnitude cancelling hence no radiation We know that the transmission line has a E M field around it simply because there is current moving That E M field around an open transmission line is why coax was invented An open transmission line has some inconvenient aspects We should not install an open line near obstacles especially metal because the E M field around that line will be influenced In a coax cable the E M wave remains inside the coax ideal This is due to the Faraday Shield Effect So what happens if we start bending the ends of the transmission line If we start to bend the last 1 4 wave ends of the transmission line and set them 180 degrees apart we can see that the currents are in the same direction and no cancelation takes place its free to radiate And above is the initial explanation of why a dipole antenna can radiate 24 www cb antennas com Sigma 4 1 01 Jan 2015 4 THE CONE NOT A TRANSMISSION LINE The transmission line effect is the same thing that is
17. THE AVANTI SIGMA 4 A vertical with some collinear gain Henry HPSD 1 01 2014 www cb antennas com Sigma 4 1 01 Jan 2015 The aim of this article is to provide insight into the antenna known as the Sigma IV or AV 174 An antenna first produced by Avanti The Antenna Specialist Co and cloned by many under various names Instead of just explaining how the antenna works the intention has been to approach the theory by looking at the information supplied on the internet and investigate the claims made This for the primary reason during the background research it became clear that there is confusion about the information provided from several sources hope the efforts in doing so will be successful and that it is useful for those with interest about the Avanti Sigma IV Kind regards Henry Poelman PGODX 19PA348 www cb antennas com Sigma 4 1 01 Jan 2015 CONTENT 1 Overall description of the antenna and a summary of the claims found Page 4 5 Page 6 Overall description of the antenna What are the claims found 2 Can the Sigma IV be modelled Page 7 Page 8 Page 8 Page 9 Page 10 Page 10 Page 11 Page 11 Which method needs to be used Cebik said difficult to model Limitations of MoM NEC Close spaced wires Wires attached to each other in an acute angle Source placement at segment junctions The tapering of aluminium Confirmation 3 An antenna has sev
18. ace a good RF choke at the bottom of the antenna and add 4 radials to provide a ground for the antenna We can discover the signal strength received at a 30 Km away from the Sigma IV at a height of 10 meters can be improved by almost 2 dB depending on height Please notice that the difference between the two will become larger as we move the antenna further away from earth 7 0 8 0 9 0 10 0 11 0 12 0 13 0 14 0 15 0 16 0 17 0 18 0 19 0 20 0 21 0 22 0 23 0 24 0 25 0 26 0 27 0 28 0 29 0 30 0 www cb antennas com Mi i co TS E Lele f Y Sen Nn Sigma IV Sigma IV Radials As mentioned an RF choke is always advised 5 turns with a 10 cm diameter RG213 will provide a good choke to stop the coax from being a radiating part Another possible improvement would be to eliminate the gamma match and direct feed the antenna As the cone acts as a transmission line there is nothing to stop us from finding the 50 ohms resonant point removing possible loss in that region 41 www cb antennas com Sigma 4 1 01 Jan 2015 7 Overall Conclusion During our search we were able to realise Antenna software based on MoM can model the antenna as long as we know what we are doing The results of free or relative cheap software compared to the best analysing software that will cost in the direction of 100 000 are equal We have discovered a lot of the claims made for the sigma IV
19. aim for a 5 8 wave vertical is without a doubt one of the best myths on 27 MHz and am afraid it will remain that way for a long time 39 www cb antennas com Sigma 4 1 01 Jan 2015 6 REAL LIVE TESTING There is as much debate on how to measure antennas as there is about antennas themselves The initial thoughts were to write a second article to provide the measurement results gathered for the Sigma IV However one could consider it a lack if this article did not contain at least the findings of those results For that reason a future article will deal with how to measuring antennas in real situations And for now we will refer to the results provided in this paragraph The measurements made were done using different measuring methods using data gathered from a spectrum analyser with a tracking generator and a relatively simple method using field strength measurements The trick with all measurements is to eliminate the data you do not want and be confident the data isn t influenced and you have an accurate method of verification Imagine that we are travelling and at the end of our journey we are provided with the distance and time we needed to get there Then you will be able to calculate how fast you have travelled that sounds logical doesn t it But what if that distance is provided in a straight line And we actually went over a couple of mountains What about the conditions we were travelling in Was it snow rain win
