Mobile antenna notes
One of the most common jobs of the land mobile radio technician is the installation and testing of mobile radio antennas. The proper placement of the antenna on the vehicle involves several factors: the vehicle’s primary use, the customer’s preference, other obstructions already installed on the vehicle, the type of antenna and the optimum location from an operational standpoint. This column deals with the proper installation and tuning of the mobile antenna.
Type of vehicle use If you are installing an antenna on a “street” vehicle, then the choice of antenna or location may not be so critical. If the vehicle is to be used in locations where it passes under low-hanging tree limbs, then consider placing the antenna in a location other than the roof. Long “gain” antennas can catch on limbs or garage doors and have been known to pull completely out of the roof of a vehicle, damaging it. (They don’t make them like they used to.) I have seen this happen many times, and it makes an ugly hole in the roof. When using 5/8l or other long antennas on vehicles, you should consider locating the antenna at a point other than the roof. If the vehicle is an off-the-road vehicle, you should never place a long antenna on the roof.
Figure 1 below left shows possible locations for installation of antennas on a car. Unless there are other prohibiting factors, the best location for the antenna is on the roof_top, dead-center. If possible, keep the antenna away from other objects that may interfere with the radiation pattern of the antenna. Remember, the roof has to serve as a ground plane for the antenna. If one of the lower locations is chosen for the antenna installation, then a 5/8l antenna might be more efficient.
Customer preference Sometimes one of the toughest obstacles is the customer’s preference. He wants maximum coverage but doesn’t want the antenna installed on the roof. He doesn’t want holes drilled in the vehicle and is more concerned about appearance than reliability (at this point). Obviously, some compromises have to be made, and the customer should be apprised of all the facts so that he is able to make an informed decision.
Some customers are just totally opposed to holes in their vehicle, especially if the vehicle is new. I was once asked to place a VHF antenna on a long bracket behind the cab of a pickup truck. The long bracket would place the antenna at a level equal to the top of the roof of the pickup truck. However, the long bracket interferes with the tuning of the antenna because the bracket itself becomes part of the antenna. Ever tried tuning an antenna for VHF high band (or higher frequency) mounted on one of these? Forget it.
Sometimes a magnetic antenna can be used to great advantage when consulting with a customer about the best location for the antenna. If you know that the customer doesn’t operate in the far fringe areas of his coverage area, then the radio might work well at a location other than the roof. However, if you know the customer needs the best possible range from his antenna, then you should try to persuade him to let you install the antenna in the optimum location. You can temporarily place a magnetic antenna in different locations for comparison and use an inexpensive relative field-strength meter to demonstrate signal levels and directional patterns around the vehicle. Sometimes this can be a great persuader.
A simple diagram for a relative field strength meter is shown in Figure 2 on page 8. It can be built in a small project box like the one in Figure 3 on page 8. Every radio shop should have a small, portable, relative field-strength meter available. They are handy for more than just this purpose.
Tuning the antenna How the antenna is tuned will depend on whether the radio is multichannel with wide frequency spacing, narrow frequency spacing, or single frequency. If the radio is single-channel or has narrow frequency spacing then the antenna tuning is less-complicated.
If the radio is multichannel with wide frequency spacing, set the radio to a middle frequency. Check the forward-reflected power ratio and record it. Repeat this procedure on the highest and lowest frequencies in the radio. Compare the results. The forward-reflected power ratios at the upper and lower frequency ranges should be about equal and at least 10:1. The forward-reflected power ratio at the center of the band should be better (higher) than at the frequency extremes.
The graph in Figure 4 below shows how the VSWR might vary over a range of frequencies. If the antenna rod is too long, the forward-reflected power ratio will be better at the lower frequencies. If the antenna rod is too short, the forward-reflected power ratio will be better at the higher frequencies. Thus, the radio can be used to determine whether the antenna rod needs to be shortened or lengthened. It is important that the antenna be wideband so that a satisfactory forward-reflected power ratio (VSWR) can be achieved over the entire range of operating frequencies.
The following table correlates forward-reflected power ratio with VSWR, for the VSWR values from 2 to 1.1:
FWD-REFL PWR VSWR 9 2 10 1.9 12 1.8 14 1.7 18 1.6 24 1.5 34 1.4 58 1.3 110 1.2 400 1.1
If the radio is a single frequency, or has narrow frequency spacing, you won’t be able to use the method just described to determine antenna tuning. The setup in Figure 5 above can be used to determine whether the antenna rod is too long or too short.
Procedure: Key the transmitter, and while observing the reflected power reading, move the end of the dowel up the length of the antenna rod. If the reflected power increases as the dowel approaches the tip of the antenna rod, the rod is too long. If the reflected power decreases as the dowel approaches the tip of the rod, then the rod is too short.
Summary Each antenna is different. Each has its own special needs and requirements depending on the customer, the vehicle, the type of use and the type of antenna. Spend a little time consulting and advising the customer so that he can make an informed decision. He will appreciate your concern and interest. It also might eliminate having to install the antenna a second time. Make sure the antenna is installed securely and properly tuned, so that it operates well over the entire range of frequencies of the radio. After each trimming of the antenna rod, move away from the antenna before rechecking the forward-reflected power ratio. Careful installation and tuning of the antenna can make you a hero to your customer. Until next time_stay tuned!
