Frequency Counters
by Black Axe

	The frequency counter is probably one of the most useful radio monitoring tools ever 
made. It has the ability, when used in the proper manner, to snag frequencies out of the 
air. This is much more preferable work, as opposed to sifting through FCC databases and 
personal webpages, finding outdated info, sitting in your car and scanning different 
frequency ranges, etc. The catch here is that many people see the frequency counter as 
this magical device that will instantly tell them the frequency of that agency or group 
that they want to monitor. It's a lot more complex than that, both logistically and 
technically. First, let's look at (or drool over, your pick) equipment.

	The first frequency counter you'll probably notice is the one sitting on the shelf 
of your local Rat Shack. This is about standard for what you'll see available. Range is 
AF (audio frequencies) to 1.3gHz. Hold function, selectable gate times, and a backlight 
are included. This is an alright counter, available, and not too expensive (around $100 
last time I checked). 

	When shopping for counters, there are counters, and then there is the Opto Scout. 
It has 400 memories, each with a hit counter capable of counting 255 transmissions on each
received frequency, CI-V interface, etc. Really nice. Most other frequency counters 
were made for testing radio gear to see if it's on frequency, etc.. not so with the 
Scout. The Scout is the only counter that's made specifically to snag frequencies for 
monitoring. This may explain the $350 price tag. If you have the cash, it's definitely 
worth it; however, it's not for everyone. 
Check it out at http://www.optoelectronics.com/. 

	A frequency counter, in theory, is a very simple device. Flashback to basic 
electronics and radio class. Radio transmissions oscillate at a certain frequency, in the 
shape of an AC (alternating current) waveform. What your frequency counter does, basically, 
is measure the number of times that the waveform's voltage drops from its peak to zero 
within the given gate time. After that measurement is taken, the number of times that 
the wave's voltage would drop from its peak to zero in a second is calculated, factoring 
in the length of time that the counter was counting voltage drops. This calculated value 
is then displayed, stored into memory, etc. From this, we can determine that the counter's 
gate time is a setting that will affect the accuracy of the measured signal. In most 
cases, however, the shortest gate time will prove most beneficial and will give results 
accurate within 1kHz or so. Remember that frequencies, for police departments and such, 
are allocated based on a bandplan, with predefined steps. In other words, if you got a 
reading on your counter of 155.687, one could guess that the actual frequency in use 
would be 155.685mHz (the closest frequency allocated for police activity). Same goes 
for a reading of 879.98 - that's in the cellular band, and the cellular band is 
allocated in 30khz steps, making the closest valid frequency 879.99mHz. Also remember 
that your frequency counter isn't entirely accurate. And, most likely, neither is the 
transmitter you're measuring. This inaccuracy should not harm your readings at all - 
so don't think that your counter is screwed when it reads 155.68592 when counting your 
local PD. 

	So far, it seems fairly easy to use the counter, right? Wrong. Here comes the bad 
news, the part that you wish just wasn't true. In order for a counter to operate 
properly, it needs to see the cleanest AC waveform possible. Think of your average 
communications tower. Think of all the antennas there that are transmitting 
simultaneously. When your counter sees 2 AC waveforms at about the same strength, 
it doesn't know what to do. Some counters may produce some sort of an odd average 
of the two frequencies. Some may lock up completely and not display anything. And 
on a communications tower like that, there's _always_ someone yapping. In order for 
a counter to operate properly, it needs to "see" the desired AC waveform at least 15 
to 20db stronger than the rest of the clutter. At your average communications tower, 
there's probably a cellular base station there. Or a paging transmitter. We all know 
that a cellular tower is constantly transmitting on its control channel, and that 
pager transmissions rarely cease. Thus, your stock counter will be unable to snag 
the frequency of the police repeater amidst all of this clutter. The solution? 
Engineering the signal before it enters the antenna jack. This is accomplished 
through the use of filters and tuned antennas. Tuned antennas are, well, tuned to 
receive best in a specific frequency range. This will "magnify" the AC waveforms 
seen in that range by the counter, and de-emphasize the other signals. This will 
only work if one's target frequency is known to be in a specific band. Filters will 
attenuate (knock down, in other words) signals at certain frequencies. For example, 
a commercial 88-108mHz filter is available, to de-emphasize the effects of broadcast 
FM transmitters. Other filters can either be bought or homebrewed. Probably the most 
useful filter, for the monitoring enthusiast, would be one that attenuates anything 
over 512mHz or so, leaving most of the public safety band intact, and eliminating a 
lot of pagers and cellular interference. Don't even bother with preamplifiers or 
broadband attenuators; what we're trying to do is increase the desired signal's 
relative signal strength in relation to other signals in the spectrum. Simply 
amplifying or attenuating everything doesn't change strengths relative to each other.

	Now let's look at the field end of things, i.e., not hanging out under a comm tower. 
Things become much simpler here, as all it entails is getting close to a transmitting 
target. Once you've snagged the frequency, you're _almost_ home free. What you have 
then is the input frequency. Most listening is done on the output frequency. If the 
frequency you have is in a band with a standard bandplan (like around 460-470mHz), 
then you can simply determine the output frequency by subtracting 5mHz if the frequency 
is between 460-470mHz, or subtracting 3mHz if the frequency is in the UHF T-band 
(470 to 512mHz). Sometimes this doesn't work too well, and consulting the FCC database 
is necessary. Do a lookup by state/frequency, and input what you have. Get the callsign 
of the agency from the input frequency, and do a search on that callsign. You now should 
have a good chunk of freqs to work with. In the VHF band, there are no standard repeater 
offsets, so your only recourse is to use the database method. With counter in hand, you 
should be easily able to identify many frequencies in use in your area with a little 
elbow grease and a little logical thinking. 



