FIM-71 Operating Instructions
Contents
1. 3 7 than 0 2 dB The average detector accuracy limits must be included in estimates of peak measurement uncertainty To correct the peak detector measurements divide the volts reading by the left hand ordinate value corresponding to the scale indication and FULL SCALE setting alternatively algebraically subtract the right hand dB values from the 20 to 0 dB scale indication The peak detector correction curves only apply to the LIN linear mode and the TV wide IF bandwidths Also never use the peak detector to measure CW signals or long pulses with amplitude less than 1 mV Note that the correction curves cannot account for peak detector errors due to external inter ference such as automative ignition noise The FIM 71 peak detector response is reduced by at least 15 dB for impulse like pulses with a repetition rate of 200 Hz or less Equivalent to an 8 cylinder engine running at 3 000 RPM 3 4 FIELD STRENGTH MEASUREMENTS WITH THE ANT 71 3 4 1 Mechanical Features For storage and transportation and for near ground measurements the ANT 71 dipole antenna is normally attached to a telescoping mast which is supported by a swivel mounted on the right side of the FIM 71 see figure 3 5 The mast swivel locks in three positions vertically upward at an angle of 60 de grees with respect to the front panel and vertically downward for storage The telescopin2. SAN pasn sugsueT s uq fHMSA peansPeu H HER E unutup 04 qu u eT squrod p 3qold Hr 2 1 HH ST it rpo gt 2 TIS TITI Tp 2 bey PETIT 21 12 4 21 i arani popapo ripo eee m F cea 21441111 Ep ETIS 141 gt ELEL LY 217 i Dy Pipe ririo 12 SSE Se ee z s 1 1 14110111 21112 api PS et 11 4 4 EEE Sed oi 1 iri EE po m SoA Epen erp 1 nirin Pn A TA 2 ME zq Iyoo 11112 te beep pea ep Diy d on oo 09 z LIPIS EMEI 17 a 151011111 3 L 21 11 1111134 Z 2 ro os 4 4 t s PLiit NGT EITT X m none 2 SERS os a Drill pl z SG m 2 1 r 4 fee tt 3 SS och m 2 lt 2 m H T i pod Jee 19 31 dN INVW OL 5 011235 Y3L3WVIG 1539 1 3Sn L JASNO WO 7 H19N31 OS OL
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5. quBTey AUNO ot TA INY THOON VNNHINV ZIOdIG SA 58401094 VNNELNY 4 i i 1 tr al i ta k i I i i a Set the FULL SCALE switch to the 100 MV CAL position Set the OSCillator switch to CALibration after a slight hesitation the panei meter should indicate up scale Rotate the 10 turn GAIN control for a reading of exactly 0 dB on the meter scale corresponding to the METER switch position full scale for LIN FS REF on green scale for LOG d Return the OSC switch to OFF The instrument is now calibrated for reading voltage e If applicable reconnect signal source to RF INPUT NOTE The above calibration steps a through e does not have to be repeated before every reading particularly when only relative readings are required Receiver gain may change with frequency with the position of the IF BW and MTR switches and with en vironmental conditions Calibration routine should be based on experience with the instrument and the measurement conditions 9 After calibration set the FULL SCALE switch for an on scale reading and carefully retune for a maximum panel meter indication 10 With the METER switch in the LINear position the input voltage can be read directly on the VOLTS scale by noting the position of the FUL
6. Amphenol 69475 for RG 174 U cable 3 Connect a BNC adapter UB 274B U directly to the RF INPUT connector on the side of the FIM 71 Connect the quarter wave stub to one input of the T adapter using a short 50 ohm cable connect the other input of the T adapter to the OSC OUT connector 4 Set the FIM 71 controls as follows POWER ON MTR LOG OSC OUT DET AVG FULL SCALE 10 mV 5 The panel meter should indicate up scale Starting at the fundamental frequency slowly tune the voltmeter in the direction of lower frequency to find the frequency cor responding to the minimum panel meter indication the notch frequency Decrease the setting of the FULL SCALE switch if necessary 6 Using a sharp cable cutter or knife cut a small piece off the end of the cable and tune the voltmeter to find the new notch frequency Continue this process until the notch frequency is equal to the fundamental frequency Set the MTR switch to LIN to obtain greater resolution as the notch frequency approaches the funda mental frequency and be careful not to cut off too much cable If the cable is to be coiled up or otherwise contained this should be done before the final cut 7 finish the trap dip the end of the cable in an insulating varnish such as RED GLPT G C Electronics catalogue number 90 2 when dry tape the end to prevent fraying The trap should be labeled with the notch fundamental frequency to 3 17 8 use the
7. holes which comfortably position the antenna elements approximately 7 feet above the ground with the mast fully extended Connect the short coaxial cable from the antenna output connector on the bottom of the black balun housing to the RF INPUT on the receiver 3 4 5 Elevated Measurements For higher measurements such as the F C C approved 30 foot TASO tests the antenna is re moved from the telescoping mast by unscrewing the 24 20 stud from the support bushing on the bottom of the antenna balun housing a 7 16 inch wrench is required The antenna can be attached to any mast fitted with 4 20 screw not more than 7 16 inch long the balun support bushing contains a self locking insert Connect the 34 foot coaxial cable from the antenna to the RF INPUT on the receiver Calibrate the cable as described in paragraph 3 4 6 Tape or otherwise attach the cable to the mast to relieve the stress on the cable connector and to maintain the electrical symmetry of the antenna 3 4 6 Cable Attenuation NOTE The ANT 71 ANTENNA FACTORS given in CURVES A and B relate the field strength to the antenna output voltage at the balun output connector terminated with 50 ohms The following procedure provides a means of measuring and compensating for the losses of the cable connecting the an tenna to the receiver This procedure insures that the accuracy of field strength measurements are independent of the particular cable in use and will also quickly show