20. d For sure that will affect the effective distance and therefore average speed And so is the case with measurements we need to be confident we have the correct data and all the data and do not have any side effects The antennas under test were J pole Sigma IV copy Dipole Sigma IV with extra wave on top Same tip height RF choke attached Same mast changed several times Far within far field region 30 lambda During the test was able to measure the gain difference in the J pole pattern as such a pattern is not fully Omni directional from that have drawn the conclusion small changes were measurable The end result was That gain from an antenna based on the principle of the Sigma IV was equal to that of the J pole As mentioned a future article will provide information 40 www cb antennas com Sigma 4 1 01 Jan 2015 6 ROOM FOR IMPROVEMENT We have already noticed that the antenna could very well be an improvement compared to others There are situation where that difference could become remarkably large And in other situations the improvement will not be so noticeable We also noticed that in an ideal situation with added antenna height the difference will become less That is because of ground A possible improvement would be to improve the ground of the Sigma IV If we eliminate the mast and think of the Sigma IV being attached to a non conductive mast And if we pl
21. e Sigma 27 www cb antennas com Sigma 4 1 01 Jan 2015 5 WHAT GAIN CAN WE EXPECT FROM THE SIGMA IV Gain from a monopole 3 4 radiator If we start approaching the question logically we know a centre fed wave radiator is capable of producing about 3 dBi free space If we use such an antenna and feed it at the bottom end fed the roughly 1dB gain over a dipole will not be noticed because the main lob is pointing to the sky Total Field E NEC Pro 4 E NEC Prov4 27 185 MHz Elevation Plot Cursor Eley 0 0 deg Azimuth Angle 0 0 deg Gain 3 01 dBi Outer Ring 3 01 dBi 0 0 dBmax slice Max Gain 3 01 dBi Elev Angle 0 0 deg Front Side 99 59 dB Beamwidth 59 0 deg 30B 3430 5 279 5 deg Sidelobe Gain 3 01 dBi Elev Angle 180 0 deg Front Sidelobe 0 0 dB Above a 3 4 wave centre fed dipole The Sigma IV is a 3 4 wave antenna where the bottom part 3 4 wave is acting as a transmission line where not all currents are cancelled but most of them are For that reason the gain of a sigma 4 can never be higher than 3 dBi but will be lower And gain will be higher compared to a dipole 2 14dBI because we do have some radiation from the cone Total Field JE EZNEC Prov 27 185 MHz Elevation Plot Cursor Eley 0 0 deg Azimuth Angle 0 0 deg Gain 2 14 dBi Outer Ring 2 14 dBi 0 0 dBmax Slice Max Gain 2 14 dBi Elev Angle 0 0 deg Front Side 99 99 dB Beamwidth 6 2 deg 116 320 9 39 1 deg side
22. e distinct difference between the colour is On one side we have a sign and on the opposite side a sign With the information provided from the previous pages there is only one conclusion The discussed CST plot is showing The direction and magnitude of the near magnetic field H field www cb antennas com Sigma 4 1 01 Jan 2015 20 4 HOW DOES IT LOOK WITHOUT DIRECTION have been in a position to ask those with access to CST if they could model the Sigma IV and provide the plot without magnetic flux direction indication The plot that remains can be seen below H Field gt h field f 27 205 1 gt Abs h field f 27 205 1 peak e Cutplane name Cross Section A Cutplane normal 1 0 0 Cutplane position 0O omponent Abs 2D Maximum A m 13 62 dB Frequency 27 205 Phase 0 And now we have a view of something that can be seen using free software like 4nec2 MAGNETIC H FIELD SIGMA WI WITHOUT MAGNETIC FIELD DIRECTION CST provided on the left and 4NeczZ on the right And now we can see that besides the slight difference due to different software freq power max scale etc They are telling us the same story CST AND 4NEC2 ARE PROVIDING SIMILAIR RESULTS 21 www cb antennas com Sigma 4 1 01 Jan 2015 4 WHAT CAN WE LEARN FROM THE PLOT Now that we realise what it is that we are watching Perhaps we can gather knowledge about the antenna from the plot The reason why the colour
23. e used ideal antennas no loss no common mode current etc In a real situation sadly things are not ideal Let us have a look at the other commonly used antennas 6 COMPARED TO A HALF WAVE END FED VERTICAL We are expecting a half wave end fed vertical to provide 2 14 dBi gain but is that true in all cases The 2 14 dBi gain mentioned is for a resonant half wave dipole without any losses The antenna we commonly use as a half wave is the half wave vertical end fed In that case There always is some form of matching system coil and sadly not all are constructed that well Besides the possible matching losses not all manufacturers use materials with performance in mind Some manufacturers true in commercial FM broadcast applications as well put the commercial aspect above performance So from a construction point of view we could have less performance than we initially would expect Another possible fact that could easily influence the performance of an end fed half wave antenna is the basic of the antenna It lacks the second part of the antenna The more knowledge you will gain investigating antenna theory the more you will find out that symmetry is important A half wave end fed vertical doesn t have a ground plane That will cause common mode currents that could easily influence performance Most of the CB enthusiast will have heard about common mode currents they have heard they can cause is
