Optoelectronics, Inc. What The Scout Will Count

User Expectations
vs
Reality

A Magic Box?
Frequency counters are often viewed as a magic box with an antenna that when held in the air will quickly read the frequency of just the signal you've been wanting the frequency of. Obviously, this sarcastic scenario is unrealistic, but let's face it - it's just what we wish the counter would do.

In this article, we'll look at how close we can come to this and why.

How Does A Counter Work?
Simply, the counter counts the number of positive zero crossings of its input signal in a specified period of time.

Thus, if the input signal is a 100 MHz sine wave and the chosen gate period is one second, the counter will count the number of times the input voltage goes from a negative to a positive voltage in one second, and display that as the frequency of the input in cycles per second, or Hertz (Hz).

So Many Zero Crossings!
Getting an accurate frequency count depends on the counter seeing only zero crossings due to a single signal. If two or more signals are present at the input to the counter, the voltages of the two signals add and the zero crossings are distorted, resulting in erroneous counts.

Exactly how much signal it takes to screw up counting of the desired signal depends on the both the relative amplitudes and frequencies of the signals, but a good rule of thumb is that the desired signal should be 10 to 15 dB higher than all others to get a usable count.

If more than one interfering signal is present, the situation gets worse quickly.

Real World Impact
So, how does this impact frequency counting in the field?

Transmitter sites with many transmitters operating simultaneously can be particularly difficult for a frequency counter.

The multitude of signals produce a mixture
of zero crossings, so that the counter does not see any single signal.

If you are lucky enough to catch the site when only one transmitter is operating, a counter will read correctly, but this will be the exception.

The same conditions apply at cell sites because the control channel as well as numerous individual channel transmitters all operate simultaneously, making successful counting very unlikely. Add to that the number of digital cell sites, and the odds get even worse.

Filters may be helful if the transmitters happen to be widely spaced in frequency, but this is not often the case.

One Lone Antenna
Even if a single transmitter is operating, a high-gain vertical antenna may direct very little radiated energy toward the base of the tower. It seems intuitive that the signal should get stronger as you approach the tower, but in fact you may just be getting further into the null of a very directive antenna.

Don't expect to count anything coming from a "dish" antenna - they are very directive and probably operating at a very high frequency above the capability of your counter.

Cellular Uncertainty
Cellular phones can be difficult to count since the phone transmitter power output varies over a wide range under command of the cell site. This makes cellphone range testing very ambiguous and generally unrepeatable.

Digital cell phones, and other pulsed or TDMA (time division multiple access) signals may look like a nice strong signal on the counters level meter, but because the signal actually is only present part of the time, cannot be correctly counted with a typical counter. The Digital Scout can pick up a TDMA signal, however it will not lock on to a CDMA (Direct Sequence Spread Spectrum) signal.

TV stations are also usually uncountable because of the AM present and the presence of the aural transmitter carrier which messes up the zero crossings.

Transmitters in Motion
Counting while in a moving vehicle (or counting a moving emitter) can cause problems due to multipath fading. The brief nulls in the signal caused by this fading my be barely perceptible noise burst on a scanner but chop big holes in the short gate period of a counter and render counts inaccurate.

Using antennas larger than a half wavelength long can help this situation since the a longer antenna will no be completely contained in the spatial null.

So what CAN you count?
Mobile and handheld two-way radios make excellent targets for counting because they are typically the only transmitter operating in a given area and result in a relatively pure signal.

These include police, fire, utilities, hams, cellular phones (sometimes), traffic lights, telemetry, etc. Broadcast FM transmitters can also be counted, but it's usually easier to look at the sign on the side of the transmitter shack.

Don't be discouraged if you can't count something on demand - there probably just isn't anything to count. We can leave a counter our my workbench for hours without a single hit and know there's no problem with the equipment.