8. long mast sections into the antenna balun box screw the second long section into the first and screw the base section into the second long section 7 Install the antenna and mast on the FIM by reassembling the rotating joint in the base section of the mast Orient the antenna assembly as desired and tighten the thumbscrew 8 Connect the 45 inch RG 223 cable supplied in the FIM cover between the antenna connector and the FIM s RF INPUT connector 9 Adjust the antenna element length as required TO RETURN THE ANTENNA TO THE STORAGE POSITION Carry out the procedure above in reverse At Step 4 when reassembling the antenna and stub to the base section orient the antenna box with its long side parallel to the end of the case and tighten the thumbscrew Potomac Instruments inc Date of issue 8 1 90
9. treet tee be ped pridi porch ie fo rag ts Heir HH H e h 210177 0241 6 8 4 9 s 3TVIS SLTOA 3 9 1 LOOSEN THIS KNOB AND PULL THE BODY OF THE ANTENNA TOWARDS THE 2 CERTAIN THE BUTTON ON THE BALUN HOUSING CLEARS THE SUPPORT GROOVE Figure 3 5 Antenna Deployment Sheet 1 3 10 3 ROTATE ANTENNA BODY IN CLOCKWISE DIRECTION UNTIL THE DETENT PINS SNAP INTO THE DESIRED POSITION VERTICAL 609 ANGLE TIGHTEN LARGE KNOB TO LOCK IN POSITION RF INPUT 986 QUT 4 LOOSEN KNOBS AND PULL ANTENNA ELEMENT TOWARDS THE KNOB SWING ELEMENT IN CLOCKWISE DIRECTION UNTIL DETENT PINS LOCK INTO DESIRED POSITION VERTICAL OR HORIZONTAL WITH 2 59 UPLIFT TIGHTEN KNOB TO LOCK POSITION Figure 3 5 Antenna Deployment Sheet 2 3 11 3 12 5 ADJUST ELEMENT LENGTH AND MAST HEIGHT AS REQUIRED REVERSE THE SEQUENCE SHOWN IN STEPS 1 THROUGH 4 TO RETURN THE ANTENNA TO THE STORAGE POSITION Figure 3 5 Antenna Deployment Sheet 3 the antenna elements positioned 7 feet above the ground To hand hold the instrument lock the antenna mast in the 60 degree position so the receiver front panel slopes away from the operator when the mast is vertical Attach the neck strap to the black kleats on the sides of the instrument use the strap
10. 00 mV calibration signal is internally switched to the input of the RF attenuator also the RF INPUT BNC connector on side of unit is internal ly disconnected and bypassed to ground In the OUT position the Cal Osc is on and the calibration signal is switched to the OSC OUT 100 mV output BNC connector on side of unit also the RF INPUT is inter nally switched to the input of the RF attenuator and the receiver functions normally GAIN Control Adjusts the overall gain of the receiver Usually used in conjunction with the 100 mV calibration oscillator to compensate for variations in receiver gain or to compensate for external losses such as antenna cable attenuation PHONES Jack 75 ohms source wideband output from the selected AM or FM demodulator Variable with AUDIO control up to approximately 5 V peak to peak into 75 ohms 30 Hz to 100 KHz no deemphasis 1 4 inch diameter phone plug disconnects speaker RECORD Jack Provides two outputs Tip contact on standard 2 conductor or 3 conductor 4 inch diameter plug provides DC voltage proportional to panel meter indication approximately 0 8 V to 8 0 V 2000 ohms source resistance Ring contact on 3 conductor plug provides calibration oscillator AFC voltage 5 5 V 3 5 V 10 000 ohms source resistance EXTernal PoWeR Jack Located on rear of instrument accepts input from external power source such as the PI AC Power Supply Requires 11 5 V to 19 0 V DC with ground Mates wi
11. 6 With the OSCillator switch in the OUT position adjust the GAIN control for a meter reading of 0 dB This pro cedure increases the gain of the receiver to compensate for the loss in the antenna cable and can be used in either the LINear or LOGarithmic modes as long as the cable attenuation does not exceed the range of the GAIN control When measuring antenna output voltage according to paragraph 3 2 the calibration procedures in step 8 should nor be performed since the procedure given above effectively calibrates the receiver including the antenna cable and the GAIN control must nor be readjusted The adjustments of steps 1 through 7 should be checked periodically NOTE If the cable or measurement frequency is changed the procedures described in paragraph 3 4 6 must be repeated 3 4 7 Measurements and Antenna Factors Note the large arrow on the top of the antenna balun housing When measuring a signal the antenna must be rotated with the vertical mast as an axis for the maximum reading However two slightly different maximums usually with less than 0 5 dB difference will be found 180 apart Norm ally the higher maximum will be obtained with the balun arrow pointing approximately toward the source of the signal being measured and this antenna orientation should always be used Treating the 71 as any signal source measure the antenna output voltage with the pro cedure given in paragraph 3 2 Also review Paragra
12. FIELD INTENSITY METER MODEL FIM 71 OPERATING INSTRUCTIONS POTOMAC INSTRUMENTS INC 932 Philadelphia Ave Silver Spring MD 20910 Phone 301 589 2662 Fax 301 589 2665 www pi usa com SECTION 3 FIM 71 OPERATING INSTRUCTIONS 3 1 CONTROLS INPUTS amp OUTPUTS PANEL METER FUNCTIONS The following paragraphs describe the controls indicators and connectors for the FIM 71 Refer to figure 3 1 POWER Switch In the OFF center position the battery and external power source are dis connected also a conductor is switched across the panel meter terminals for maximum damping during transportation If the Rechargeable Battery kit is connected to the AC line the charging circuit remains ener gized with the POWER switch OFF In the TEST position the panel meter indicates the battery voltage or the external voltage when the external power plug is inserted The FIM 71 will not function properly if the TEST indication is below the green BATTERY EXTernal bracket also the instrument can be permanently damaged if the external voltage exceeds the EXT bracket In the ON and TEST position battery or external power is applied to the unit DEMODulator Switch Selects the AM amplitude or pulse modulation or FM frequency modulation demodulator The output of the selected demodulator is applied through the AUDIO control and demodulator output amplifier to the loudspeaker and PHONES jack AUDIO Control Adjusts the output level of th