24. ec manual EYNEC Prov EYNEC Pro d Above we have two examples The first with the source rather close to a segment junction and the second with the source moved slightly upwards away from that junction resolving to more accuracy 10 www cb antennas com Sigma 4 1 01 Jan 2015 2 4 The tapering of aluminium quote Eznec manual NEC 2 is known to be inaccurate in modelling connected wires having different diameters Note This is sometimes called being tapered It shouldn t be confused with EZNEC s segment length tapering end quote Eznec manual A possible solution to this problem is Not to use different diameters but model the antenna with a single diameter Although we are not able to copy the exact antenna provided by a manufacturer we are able to analyse the performance of it There are ways to calculate the total electrical length from a radiator using tapered aluminium tubing That total length can then be applied to a single diameter and you would have the same outcome Another option would be to use another engine NEC 4 though more expensive is capable of handling this with better accuracy It is beyond this article to provide information on how one should handle all the different situations But in this case they are not a limiting factor of the accuracy of NEC as long as we realise what we are doing and how things should be done 2 CONFIRMATION myself do not have a deep technical background like many of
25. eral regions Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Regions The near field can be divided into two different fields The famous CST plot The colour indication Plot E field dipole Plot H field dipole Plot H field cone Sigma IV Plot Sigma IV far field and magnetic H field 4 Perhaps now we are able to understand what the CST plot is providing Page 20 Page 21 Page 22 Page 23 Page 25 Conclusion from page 12 19 How does it look without direction What can we learn from the plot We are going to start drawing Finally getting somewhere The cone not a transmission line From page 25 27 5 What can we expect from the Sigma IV and additional information Page 28 Page 29 Gain from a 3 4 wave monopole radiator Sigma IV free space gain page 29 30 6 Reasons for gain but not gain itself Page 31 Page 32 Page 33 Page 36 Page 38 Page 41 Page 42 Page 43 Page 44 The average user Reasons Real and ideal Compared to an end fed half wave vertical Compared to a 5 8 wave ground plane antenna Real live testing Room for improvement overall conclusions Thank you www cb antennas com Sigma 4 1 01 Jan 2015 1 OVERALL DESCRIPTION OF THE ANTENNA The antenna A google search with just the name of the antenna vector 4000 will provide about 50 million hits If we add the words forum or debate to google it still provid
26. es about 500 000 hits an indication it is well known Overall the antenna is most commonly seen on the CB band and can be found under various names like Sigma IV Vector 4000 CTE saliut lw 150 Targa BT 104 Q 82 etc Though there are examples since 1996 available for other bands up to VHF region like FM broadcasting Almost every present day CB user has heard of the antenna In fact many have used or are still using the antenna today So what does it look like www cb antennas com Sigma 4 1 01 Jan 2015 1 LOOKING AT THE ANTENNA WE COULD DESCRIBE IT AS In the centre we have a 3 4 wave long conductor some manufacturers speak of a 7 8 wave radiator with a physical lengths for 27 MHz between 8 5 and 9 15 meters Around that for CB terms relatively long radiator we have a cone with 3 or 4 radials which are about a 1 4 long and electrically connected to the bottom of the 3 4 wave vertical section at an acute angle At the top end of the radials there is a horizontal loop attached At the bottom you will often find a matching system in the form of a gamma match And the antenna performs as a vertical omnidirectional antenna that s basically it HISTORY The origin of the antenna lies in the hands of a company called Avanti Research amp Development Inc Who were later bought by The Antennas Specialist co In 1980 Avanti gained a Patent number 4282531 That patent speaks of A communicat
