Generally speaking, land mobile radio technicians favor forward and reflected power measurements to determine the operating condition or tuning of an antenna. During the installation of a mobile radio antenna, a wattmeter is inserted into the transmission line near the transceiver. The transmitter is keyed, and the forward power is measured. Next, the wattmeter is set to measure reflected power, either by turning the “slug” in the reverse direction or by turning a switch to the “reflected power” position. The less the reflected power (relative to the forward power), the better the antenna match. This method is straightforward and works well. However, there are times when other techniques can aid the technician in checking or tuning the antenna.
Trimming the whip When a new antenna is installed on a vehicle, it must be tuned at the frequency of the radio to perform properly. Too much reflected power will cause the “foldback” circuit to reduce the transmitter power to protect the final stage from excessive reflected power. Thus, it is essential that the antenna be “tuned” so that it presents a 50-ohm load as seen by the transmitter.
The directional wattmeter is used, as previously described, to tune the antenna, while the whip is trimmed, until the reflected power is at a minimum (or low, compared to the forward power). Usually, the process is simple. However, confusion can sometimes arise over whether the antenna rod is already too short or it needs further trimming. Remember, you can’t “put it back” if it is cut too short. So, the question becomes: “To cut or not to cut?” If you cut it too short, you have to start over with a new antenna rod, wasting time and money. There are ways around this problem, however.
Multifrequency radios If the radio being used is a multifrequency radio, then the high and low frequencies can be used to tell whether the antenna is too long or too short-provided that the frequency separation between the highest and lowest frequency is sufficient. Simply set the radio to the lowest frequency and check the SWR or percent reflected power. Then, do the same at the highest frequency. If the SWR or the equivalent percent reflected power is better on the high frequency, then the antenna is too short. If the SWR or equivalent percent reflected power is better at the lower frequency, then the antenna is too long. The antenna rod should be cut so that the best SWR is seen at the mid-frequency range, with about the same rise in SWR at the lowest and highest frequencies. See Figure 1 below. Table 1 below shows SWR vs. percent reflected power. If the antenna is a wideband type, then the measurements at the low and high end of the frequency range may not be sufficiently different to indicate whether th e antenna rod is too l
The tuning wand A tuning wand can be made by wrapping three to four inches of aluminum foil around the end of a wooden dowel and then securing the foil with electrical tape. Then test the antenna by moving the wrapped end of the dowel up the antenna rod while observing the reflected power on the wattmeter, as shown in Figure 2 on page 20. If the reflected power increases as the tuning wand approaches the tip of the rod, then the rod is too long. If the reflected power decreases as the tuning wand approaches the tip of the rod, then the rod is too short.
The MFJ SWR analyzer The MFJ model 259B SWR analyzer (Photos 1 and 2, above) is popular among radio amateurs. However, the usefulness of this device is not limited to ham radio use. The analyzer is simple to use, inexpensive and portable, and it has a self-contained signal generator for finding the resonant frequency of antennas and other frequency-sensitive devices or circuits. Hook the coax line to the analyzer and tune the frequency of the analyzer to produce the lowest SWR indication. This indicates the resonant frequency of the antenna. You can also check the bandwidth of the antenna by adjusting the frequency until the SWR indicates “2:1” on the upper and lower side of the resonant frequency. Note the frequencies at which the SWR is 2:1. This is the 2:1 SWR bandwidth of the antenna. Analyzer models are available that handle VHF highband as well as UHF land mobile frequencies.
Photo 3 at the right shows the analyzer locating the resonant frequency of an antenna. As shown, the resonant frequency is 151.34MHz. The SWR at this frequency is 1:1, as shown on the lower left meter. Photo 4 on page 22 shows the frequency ranges available on the model 259B.
Return-loss bridge The return-loss bridge is also useful for determining the resonant frequency of an antenna or other frequency-sensitive devices. Figure 3 on page 22 shows the setup for using a return-loss bridge. Here, a tracking generator is used with a spectrum analyzer to display the response of frequency vs. return loss of the device (the antenna) under test (DUT). First, the reference is established by shorting, or leaving open, the port where the DUT is connected. If the output of the tracking generator is set to 0dBm, then the reference should reach almost to the 0dBm mark on the display. Then, the antenna is connected and the tracking generator is set to sweep the proper frequency range to be checked. The response curve of the return loss will look like the one shown in Photo 5 on page 24. The resonant frequency will be the lowest point on the curve (the point where the return loss is the greatest). Although this method is quite accurate, it is seldom used to check mobile antennas because of th e required equipment.
The time-domain reflectometer The time-domain reflectometer (TDR) is used to determine where a fault lies along a transmission line. It can also be used to determine the length of a transmission line. Figure 4 on page 22 shows how the TDR is connected and used with an oscilloscope to check a transmission line. Photo 6 on page 24 shows the oscilloscope waveform produced by a TDR connected as shown in Figure 4. The initial, or incident, pulse is shown to the left on the display. The horizontal timebase is set to 0.1ms per division. The transmission line being tested has a velocity factor of 0.66, or 66%. This means that a wave traveling on the transmission line will travel at only 66% of the velocity it would travel in free space. The free-space velocity of a radio wave, in meters, is 300,000,000mps. Thus, the velocity of a wave on this cable will be 0.66 X 300,000,000, or 198,000,000mps. Note that on the oscilloscope display, the return pulse is delayed by two divisions, or 0.2 microseconds, from the incident pulse. In 0.2 microseconds, t
Each of the test and measurement procedures presented here has its appropriate use. You may not need to use some of these procedures or setups every day, but don’t overlook some of these alternate methods of doing things when the need arises. We tend to get used to doing things the same old way and to resist changes in our methods and techniques. Some of these techniques can be quite revealing and interesting. Until next time-stay tuned! n