13. Hz mark above the dial window Greater accuracy can be obtained by tuning to a signal of known fre quency within approximately 7 of the desired frequency and adjust ing the hairline cursor to the known frequency on the MHz dial The FIM 71 demodulators can be used as an aid in identifying a signal when when tuning the instrument Set the DEMODulator switch to AM or FM as appropriate for the signal to be measured and rotate the AUDIO control clockwise for the desired sound level at the phones or loudspeaker Calibration Steps a through e NOTE Measured signal voltages in excess of I V RMS may cause a small error when calibrating the instrument Also with the OSC switch in the CAL position the RF Input is bypassed to ground with a 001 microfarad capacitor For these reasons the signal source should be disconnected from the RF INPUT for calibration steps a through d in the following cases 1 Measured voltage exceeds 1 volt RMS 2 Signal source will be damaged by low impedance load and 3 Signal source can generate more than 0 5 watt across low impedance load taking into account im pedance transforming properties of transmission lines Ove 024 002 081 v 10122 PUUSJUY 7 6 911814 Z183HV93W NI ADNIN03yd 091 3 5 Ovi 02 001 1 HEE 024608 Jo spxepueqg nee
14. In the LOGarithmic position DC feedback is applied to the receiver to obtain a measurement range of 1 to 1000 or 60 dB and the panel meter deflection green scale is in equal decibel increments Also the RECORD output is proportional to the logarithm of the measured signal allowing a true dB plot on standard linear recording grid The LINear mode provides better measurement accuracy and lower AM demodulator distortion the LOGarithmic mode provides greater measurement range and better demodulator signal to noise ratio FULL SCALE Switch 7 position rotary attenuator provides RF and IF attenuation to set the full scale meter sensitivity in both the LINear and LOGarithmic modes 3 1 FREQuency Control and MHz Dial The FREQuency control adjusts the frequency of both the receiver and the calibrating oscillator over the entire frequency range without bandswitching The tuned frequency is indicated on the 6 turn spiral MHz dial under the hairline cursor and to the left of the RED pointer The calibrating oscillator frequency is maintained at the center of the IF passband with auto matic frequency control for any on scale panel meter indication CURSOR Control Adjusts the angular position of the hairline cursor to calibrate the MHz dial on a known frequency near the desired frequency OSCillator Switch In the OFF position the calibration oscillator is off and the receiver funct ions normally In the CAL position the Cal Osc is on and the 1
15. L SCALE switch and interpreting the VOLTS graduations as either 0 1 to 1 0 or 1 0 to 10 depending on the FULL SCALE multiplier In this mode input voltage can also be read in decibels from 20 dB to 0 dB with respect to the FULL SCALE multiplier ll With the METER switch in the LOGarithmic position an input voltage numer ically equal to the FULL SCALE switch setting will be indicated on the green dB scale at the FS REF full scale reference index at 0 dB In this mode input voltage is read in decibels from 20 dB to 40 dB with respect to the FULL SCALE multiplier 3 3 PEAK DETECTOR CORRECTION The peak detector is less linear than the average detector also for signals less than 10 micro volts internally generated noise causes a positive error which increases rapidly as the input voltage is reduced The peak detector characteristics are shown graphically in figure 34 for different settings of the FULL SCALE switch A minor change affecting the peak detector was incorporated in the 71 design starting with serial number 141 the dashed curves apply to instruments with serial number less than 141 The left hand ordinate of figure 3 4 shows the ratio of the peak detector reading to the actual peak value of the measured pulse corresponding to the meter indication the right hand ordinate shows the same ratio in decibels Actual Peak Value refers to the RMS value of the signal during the carrier on interval and is equ
16. MCC 71 The base section of the new mast is mounted to the case as the old mast was by a pivot assembly consisting of two cylindrical parts an inner part to which the mast is attached and an outer knob which clamps the inner part in place The inner part has detent pins which allow it to be locked in three positions straight down for storage straight up for use with the meter panel vertical and an intermediate position for hand held measurements using the neck strap The base section of the mast has a rotating joint clamped by a thumbscrew which permits the antenna to be oriented as desired This joint may be pulled apart when the thumbscrew is unscrewed is unscrewed two to three turns do not completely remove the thumbscrew TO SET UP THE ANTENNA ON THE SHORT MAST 1 Fully unscrew the outer knob of the pivot approx two turns 2 Pull the inner part of the pivot away from the case to free its detent pins and swing the mast and antenna forward and up until the pins fit into holes at the desired operating position 3 Tighten the outer knob to lock the mast in position 4 Unscrew the thumbscrew in the mast base section 2 3 turns and remove the antenna with the base section stub the shorter part of the mast base section attached 5 Unscrew the base section stub from the antenna balun box For the FIM 71 first swing the antenna elements up into their operating position leaving them fully compressed 6 Screw on of the