27. fficulties we could encounter modelling the antenna with MoM it becomes clear There are some details that need to have attention if we want accurate information It is easy to make mistakes and the term difficult to model could be applied to the antenna for a beginner Although we will never know for certain it is very plausible he indicated with his words some possible common made errors with antenna modelling using NEC MoM Some of them are 1 Closely spaced wires 2 Wires attached to each other at an acute angle 3 Source placement at a segment junction 4 The tapering of aluminium 5 The ring attachment to the upwards folded radials But there are others To give a brief indication we will look at the five mentioned www cb antennas com Sigma 4 1 01 Jan 2015 2 1 Closely spaced wires The Eznec manual 10 provides an insight quote When modelling parallel or nearly parallel wires which are closely spaced it can be very important to align the segment junctions That is they should be directly across from each other This is particularly true if the segment length is greater than the line spacing end quote Eznec manual With the Sigma 4 at the bottom of the cone we have a region where parallel wires exist It is important to be aware of the individual segment lengths and apply the precaution provided in the manual I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
28. ground plane gain figure If we compare the Sigma IV against the world s best 5 8 wave without any losses both at the same mast the conclusion will be Total Field EZNEC ww cb antennas com Black Sigma I v Blue 5 8 wave Both antennas situated at wave length above real average ground 27 185 MHz Elevation Plot Cursor Eley 3 0 deg Azimuth Angle 292 0 deg Gain 4 16 dBi Outer Ring 4 16 dBi 0 0 dBmax 0 0 dBmax3D 3D Max Gain 4 16 dBi Slice Max Gain 4 16 dBi Elev Angle 3 0 deg Beamwidth 20 8 deg 30B 3 4 24 2 deg Sidelobe Gain 4 14 dBi Elev Angle 172 0 deg Fronv Sidelobe 0 02 dB The Sigma IV can outperform a 5 8 wave vertical For as long as know there has been an ongoing battle between the best 5 8 wave or 64 wave antenna Please do not be fooled by how it looks how expensive it is or what others have said about the best 5 8 wave antenna If a person indicates his performance went up it only means his reference material was not functioning as he thought it was We cannot construct a 5 8 wave antenna that performs beyond the capabilities of a 5 8 wave We can only construct it in such a way that it is has less loss compared to others By looking at the above plot and the diagram provided on page 33 we notice one of the claims mentioned on page 6 point6 is very plausible p6 6 The gain will become more obvious at the distance horizon The more gain at low angle cl
29. however we can t draw a conclusion on how the far field will be formed from the near field region as the far field is not established yet The antenna is still working on its far field radiation pattern We can t provide any details about antenna gain in the far field region from that near field plot Perhaps that s difficult to understand So imagine we are looking at a car It is parked somewhere and the engine is running we take a glimpse inside and a look at the gasoline indicator Looking at near field From that we draw the conclusion It will be driving 100 Km with speed of 50 Km hr far field It is obvious that would be a rather strange conclusion We would have no idea what is going to happen Perhaps we will just turn the engine off and wait until the owner returns A picture from Wikipedia tries to explain the interaction NEAR FIFLD FAR FL F HOH RADATNMVE AADIATWE RCAC TIVE FRES HEL 13 www cb antennas com Sigma 4 1 01 Jan 2015 3 THE CST PLOT A very interesting film has been provided by Sirio and can be found online from several hosts Although to my knowledge Sirio has not claimed to prove anything with the film there are others who have based their theories on it The collinear theory is for example partially based on that film The animated film we are talking about looks like this single frame as the film can be found online Why is the famous CST plot of interest
30. ions antenna is disclosed having improved mechanical strength gain and reduced lateral dimensions The antenna includes a three quarter wave length vertically disposed elongated radiating element and a plurality of one quarter wave length diverging elements connected at their bottom end to the bottom o the vertical radiator The diverging elements flare upwardly and outwardly from the radiator at an acute angle typically between about five degrees and about thirty degrees The band width of the antenna may be broadened by interconnecting the free ends of the diverging elements such as by a conductive hoo SIGMA IV OO INDUCTIVE 3 4 WAVE OMNI OIRECTIONAL On ANTENNA Now we have a rough indication which antenna we are talking about we will start digging www cb antennas com Sigma 4 1 01 Jan 2015 1 WHAT ARE THE CLAIMS FOUND Intense work has been done by several enthusiasts who are active on The worldwidedx forum A forum which for the average CB enthusiast could be considered one of the best in the world due to the knowledge and the way of debating from the active members There are several topics where the antenna is discussed including links to different sources A summary of the claims found are 1 The Avanti patent speaks of 2 2 dB gain over a dipole begin quote In one disclosed embodiment for use in the C B band the maximum radial dimension between the antenna radiator and the diverging elements is about
