Time is money

Dec 1, 2007 12:00 PM, By Rob Barden

Today's RF engineers and technicians are faced with myriad issues related to keeping radio communication systems operational. Whether they are dealing with vehicle-based radio systems, fixed repeater sites or airborne radio communications, their primary focus is keeping the system online for maximum readiness.

In addition, many large-volume users of radio communication equipment, as well as radio original equipment manufacturers, face substantial costs when dealing with warranty support for radios that have been pulled from a system and returned, only to have the radio diagnosed as “no fault found.”

But isolating problems in radio communication systems can be a time-consuming process if you are not properly equipped. Using the latest equipment can save you time, money and effort in isolating problems in RF systems.

A typical radio communication system consists of a radio, amplifier, cable and antenna. When isolating problems, it is best to start from an operational standpoint with a simple over-the-air test to verify basic system performance.

While some engineers might disagree, it is possible to perform an over-the-air test on an installed radio and obtain useful results. With a simple procedure, you can perform over-the-air tests on both the transmitter and the receiver of the radio being tested, including:

RF frequency error.
Percent modulation for AM transmitters.
Frequency deviation for FM transmitters.
Relative power.
Relative sensitivity of the receiver.
Relative squelch on and squelch off level.

Some of these measurements are referred to as “relative” because the measurements are made over-the-air. Relative means that the measurement should be the same for a “known good” radio made under the same conditions. For this reason, when conducting an over-the-air test, it is very important to follow a consistent procedure. The most important parts of this procedure are:

The position of the radio test set with respect to the radio. This should be exactly the same each time the test is performed.
The frequency and deviation of the modulation source from the test system.
The type of antenna on the radio test set.
The RF frequency or channel used.

To determine whether a radio transmitter or receiver passes, a list of pass/fail criteria needs to be established. The criteria for frequency error and modulation are either specified by the manufacturer or derived from known test data. Over-the-air measurements will not affect frequency error and deviation (or modulation index for AM) measurements.

Relative measurements will be based on a “golden” radio (one that is known to be good). Results then will be compared to the results derived from the golden radio to indicate whether a problem exists.

The biggest enemy when making an RF measurement over-the-air is multipath. Multipath is the result of the transmitted or received signal following two or more paths to or from the antenna. The cause of the multiple paths is the RF signal reflecting off the ground, walls, ceilings, cars, equipment and buildings.