17. Pack 4 Identify the 4 conductor receptacle 1701 mounted the Battery Charger printed circuit board near the fuse holder Also straighten out the leads with the yellow and green pin plugs P701 and P702 5 Holding the Battery Charger in one hand above the battery compartment insert the yellow and green pin plugs P701 and P702 into the same colored jacks on the battery compartment board Bend the yellow and green leads down over the plugs After the pin plugs are inserted connect the 4 conductor Battery Pack Plug P801 to J701 3 19 3 9 2 3 20 6 Short circuiting the battery can cause damage to charger or battery Do not allow P701 to touch P702 after P801 is connected Also reversal of P701 and P702 will reverse the battery polarity and severely damage the FIM 71 Also some units have a third blue pin jack which is not used with the Rechargeable Battery Kit If there is any question concerning the proper connection of P701 and P702 contact Potomac Instruments don t guess Carefully lower the Battery Charger into the space to the left of the Battery Pack Dress the Battery Pack cable into the gap between the charger and the battery As the Battery Charger cover is lowered there will be some resistance when the foam backing presses against the battery however it should not be necessary to force the unit into place Pressing the Battery Charger panel flush with the rear of the FIM 71 complete the installatio
18. WER TEST reading below the BATTERY bracket battery damage will be prevented by the LOW BATTERY BREAKER which will release trip and disconnect the battery from the FIM 71 A tripped battery breaker is indicated by the red plunger which out almost flush with the end of the cylindrical guide Holding the LOW BATTERY BREAKER plunger in when it will not latch will severely discharge the battery and may cause damage to the battery the charger or the FIM 71 see step 8 10 sharp blow to FIM 71 such as dropping the instrument may trip the magnetically latched LOW BATTERY BREAKER even though the battery is fully charged If this happens simply reset the breaker by pushing in the red plunger To recharge the battery plug the female end of the line cord into the line connector J702 on the BC 71 panel Plug the line cord into a 3 prong U grounded receptacle with nominal AC voltage equal to the VOLTS AC engraved on the Battery Charger panel The red BATTERY CHARGING light on the charger panel will be energized whenever AC voltage is applied See WARNING at start of this section The FIM 71 can be switched off or it can be operated while the battery is charging Operating the instrument increases the recharge time by approximately 20 percent As the battery is being charged the POWER TEST indication should gradually rise to above the E BATTERY Also when charging a deeply discharged battery
19. al to the average detector reading of a CW signal precisely set to the pulse amplitude These curves are based on measurements utilizing a 4 8 microsecond pulse at a repetition rate of 15750 Hz the pulse amplitude was set 6 dB above a CW level simulating an grey TV picture The curves are typical being derived from many measurements the unit to unit spread in the data is believed to be less 3 6 2 daIng slope BUUJJUY E E NI 2183 AONANOSYS 091 oz xog NNIve Hdi 4 0 i i i 4 1 moto tt 3 1 Ei 4 free pet ppt yd faethe fad t on i pa d i t EHE EE 4 q t
20. at the fundamental and harmonic frequencies Antenna factors and antenna element lengths corresponding to the fundamental and harmonic frequencies are obtained from CURVES A B and C of figures 3 2 and 3 3 1 Using the procedure of paragraph 3 3 at the fundamental frequency adjust the antenna element length and calibrate the voltmeter including the antenna to voltmeter cable Measure the antenna output voltage and calculate and record the fundamental field strength alternatively see step 3 record the voltmeter reading and the antenna factor 2 At the harmonic frequency readjust the antenna element length and calibrate the voltmeter including the antenna to voltmeter cable and if applicable the band stop filter or trap 3 The ratio of the harmonic field strength Ej to the fundamental field strength can be expressed in decibels as follows Alternatively to reduce the number of calculations the ratio of En to be obtained directly in dB by recording the ratio of harmonic to fundamental 3 16 voltage directly in dB as done in step 4 of paragraph 3 6 2 and then adding the ratio expressed in dB of the harmonic to fundamental antenna factors Vi Vr dB 20 LOG V dB Ve dB 20 FULL SCALE steps CCW from Vr to 20 LOG KI Kp where Kp harmonic and fundamental antenna factors respectively 3 6 4 A Fundamental Frequency Trap A very simple yet effectiv
21. d harmonics will be at least 100 dB below a fundamental voltage up to 100 mV at any FULL SCALE position If the fundamental voltage is attenuated by at least 30 dB the band stop filter or trap attenua tion at the harmonic frequency should not exceed 1 5 dB when the voltmeter is tuned to the harmonic frequency internally generated harmonics will be at least 120 dB below any measured fundamental voltage up to 10 V If the fundamental voltage exceeds I V harmonic measurement made on the 10 uV FULL SCALE range may be inaccurate For fundamental voltage of 50 mV or less a fundamental band stop filter or trap is normally not required For fundamental voltage above 50 mV the need for a filter depends on the fundamental fre quency the order of harmonics 2nd 3rd etc the harmonic to fundamental ratio and the required measurement Consequently for the higher input levels the necessity for fundamental attention is best determined experimentally In general if the change in harmonic reading with and without the device is less than the required accuracy then the filter or trap is not necessary An inexpensive easily constructed trap suitable for second harmonic measurements is described in paragraph 3 6 4 below 3 6 2 Measuring the Harmonic Output of Voltage Sources 1 Using the procedure given in paragraph 3 2 calibrate the voltmeter including the signal source to voltmeter cable Measure and record the fundamental frequ
