Why isn’t my walkie working?
Ira Wiesenfeld, P.E.
Have you ever used a walkie-talkie and felt there was something wrong with the particular radio you were using, or with the system, because the range was reduced? Have you ever wondered why some walkie-talkies seem to have much farther range than other portable radios on the same system?
Case in point: A new VHF radio system recently installed for a public-safety agency worked sometimes, and sometimes not. Specifically, it had poor range with some of the walkie-talkies but great range with others. Why did some of the units work with a range of better than 20 miles, while others exhibited a range of less than 2 miles?
The range of a radio system is influenced by the aggregate of several factors, including the following: transmitter power; receiver sensitivity; transmission line losses; antenna gain, efficiency and height; line of sight; obstructions; terrain; noise floor; and RF interference. But the items dealing with the antenna have the greatest effect on the range.
Because the fixed part of the system, often referred to as the system infrastructure, is constant for all users, the system’s base station and mobile and portable radios have the same operating parameters. The main difference is that the external environment in which a portable handset operates can change simply by the user turning around. Reception also can be affected by where the radio is worn or where the user’s hand is placed in relation to the walkie-talkie antenna.
Even though antenna efficiency has the greatest influence on both the transmitting and receiving paths, it is rarely tested. While it is easy to measure a radio’s transmitter and receiver parameters, it is quite difficult to measure the antenna parameters.
When testing antennas for walkie-talkies, especially for VHF radios, it is very important that the antenna always be at least 1 to 2 inches away from the body if the radio is worn on the belt, as close proximity of the body to the entire length of the antenna seriously detunes the antenna. In fact, it is not uncommon for the range to shrink to 1% of normal should the antenna come in contact with a person’s body. Most of the radios on the market today have the antenna mounted to the case in a manner that creates suitable distance from the body, or they have leather or canvas cases that provide adequate separation. Those of us who have “extra handles” at our midsections must use the external cases or pouches, even on the radios that have the distances built into them.
The best way to determine how well an antenna performs standing in free space compared with sitting just above a belt is to use one of the antenna analyzers — such as Anritsu’s Site Master or Bird Technologies’ Site Analyzer — with a phase-stable cable, along with the proper adapter to match the end of the phase-stable cable to the antenna.
An antenna analyzer will give a graphical representation of the antenna performance, which is better than just looking at the reflected power on a specific frequency using a wattmeter.
Under normal circumstances, a standing wave ratio (SWR) of under 1.5:1, which is equivalent to -14 dB return loss, indicates a good match at the operating frequency. A SWR of 2.0:1, which is equivalent to a -9.0 dB return loss, is the worst case that would be tolerated. At 2.0:1 SWR, the antenna is operating only at 90% efficiency, which is the bare minimum.
Field tests show that many antennas operate only within a 5 MHz window where the SWR is less than 2.0:1, and the antenna manufacturers normally label such limits on the antenna or by the model designation.
In further testing, when an antenna that is tuned correctly and has a good SWR on the normal operating frequency is placed directly next to a body — as would occur when a walkie-talkie is mounted on a person’s belt and the antenna is touching the body — the SWR goes from acceptable to terrible. In the case of the public-safety agency installation mentioned earlier, the antenna was detuned to the point that it was operating at less than 1% efficiency.
Using an antenna analyzer, it was shown that just moving the antenna away from the body by at least 1 inch caused the antenna to behave as designed and the SWR returned to the acceptable range. Some manufacturers, such as Motorola and Kenwood, have addressed this problem with antennas that are mounted so that they are moved away from the body even when there is no holster case and only the belt clip is used.
Radios manufactured by other companies that were outfitted with the recommended holster cases — resulting in the antennas moving away from the body at the recommended distance of 2 inches or more — worked as designed. (See Figure 1 on page 34.)