Potomac Instruments FIM-21 - RF Bridge Measurements

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SECTION 2. OPERATION (Cont.)
2.5 Measuring Harmonic Ratio in Decibels (Cont.)
Example:
Let us say that we want to measure the field strength
and the second and third harmonic suppression of' a
transmitter with an operating frequency of l500 KHz.
1. Measure field strength of fundamental as outlined
in section 2.4 of this manual; note db scale.
Example: 560 mV/M & 5 db.
2. Measure field strength of second harmonic as above
at 3.0 MHz. Examp le : 0.35 mV/M & 9 db.
3, Measure field strength of third harmonic as above
at 4,5 MHz. Example: 79 uV/M & 2 db.
Calculate harmonic suppression:
db1 - db2 + 20 x l! = ratio (db)
second harmonic suppression= 9-5 + 60 = 64 db
third harmonic suppression = 2-5 + 80 = 77 db
It is not the purpose of this manual to describe the techniques
of locating radial measurement points and plotting the field data required
by the FCC. For this type of information, and for a more detailed descrip-
tion of operating techniques , the user is referred to the current edition of
the NAB Hand book.
2.6
R. F. Bridge Measurements
(Tuned
Voltmeter)
The receiver section of the FIM-21 and FIM-41 may be used as a
tuned voltmeter for applications such as R. F. Bridge null detection. As
a tuned voltmeter, these instruments exhibit an i~put i.rnpedance of 2500 ohms
shunted by less than 2 picofarads.
To use the FIM as a relative indicator of RF voltage, proceed as
follows:
1. RF ThTPUT switch
2. METER switch
3. FUNCTION switch
4. FULL SC.ALE switch
2-6
switch to EXT position
LOG or LIN - the LOG mode is helpful
for RF null detection work
set to FI -CAL TUNE
set to lOV position or as required
for meter indication