22. e fundamental frequency trap can be constructed using a T connector and a length of coaxial cable experimentally cut to 4 wavelength This quarter wave stub can only be used when measuring even order 2nd 4th etc harmonics Since the trap functions by shorting the fundamental voltage it will only be effective if the voltage source impedance matches approximately the impedance of the source to voltmeter cable Also if any of the conditions listed below are applicable it may be necessary to partially isolate the voltage source from the voltmeter and trap attenuators pads power dividers directional couplers sample probes etc are used for this purpose 1 Voltage source will be damaged if shorted by trap 2 Harmonic output of source increases significantly when fundamental is shorted by trap 3 Excessive power will be dissipated The trap described steps 1 through 8 below may be used with the ANT 71 antenna without additional isolation The maximum fundamental attenuation will be obtained when low loss cable such as RG 232 U is used however miniature cable such as RG 174 U is usually satisfactory and is very convenient 1 Cuta coaxial cable to approximately 80 of the length L given on CURVE C figure 3 3 corresponding the fundamental frequency This length should be 3 to 10 too long depending on the type of cable and the frequency Install a BNC plug on one end of the cable Use UG 88C U for RG 232 U cable or
23. e selected AM FM DEMODulator at the loudspeaker or PHONES jack With the AUDIO control set to the 0 mark power is removed from the FM discriminator and the output amplifier to conserve battery energy DETector Switch In the PEAK position the panel meter indicates the RMS value of the car rier amplitude during pulse on modulation Detector time constants are optimized for measuring the amplitude of TV syne pulses and the PEAK detector is usually used with the TV wide IF bandwidth In the AVeraGe position the panel meter indicates the RMS value of the carrier amplitude averaged over the modulation cycle and is usually used with the FM AM narrow IF bandwidth IF Bandwidth Switch Selects the TV wide or FM AM narrow bandwidth The wide band width is usually used when measuring the amplitude of TV sync pulses or to obtain the widest demodu lator bandwidth The narrow bandwidth is usually used when measuring aurally modulated AM or FM signals or to obtain the best demodulator selectivity and signal to noise ratio MeTeR Switch In the LINear position the receiver operates with constant gain and the panel meter indicates input voltage over a range of 1 00 10 or 20 dB The meter angular deflection is proportional to the logarithm of the input so voltage can be read with constant percent accuracy or in equal decibel increments Also the RECORD output voltage or current is linearly proportional to the measured signal amplitude
24. element length and antenna factor are obtained from curves C and A respectively see figures 3 3 and 3 2 for the frequency to be measured curve B is not applicable for vertical polari zation measurements 3 14 For non precise relative measurements near the ground ANT 71 can be set up to approxi mate a vertical 1 4 wave whip antenna With the arrow top of the balun housing pointing away from the user set the right hand element vertically downward and fully collapsed set the left hand element vertically upward and extend to the length given in curve C for the frequency to be measured Hand held readings can be made with the FIM 71 mast fully extended if the neck strap is used lock the mast into the 60 degree detent Antenna factors are not provided for this configuration BEWARE OF POWER LINES 3 6 HARMONIC MEASUREMENTS VOLTAGE AND FIELD STRENGTH 3 6 1 General With certain restrictions the FIM 71 can be used to accurately measure harmonic frequency signals which are more than 80 dB below a fundamental frequency signal Second and third harmonics can be measured for fundamental frequencies from 45 MHz to 75 MHz second harmonics can be measured for fundamental frequencies up to 112 5 MHz which includes the FM broadcast band The minimum measurable harmonic voltage is limited by the ultimate sensitivity of the volt meter and also by the amplitude of harmonics generated within the voltmeter due to the presence of
25. ency voltage Note that any band stop filter or trap must be removed from the set up when the fundamental voltage is measured 2 band stop filter or trap may be used to attenuate the fundamental voltage when the voltmeter is tuned to the harmonic frequency Connect the filter to the RF INPUT of the voltmeter and connect the source of voltage to the filter input 3 Calibrate the voltmeter including the source to voltmeter cable and if ap plicable the band stop filter or trap at the harmonic frequency Measure and record the harmonic frequency voltage 4 The ratio of the harmonic voltage V to the fundamental voltage Vf can be calculated and expressed in decibels as follows Alternatively the ratio of V to Vf can be obtained directly in dB by reading either the LIN or the LOG dB scale on the panel meter and subtracting 20 dB for each CCW step of the FULL SCALE switch going from Vf to dB Vp dB dB 20 FULL SCALE steps CCW from Vr to Vj Note that dB and V dB must include the sign obtained from the decibel scale readings 3 6 3 Measuring the Harmonic Content of Radiated Signals The procedure for measuring the field strength of fundamental and harmonic signals is similar to the procedure of paragraph 3 6 2 with the ANT 71 antenna providing the source of signal voltage However in this case field strength and ratio calculations must include the antenna factors which are different
26. ended readings can also be made using a 12 6 V automotive battery The terminal of the external supply must be connected to the FIM 71 case Ground Severe damage can occur if the instrument is powered from an automotive electrical system with a negative ground The instrument will also operate continuously for approximately 20 hours on the Potomac Instruments Rechargeable Battery Kit which can be recharged in approximately 8 hours See Optional Accessories in Section 1 If the instrument is to be frequently operated for periods of 30 minutes or more or if readings are to be made at low temperatures then the alkaline manganese dioxide cells privide a significantly longer battery life See List of Parts Section 6 However if typical operation is for short periods at normal or elevated temperatures the standard carbon zinc cells will provide nearly the same life at a much lower cost The minimum battery current of approximately 60 mA is obtained with the FIM 71 controls set as follows AUDIO MAX CCW AT 0 DEMOD AM OSC OFF A slight CW rotation of the AUDIO control switches DC power to the demodulator output amplifier which drives the loudspeaker and PHONES jack Depending on the speaker loudness the battery current will increase to a value of 1 1 to 1 6 times the minimum current If a low impedance load i e 75 ohms is connected to the PHONES jack a similar increase in battery current is obtained again depending on the outpu