31. is that maximum gain from the antenna will become higher if we go up in frequency Effectively making the antenna longer That is what could be expected as longer often means more gain It is important to realise these figures are provided in free space there is a catch which we will discover in chapter 6 Conclusion The Free space far field gain of the Sigma IV antenna is in the order of 2 2 to 2 5 dBi 30 www cb antennas com Sigma 4 1 01 Jan 2015 6 REASONS FOR GAINBUT NOT GAIN ITSELF COMPARED TO OTHER ANTENNAS So far we haven t provided an indication if the antenna could really outperform traditional antennas Traditional antennas for a CB user for example a 1 2 wave or 5 8 wave end fed vertical As we know there are many who claim the antenna does Amongst these claim there are independent antenna user Being independent It is not that strange to think there must be something going on that is responsible for these good results Those lacking knowledge will be tempted to take the explanation from a manufacturer for granted including the gain figures in order to provide reason for the given better performance That brings us to the questions What could be the reason the antenna has gained such a good reputation 6 THE AVERAGE USERS Before we start comparing as we should do there are some situations that could happen to the average user Situations that could possibly influence the
32. lobe Gain 2 14 dBi Elev Angle 180 0 deg Front Sidelobe 0 0 dB Above the elevation plot of a dipole in free space providing 2 14 dBi 28 www cb antennas com Sigma 4 1 01 Jan 2015 5 SIGMA IV FREESPACE GAIN Modelling the antenna with a Mininec3 engine developed by J C Logan and J W Rockway at the Naval Ocean Systems Centre in San Diego gives us the result provided in the plot below The software used was antenna model software from Teri software sigma 4 Free Space Avg Gain 0 9900 Max Gain 2 26 dBi zA 27 185 MHz gt 29 www cb antennas com Sigma 4 1 01 Jan 2015 The far field gt far field plot analysed using CST is providing 2 22 dBi EZNEC Prov 27 185 MHz Elevation Plot Cursor Eley 23 0 deg Azimuth Angle Gain 22r dBi Outer Ring 2 2F dBi 0 0 dBmax Slice Max Gain 2 2 dBi Elev Angle 23 0 deg Front Back 3 15 dB Beamvwidth T4 3 deg 308 339 7 54 0 deg Sidelobe Gain 2 24 dBi Eley Angle 157 0 deg Front Sidelobe 0 03 Above is the free space elevation plot provided using a NEC4 engine in Eznec Pro again indicating 2 27 dBi Gain from a monopole antenna will vary with the frequency of operation If we analyse the gain provided from the Sigma IV versus frequency in free space we can provide Freespace _ Shee Max Gain dBi vs Frequency MHz frequency versus Max gain a p a Lb i Frequency MHz The indication provided
33. nd fed half wave could very well not have been what we expected Those reasons are often of more impact than we would expect Important thing to remember is Although we think we have a good setup with our present setup it could very well not be the case Do not take for granted your end fed half wave is always performing as we would like to even though we have no indication there is something wrong 36 www cb antennas com Sigma 4 1 01 Jan 2015 6 HOW WILL THE ANTENNA FUNCTION COMPARED TO A 5 8 WAVE VERTICAL A lot of the performing errors mentioned in aspect to the end fed half wave are true for the 5 8 wave and could have a negative influence on the Sigma IV as well But besides those there is a very stubborn myth about the 5 8 wave that just will not go away As often with antenna myths there is some truth Many of us have heard in the past well theory is theory and practice is what is important however If theory and practice are not providing the same answer it is either that theory is not understood or practice is wrong And the case of the 5 8 wave is certainly a case where theory is not always fully understood For example Most of us have the impression the 5 8 wave vertical has say 3dBi gain give or take a bit Or we have seen a plot somewhere in a theory book that is similar to this one EZNEC MA COS ieee CO black VI weve blue 5 8 wave Both antennas situated over perfect conducting
