Potomac Instruments FIM-21 - Measuring Field Strength

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SECTION 2. OPERATION (Cont.)
2.3 FIM Calibration (Cont.)
2.. Rotate the FUNCTION switch to the BATT position and see that
the rea d ing is above 5.0.
3. Rotate the FUNCTION switch to the FI position.
4. Switch LOG-LIN switch to LIN position.
5. Set the FULL SCALE switch to the range which covers the ex-
pected value of field strength.
6. Turn on the AUDIO control and tune the RECEIVER. d ial to the
signal to be measured. Use the meter indication to obtain
peak tuningJ adjusting the GAIN control and FUIL SCALE switch,
and rotating the instrument to obtain a reading within the
range of the meter . Use the audio signal to ident ify the sta-
tion. The RCVR Tuning Control has vernier drive for 3/4th of
one turn; it changes to direct drive and becomes harder to turn
beyond the vernier range, Adjust it so that the desired station
i'a.lls in the vernier range.
7. Ro t ate the instrument to obtain reading below 10 mV. In strong
fields the m eter reading may be reduced below 10 mV by holdi ng
the FIM horizontally or by changing RCVR Tuning away from the
desired signal by no more than 10 KHz. (This is permissible
because the receiver gain variation with frequency is very grad -
ual.) An indistinct dip in the meter reading in Step 8 indicates
inade qua te reduction of the receive d signal, or interference.
8. Place the FULL-SCALE switch in the CAL position: adjust GAL OSC
Tuning for highes t meter reading, or an audio beat note low enough
in frequency to be inaudible. (The meter reading should be 3. 5-
4.8 when the Gain control is properly set.)
9. Rotate FUNCTION switch to CAL NULL position: adjust GAIN control
for lo·,rest meter reading. The meter reading should be below 3.0.
10. Return FUNCTION switch to FI-CAL-TUNE . Retune RCVR tuning for
ma.x:i.Jnummeter reading on the desired signal. The Field Strength
Meter is now calibrated at the frequency to which it was origi-
nally tuned.
2.4 Measuring Field Strength
In use, the field meter is generally held in the hand, or mounted
on a tripod or unipod. Of the latter two, the u.nipod is preferred, since it
can be easily rotated, and can remain attached to the instrument. A plate
ha ving a hole tapped for a¼ - 20 screw is ~ astene d to the bottom of the case,
for attachment to a support .