27. g antenna elements screw into smaller swivel joints contacts which also lock in three positions vertically up horizontal with a 2 59 uplift and vertically down To reposition either the mast or the antenna element swivels loosen the appropriate knurled locking knob and pull the body of the joint toward the knob to release the detent pins rotate the swivel until the detent pins snap into the desired position and tighten the locking knob Never tighten a swivel knob unless the detent pins are engaged To store the antenna the mast and antenna elements are fully collapsed the antenna elements are positioned vertically downward and the antenna mast is then rotated to an upside down position so the small button on the side of the antenna balun housing can engage the support groove on the lower right side of the FIM 71 All locking knobs should be tightened see figure 3 5 3 4 2 Antenna Accessories Supplied Identify the following accessories which are stored in the FIM 71 cover 1 3 3 4 foot coaxial cable 2 34 foot coaxial cable 2 meter tape scale 3 4 leather neck strap 3 4 3 Antenna Element Lengths Rotate the antenna element swivels to the horizontal position and tighten the locking knobs Using CURVE C of figure 3 3 determine the antenna element length L corresponding to the frequency of the signal to be measured Measuring from the center slot on the top of the balun housing with the tape scale provided carefully adjust t
28. he antenna elements to length L be sure to utilize the largest diameter element sections to make up the length as shown in the figure 3 3 3 4 4 Near Ground Measurements For near ground 7 foot measurements the antenna is normally supported on the telescoping mast attached to the receiver If an UNIPOD optional accessory is used to support the FIM 71 the anten na mast is locked in the upright position the mast is fully extended and the UNIPOD length is adjusted so 9 87 21 57381230 akg SA IU JO ALI enjoy OUAS Jo yeog poyeoIpuy pE ANEI 371725 8 01 Sl oz 1 BERES Mill SERRE 1111 111111 21111 11111 LTT Cann IITTI rt Lil 5 4 4 2041
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30. it is normal for the Battery Charger panel to run hot the panel should cool to slightly above ambient temperature as the battery reaches full charge A discharged battery will be almost fully recharged in 8 hours Because of the tapered characteristics of the charger current it may take one or more days to fully charge a deeply discharged battery The Battery Charger can be left connected to the AC line for more than a week with no degradation in battery service life Continuously float charging the battery is safe but may cause a reduction in service life The Battery charging rate decreases at both the high and low temperature extremes See the 71 BP 71 specifications for additional information on charging and discharging To protect the FIM 71 the battery breaker will also release if the charging voltage becomes excessive If the breaker trips while charging the battery check that the POWER TEST indication does not exceed the EXT bracket when the breaker plunger is pressed in with AC voltage applied Excessive charging voltage which can only occur if the charger circuit has failed will quickly destroy the battery a Battery Charger exhibiting this condition should be immediately removed from service for repair The small current required for the battery breaker circuit will discharge the battery in 18 to 24 months If the FIM 71 is to be stored for a long time with the Rechargeable Battery Kit installed the battery shou
31. ld be isolated from all loads by manually releasing the battery breaker This is easily accomplished by inserting a thin conductor such as the end of a paper clip into the small hole labeled BREAKER TRIP on the BC 71 panel The conductor must touch and ground the terminal inside the hole which will cause the breaker plunger to out The self discharge rate of the isolated battery is greatly increased at elevated temperatures The BP 71 utilizes four lead acid cells which are normally sealed However in the event the charger circuit fails a small amount of hydrogen gas may be emitted from the battery Four small vent holes have been provided in the battery cover for this contingency these holes should not be obstructed The 71 and 71 may be installed in the FIM 71 indefinitely After a number of years of service the Battery Pack may not accept or hold a charge In this event the BP 71 can be purchased separately and replaced without readjusting the BC 71 Battery Charger 3 21 3 22 INSTRUCTION MANUAL SUPPLEMENT FIM 71 Sec 3 4 1 FIM 72 Sec 2 4 1 The short antenna mast mounted on the FIM case has been changed from a telescoping mast whish is no longer available to a mast made up of separate sections which must be screwed together There are three sections a short base section attached to the unit and two long sections which are supplied loose The long sections can be stored in the bottom of Meter Carrying Case Type