34. ne should see it more or less as a direction indicator The direction will depend on the period of the RF cycle applied to the antenna This is different to the conclusion some have made that they think they are watching antenna current and the phase angle of it Although antenna current and the magnetic H field are related magnitude they are different The magnetic H field expanding away from the antenna will lose strength it is round and has magnetic flux direction depending on the phase angle of the RF wave www cb antennas com Sigma 4 1 01 Jan 2015 15 3 PLOT E FIELD DIPOLE To provide more insight about this have modelled a centre fed dipole in FEKO Below is a view of the E field near field region including arrows to indicate directions _ Instantaneous magnitude XY Z E Field dB m 0 0 6 0 12 0 18 0 24 0 30 0 36 0 42 0 48 0 54 0 60 0 16 www cb antennas com Sigma 4 1 01 Jan 2015 3 PLOT H FIELD DIPOLE And the magnetic H field near field region of a centre fed dipole in FEKO provided below including arrows to indicate direction of rr ipl test nn S E Instantaneous magnitude XY Z H Field dBA m 0 0 6 0 12 0 18 0 24 0 30 0 36 0 42 0 48 0 54 0 60 0 Please notice that the arrows point in the opposite direction of the vertical dipole Try to imagine horizontal circles around the dipole and the arrows are indicating direction 17 www cb antennas com Sigma 4 1
35. ntenna is difficult to model with Eznec One of the frequently heard claims is The gain will become more obvious at the distant horizon Another one is This antenna is beyond common knowledge as it has two currents being active on the same conductor And finally The ARRL article about the open sleeve antenna gives an indication there is a large amount of gain possible from a monopole antenna 3 4 wave length in height During our analysis we will investigate those claims We will start with modelling as that is the starting point from which every present day antenna is developed www cb antennas com Sigma 4 1 01 Jan 2015 2 CAN THE SIGMA IV BE MODELLED The question arises can the antenna can be modelled using the Method of Moments as is integrated in antenna software like 4NEC2 1 and Eznec 2 Or that we need to find another solution to provide accurate information Although there are several who have provided a NEC based model see appendix 2 The validity of the models provided has been under question This because of two reasons In the past there has been contact with L B Cebik 3 about the ability to model the antenna He responded That it will be difficult to model the antenna with Eznec and that accuracy is a point of concern Those words are given extra strength by the interpretation by some that software like CST 4 is capable as they are under the impression that the CST analysis provided by Sirio 5 pro
36. s we are using There still is a lot of room for an antenna manufacturer to improve the present day available vertical antennas This from both a mechanical and electrical point of view The ongoing battle of the best 5 8 wave or 64 wave is most likely not going to be the one that will result in the best signal for a CB enthusiast And although the Sigma IV is already a good performer we have also found some grounds to further investigate to optimise the overall performance of the antenna itself The aim of this article was to provide further insight in the claims made for the Sigma IV We have tried to cover all aspects mentioned and hope the effort done could be an asset to those with interest about the Sigma IV antenna Warm wishes Henry Poelman PGODX 19PA348 42 www cb antennas com Sigma 4 1 01 Jan 2015 7 Thank you To those active on the world wide dx forum for the intense work they have done in the past Bob DB Donald Marconi and all others for keeping the debate alive and for always helping those with questions And to those who consider experts am beyond grateful for the time they spend in the past answering questions and hope can take advantage of that privilege in the future Thank you Arie Voors Brian Cake From who I have learned so much THANK YOU CST FEKO Roy W Lewallen Steve Hunt Wim Telkamp References 1 4nec2 2 Eznec 3 L B Cebik 4 CST 5 Sirio 6 NEC MoM FEM MLFMM FDTD
37. sues with RFI or SWR and some know they can influence the antenna pattern as well As soon as we have issues with RFI or QRM we can notice they are there The thing to remember is They can be there without the average CB user having an indication they are They can influence the antenna pattern even when you do not have issues with SWR and while you do not have issues with RFI or any other indicator An example 35 www cb antennas com Sigma 4 1 01 Jan 2015 B 2D Plot sigma 4 File Edit View Options Reset Total Field 27 185 MHz Elevation Plot Cursor Eley 128 0 deg Azimuth Angle 0 0 deg Gain 2 63 dBi Outer Ring 2 63 dBi 0 0 dBmax Slice Max Gain 2 63 dBi Elev Angle 128 0 deg Beamwidth 20 8 deg 3048 116 6 137 4 deg Sidelobe Gain 2 35 dBi Elev Angle 52 0 deg Front Sidelobe 0 28 dB Above is an example of a half wave end fed vertical have attached a mast next to it As we Can notice the antenna pattern of the antenna is disturbed The main lobe is pointing upwards instead of to the horizon where we actually want to have our main focus The loss at low angle is over 6dB compared to maximum performance at same height The conclusion drawn compared to an half wave end fed It is very plausible there are many who have noticed an improvement compared to the half wave end fed vertical for several reasons Not only the advantage of height of the Sigma but also the expected performance of the e
