[BC] Frequency Measurement detector...

[Burt I. Weiner]

The most common and reliable method for off-air frequency measuring 
is what is called the heterodyne method.  In this case you tune a 
receiver to the desired signal and then zero-beat the signal with a 
local signal generator connected to the receivers antenna input 
through whatever padding is required.

Zero Beat is an oft misunderstood term.  True Zero Beat is when there 
is no (zero) beat note, essentially D.C.  From a practical standpoint 
you would tune the local signal generator for zero beat while 
watching the S-Meter on a receiver.  As you approach zero beat you 
will hear the resultant heterodyne tone becoming lower and lower 
until it's either below the ability for the receiver to reproduce it 
or your ear to be able to discern the beat note.  At this point you 
can look at the S-Meter and see it start to swing back and 
forth.  The closer you get to true zero beat the slower the S-Meter 
wavers until you bring it to a stand still.  You then measure the 
local signal generator using a counter or a dial reading, depending 
on the type of Interpolation or Transfer Oscillator used.

In the case of the Heterodyne Method, the receiver is only used as a 
detector and only needs to do two things:  Allow you to hear and 
identify the station and to give you a display of the signal strength 
for the sake of being able to let you see zero beat.  I've used 
everything from a good car radio to a SP-600JX and now my receiver of 
choice is a HP-3586B Selective Level Meter.

I used to measure two stations that were both on 1340 kHz.  Both were 
over 100 miles away and equally interfered with each other.  There 
was so much interference on the frequency that watching the S-Meter 
was useless.  By having the one I was measuring modulate their 
transmitter with a 1000 Hz tone I was easily able to measure that 
individual station by determining Zero Beat by ear.

Burt

>From: Tom Dimeo <am at pa321.net>
>
>Back in the early and mid sixties I worked for WKVA 920 in
>Lewistown Pennsylvania and the station used Clarence Cheney
>for frequency measuring.  I asked him about the recieving
>setup he used and I'm pretty sure he said he used a
>Hammarlund HQ145.  Richard Johnson describes how he did it in
>another list message.  I was always amazed that he could even
>hear the station let alone measure the frequency.
>
>Several years ago there was a lengthy discussion on this list
>on the subject.  It was very interesting.
>
>Tom