32. ls must be carefully placed into the 5 RED cups on the bottom contact assembly The remaining 5 cells are installed with the terminal toward the cover contact assembly ft 3 To replace the battery compartment cover force the cover into position against the batteries and rotate the six 6 fasteners 4 turn clockwise 4 Always remove batteries when the instument is to be stored for ex tended period 3 9 INSTALLATION AND OPERATING INSTRUCTIONS RECHARGEABLE BATTERY KIT NOTE The Rechargeable Battery Kit is made up of the BC 71 Battery Charger and the BP 71 Battery Pack Both units are connected together electri cally and installed into the FIM 71 battery compartment 3 9 1 Installation 1 Place the front panel cover on the FIM 71 Set the instrument front down on a protective surface and orient the case so the carrying handle is away from the user bo With a large screwdriver or coin rotate the six slotted battery cover fasteners counter clockwise and remove the battery cover and ten D cells 3 Identify the three small and the three large holes the BP 71 Battery Pack printed circuit board Install the BP 71 into the right side of the battery compartment so that the six right most battery terminals in the bottom of the compartment fit into the holes in the Battery Pack PC board the red battery terminals fit into the large holes The 4 conductor cable should be coming off the left side of the Battery
33. n by rotating the six slotted fasteners clockwise using a large screwdriver or coin AC line voltage accessible under the BC 71 Battery Charger panel can be dangerous and even lethal particularly when units are operated from 230 volts NEVER connect the line cord to the charger unless the BC 71 is properly installed in the FIM 71 battery compartment and the six fasteners are tight NEVER remove the BC 71 before first disconnecting the line cord NEVER remove or install the BC 71 fuse with the line cord connected NEVER operate the BC 71 from an AC receptacle which does not properly ground the U prong on the line cord plug Operation With the Battery Pack and Battery Charger installed in the battery compartment depress the FIM 71 POWER switch to TEST The instrument panel meter should indicate within the green BATTERY bracket If no BATTERY indication is obtained take a small diameter instrument ball point pen tooth pick paper clip etc and push in the red plunger labeled LOW BATTERY BREAKER on the Battery Charger panel the red plunger should latch approximately 14 inch into the cylindrical guide With a fully charged battery the POWER TEST indication should be some what above the first in BATTERY As the battery is discharged the TEST indication will decrease a reading near the lower limit of the BATTERY bracket indicates that the battery should be recharged If the battery is excessively discharged indicated by a PO
34. ontrols and Indicators 3 3 To check batteries or external power source press the POWER switch to TEST panel meter indication must be within the lower and upper limits of the green BATTERY EXT bracket Set the POWER switch to ON The FIM 71 requires approximately 90 seconds warm up to obtain best accuracy Set the FULL SCALE attenuator switch to the position corresponding to the maximum expected input voltage The RMS value of the input signal voltage RMS sum of all RF AC and DC components must not exceed 10 VOLTS RMS under any condition and must not exceed 1 0 VOLTS RMS with the FULL SCALE attenuator set to the 10 uV position Connect the signal source to be measured to the RF INPUT BNC con nector on right side of instrument The FREQuency control is designed to slip at the end of the range of the MHz dial without changing the frequency calibra tion However repeated forcing of the dial against the end stops will cause excessive strain and wear on the mechanism and should be avoided Using the FREQuency control set the MHz dial to the frequency of the signal to be measured as indicated by the hairline cursor and red pointer Also see following note and paragraph 7 Adjusting the FULL SCALE switch as necessary for an on scale reading tune in the signal for a maximum indication on the panel meter NOTE The MHz dial is accurate to within 0 5 of the indicated frequency with the hairline cursor set to the M
35. ph 3 4 6 concerning cable attenuation Refer to CURVE A or CURVE B 7 foot measurement of figure 3 2 and determine the appropriate ANTENNA FACTOR corresponding to the frequency being measured To obtain the field strength at the ANT 71 antenna multiply the corrected antenna output voltage by the ANTENNA FACTOR FIELD STRENGTH VOLTS METER ANTENNA VOLTS X ANTENNA FACTOR 3 5 VERTICAL POLARIZATION MEASUREMENTS The 71 was not calibrated in a vertically polarized field However it is believed that the antenna factors obtained for horizontally polarized fields at a height of 30 feet are accurate for measure ments of vertically polarized fields provided that the asymmetry introduced by the antenna support and cable is minimized The accuracy of near ground measurements of vertically polarized fields is unknown To measure vertically polarized fields the ANT 71 dipole elements are extended normally with respect to the balun housing but the antenna assembly must be oriented so that one element points towards the sky the other towards the ground The antenna support and coaxial cable must extend hori zontally at right angle to the elements for at least wavelength to the main vertical support thus forming a cantilever like structure At the vertical support the antenna cable can descend to the selective voltmeter The less metal used in the antenna support the better particularly in the upper section near the antenna The antenna
36. t level The battery current increase is negligible if high impedance 2000 ohms or higher phones are used Setting the DEMOD control to FM applies DC power to the FM discriminator and increases the battery current to approximately 1 4 times the minimum current Setting the OSC control to CAL or OUT switches DC power to both the FM discriminator and the calibration oscillator and increas es the battery current to approximately 1 5 times the minimum current Using the loudspeaker with the FM demodulator and or the Cal Osc can increase the battery current up to approximately 2 times the mini mum current The FIM 71 is not recommended as a battery powered radio 3 8 INSTALLING AND REPLACING BATTERIES NOTE Replacement of the battery compartment cover requires considerable force to compress the spring contacts so the cover fasteners can be en gaged To facilitate battery installation it is strongly recommended that the front panel cover be temporarily installed so the instrument can be supported on the cover with the rear of the unit facing upward Never support the instrument directly on the front panel without pro viding protection for the meter and controls 1 To remove the battery compartment cover use a large blade screwdriver or a coin and rotate the six 6 black fasteners on the cover turn counter clockwise The FIM 71 requires ten 10 0 size cells When installing the ceils the positive terminal of 5 cel