38. tenna has several regions 3 Regions We can divide the radiation from an antenna into several regions These fields are specific areas space around an antenna Different regions areas have different names and represent different distances from the antenna Close to the antenna we have The reactive near field region Beyond that region we call it The radiating near field region also known as the Fresnel region And eventually we have The Far field region Fraunhofer Far Field region It is difficult to express the exact point of transfer from one region to another as they gradually change from one to the other For a approximate indication antenna engineers use the description given by C A Balanis 15 C A Balanis speaks of Reactive near field region R lt 0 62v D A 5 meters from the antenna Radiating near field region R lt 2D7 A 15 meters away from the antenna Far field Region R gt 2D2 A beyond 15 meters away from the antenna Where R distance D the antenna maximum antenna size and A the wavelength all provided in meters Those regions can be divided again into other regions For now we will focus on the near field fields 12 www cb antennas com Sigma 4 1 01 Jan 2015 3 The NEAR FIELD can be divided into the The Electric field E and the magnetic field H For the purpose of analysis an indication of those fields is very interesting
39. th loads inserted provides proof there is some radiation from the cone due to the effects mentioned The gain is in the order of 28 dB which is still a lot better than a dummy load but not high enough to be a large influence The influence of that amount on the entire system will become clearer in chapter 5 Those effects mentioned are currents not completely cancelled and currents that are active due to near field coupling And that is most likely what Cebik meant with non apparent collinear effect which was one of the questions we were hoping to find an answer to point 4 page As the term collinear gives an idea we have two currents working resulting in a single current net result And non apparent is most likely to indicate it is not in a way most think about collinear theory When Cebik mentioned the non apparent collinear effect he never mentioned that it would be of a significant amount or that it will be beneficial though he did mention it That he preferred not to go into detail as he imagined it would be difficult to understand for the average Sigma4 user We now have an indication there is some radiation from the cone but we still need to find out if it is contributing to the performance of the entire antenna Will it contribute to the radiation at the angle we want it to be or is it pointing towards the sky for instance That brings us to the question what gain can we expect from th
40. the VERON ANTENNE MEETDAG an old large UHF Yagi was tested for gain They noticed gain went up as soon as they start polishing the antenna 2 And who knows besides changing the antenna We also may have changed the coax 31 www cb antennas com Sigma 4 1 01 Jan 2015 can imagine someone to think The free space gain figure seems rather low it is only in the order of 2 2 to 2 5 dBi From that perspective the advantages of the Sigma IV seems rather small Not only the social aspects or mechanical aspect can contribute the antenna in a real situation could perform beyond what we expect from the indicated free space gain figure Again we are asking why why has the antenna gained such a good reputation Perhaps there are other reasons that could be beneficial to the performance of the antenna Reasons that would not be so obvious at first sight but perhaps explain why there are those who have noticed several dB s difference when they compared the Sigma IV against another antenna And there is 6 REASONS First we will go back to the free space gain versus frequency plot page 30 But now we will set the antenna in a real situation not in free space but above actual earth If we calculate the gain in a real situation and measure it at a low angle say at a 3 degrees take off angle We can notice Slice Max Gain dBi vs Frequency MHz frequency versus Max gain above real ground at 3 degrees take off angle P