37. th Switchcraft plug 760 insertion of plug disconnects battery RF INPUT 10 V MAX Connector BNC located on right side of instrument 50 ohms input to receiver through RF attenuator section of FULL SCALE switch OSCillator OUTput 100 mV Connector BNC located on right side of instrument 50 ohms calibration oscillator output with OSC switch in OUT position 100 mV into 50 ohms When fraction of Cal Osc signal is applied to receiver for on scale reading of at least 5 uV Cal Osc frequency AFC d to receiver frequency otherwise Cal Osc sweeps approximately 1 MHz 3 2 CALIBRATION AND RF VOLTAGE MEASUREMENTS The following steps 1 through 11 describe the procedure to be followed when performing an RF voltage measurement Step 8 with substeps a through e describe the procedure for calibrating the FIM 71 Paragraph 3 3 describes the Peak Detector Correction Procedure and paragraphs 3 4 through 3 6 describe special measurement procedures 1 Set the following controls to the positions appropriate for the intended measurement see paragraph 3 1 on control functions DETector PEAK TV sync or other pulse modulated signals AVeraGe CW FM AM signals IF Bandwidth TV wide FM AM narrow METER LINear read black scale 1 to 10 volts or 20 to 0 dB LOGarithmic read green scale 20 to 40 dB 2 ip 29 x 00 CAL FULL SCALE Figure 3 1 FIM 71 Operating C
38. the fundamental For some measurements a band stop filter or trap may be required to attenuate the fundamental when the voltmeter is tuned to the harmonic frequency Internally generated harmonics tend to increase rapidly as the fundamental voltage exceeds some threshold level which depends on many factors Also the internally generated harmonics and the measured harmonics add as vectors so the error associated with a particular measurement depends on the relative amplitude and the phase of the two components and partial cancellation of the measured harmonic is a practical possibility For example assuming that the detector is linear if all internally generated har monics are 20 dB below the measured harmonic the maximum measurement error due to the internally generated components is limited to 1 dB Harmonics generated within the FIM 71 will be at least 100 dB below any fundamental voltage up to 50 mV at any FULL SCALE position NOTE This statement is quite conservative For example with fundamental frequencies within the FM broadcast band internally generated second harmonics are typically measured at more than 100 dB below a funda mental voltage of 316 mV When measuring the field strength of harmonic signals with the ANT 71 antenna the antenna output voltage at the fundamental frequency will be reduced approximately 6 dB when the antenna ele ments are adjusted for the harmonic wavelength in this case internally generate
39. trap connect the T adapter directly to the RF INPUT of the voltmeter connect the voltage source or antenna to one input of the T adapter and connect the quarter wave stub to the other input of the T adapter When measuring the funda mental voltage the trap must be removed this is done by simply unplugging the quarter wave stub from the T adapter 3 7 NOTES ON BATTERIES AND BATTERY LIFE Assuming that it takes approximately three minutes to calibrate the instrument and perform a voltage or field strength measurement the FIM 71 should obtain at least 1500 readings from a standard battery consisting of 10 carbon zinc D size dry cells See List of Parts Section 6 NOTE The estimated battery life is based on operation at normal or elevated temperatures Battery life is decreased at low tempera tures The instrument will also operate continuously for at least 24 hours on a standard battery although continuous battery operation becomes somewhat expensive Continuous operation is more practi cal using external AC powered DC supply Any power source providing between 11 5 V and 319 0 V DC at not less than 150 be plugged into the EXT PWR jack the rear of the FIM 71 Use Switcheraft 4760 plug Plugging in an external power supply disconnects the internal battery Potomac Instruments provides an inexpensive and very convenient AC power supply for this purpose Sec Optional Accessories in Section 1 Ext
40. up a defective cable 1 Connect end of the antenna cable to the RF INPUT connector and the other end to the OSC OUT 100 mV connector 2 Set the following switches to the positions indicated POWER ON MeTeR LINear FULL SCALE 100 mV CAL OSCillator CALibrate 3 Set the MHz dial to the frequency of the signal to be measured For this step the dial accuracy is sufficient so it is not necessary to tune in the signal 4 After about 90 seconds warm up adjust the GAIN control for a meter reading of exactly O dB full scale 5 Set the OSC switch to OUT The reduced meter reading shows the cable loss which be read directly either in decibels on the dB scale or as a fractional ratio on the VOLTS scale NOTE Depending on the measurement conditions two alternate pro cedures are given for compensating for the antenna cable attenu ation When it is inconvenient to periodically disconnect the an tenna cable from the antenna step 6 can be used Alternatively if both ends of the antenna cable are accessible step 7 ALTER 3 13 NATE eliminates the need for calculated corrections 6 Record the cable attenuation obtained in step 5 above When measuring antenna output voltage according to paragraph 3 2 all readings must be mathematically corrected to account for the cable loss When using the short antenna cable the cable loss may be negligible particularly at the lower frequencies 7 ALTERNATE TO STEP
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