41. tion possible for good reason As soon as we place the antennas at a reasonable height the advantage of the 5 8 wave compared to a dipole becomes less Above say 12 13 meters the dipole will actually start to outperform the 5 8 wave vertical Same tip height and the advantage of the dipole will increase beyond that height And now we have a situation the average CB can relate to It is not that complicated for most to install an antenna at those heights It will be difficult to find a manufacturer who will tell you a 5 8 wave is not always providing maximum gain at low angle and it is not providing say 3dB s over a 1 4 wave vertical in a real situation And that myth is backed up by theoretical articles where most of them are all based on the infinite ground theory or as mentioned are written with low band broadcasting in mind near a created good earth That is also the case with the claim made page 6 point 7 about the ARRL open sleeve antenna It is a theoretical approach The trick they apply is to place the 5 8 wave over a perfect conductor 38 www cb antennas com Sigma 4 1 01 Jan 2015 Though there are situations where the infinite ground conditions are of interest it is not for the average CB user It is obviously of far more interest from a commercial point of view not to tell what we can expect in a real situation For that reason they tend to keep referring to the theoretical infinite
42. us and will never consider myself an expert But there are those who if they want or not in my eyes can be considered as such Due to circumstances had the pleasure to have been in contact with G J Burke of the Lawrence Livermore National Labs in California 11 For those who do not know of him He is one of the code writers for NEC Someone can imagine software writers would go to to discuss a certain subject about NEC capabilities When asked If he saw any difficulties modelling the Sigma IV with NEC His reply was that the antenna doesn t look like something NEC couldn t handle Other known people experts within the modelling software community like Roy W LeWallen writer Eznec 12 ArieVoors writer 4NEC2 13 as well as Brian Cake 14 and several people from CST all provided the indication that MOM is capable This is of course if done correctly Thankfully there are several ways to verify if the model is correct without asking a knowledgeable one One of them is the Average gain factor Software like 4NEC2 and Eznec and others have such a verification tool The average gain factor is the total far field power divided by the power applied to the source That factor should be in the order of 1 if it isn t there is a good indication that there is something wrong with the source placement There are more options which go beyond the nature of this article 11 www cb antennas com Sigma 4 1 01 Jan 2015 3 An an
43. various antennas on that mast We also notice that the difference will become less with added height 33 www cb antennas com Sigma 4 1 01 Jan 2015 Height 7 0 80 90 10 0 11 0 12 0 13 0 140 150 160 17 0 180 19 0 20 0 21 0 22 0 23 0 240 25 0 26 0 27 0 28 0 29 0 30 0 Signal strength received at 30 Km distance at 10 meters height From transmitter Variable antenna tip www cb antennas com height equal for all S M 0 005 Diel const 13 antennas soil type medium This second diagram above is showing the signal strength in dB s This measured at a single point 30 Km away from the transmitting antenna At that receiving distance it is measured at a height of 10 meters All antennas again were ideal no losses and no other influence The soil type between the antennas was medium The variable is the mast height of the transmitting station Where each time the tip height of the antenna was kept equal Conclusion Antenna height is a factor that should not be underestimated Antenna height can make the difference between having several dB s advantage or not When placed at the SAME mast height and all antennas being ideal the observer is most likely to notice several dB s of gain when he compares the Sigma IV against other commonly used verticals When placed at the SAME tip height the difference will not be that obvious 34 www cb antennas com Sigma 4 1 01 Jan 2015 So far we hav
44. vides different results when compared with NEC Which Method needs to be used To answer the question we need to know the limitations of NEC MoM and find out if other approaches like FEM MLFMM FDTD would be of beneficial use 6 A first indication came from a seminar in which had a conversation with Frank from CST asked him What would be the best method to model an antenna like the Sigma 4 There was no hesitation the answer was by using the MOM method Advanced software like CST and FEKO 7 and IE3D 8 are capable of using different methods of calculating The MOM solution and advanced MOM solvers are integrated as well To understand which different solvers could be used another EM field software FEKO provides the below given indication on their website Hybridisation to solve large and complex problems tbl N bt V on a Q p a _ O LLI LLJ COMPLEXITY OF MATERIALS FEKO published with permission From the above picture provided by FEKO image courtesy of Altair www altair com We are able to extract that MoM and FDTD are the initial methods for solving most antenna electromagnetic field equations Each solver has of course its limitations for us to find out how these apply to the Sigma IV www cb antennas com Sigma 4 1 01 Jan 2015 2 CEBIK SAID DIFFICULT TO MODEL As with all calculations the input needs to be correct to have a reliable outcome And so
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