Mobile GMRS Station Build Guide

How to Build a GMRS Mobile Station

Now that you have your GMRS license and have, most likely, been operating from a handheld radio for some time, you are ready for a dedicated mobile station. A mobile radio station can actually be a significant upgrade from a handheld, even if the antenna isn't any higher than it would be while standing and holding your handheld. When moving into a mobile radio, you'll be able to use up to 50 Watts of transmit power and an antenna with gain which will effectively multiply the output power of your radio. A proper match between radio antenna and feedline (coax) is essential in ensuring optimal radio performance. Let's go over the basics of each component, keeping our priority on the antenna and feedline components.

Mobile GMRS Antenna Considerations

The antenna you choose and where/how you mount it will have more of an impact on your station than anything else you include. Different antennas are available for different purposes, but a 5/8 wave, high gain antenna is the common choice for those wanting to maximize their range and performance. The mobile antenna will depend on a horizontal metal surface on your vehicle as a ground plane. A good ground plane will reflect much of the signal from your antenna back out into the field, rather than allowing that signal to be wasted or misdirected and reflected.

A magnet mount is the most commonly used and is a good choice for many situations. It also allows for mounting in the center of the roof of the vehicle without modification, which is usually the best location for the antenna for omni-directivity. If an antenna is mounted off-center of vehicle, it is almost certain to be somewhat directional or bi-directional. This is not necessarily a bad thing once the operator learns the radiation pattern, but it may be less desirable for many who prefer that their signal is radiated in all directions equally. In fact, a 1/4 wave antenna will have the most omni-directional radiation pattern of all as it has no gain and is also referred to as a 'unity gain' antenna. These antennas are only about 6 inches long and can serve a person well if they are operating within handheld range of the target station but are very limited beyond that. Below is a list of all antenna wavelength options available in GMRS mobile antennas and some of their attributes. If you are interested in knowing the popular choice among experienced operators, the answer is easy...the MXTA-26 Midland, 5/8 over 5/8 wave, 6 dB gain, mobile antenna. It performs very well on GMRS and decently on 70 cm as well. It is 32" long, so may not be ideal for some applications due to its length, but it is hard to beat and has to be the biggest bang for your buck out there.

Here are the primary wavelength options for GMRS (General Mobile Radio Service) antennas, which operate on the UHF band around 462 MHz and 467 MHz.

Wavelength Options

1/4 Wave (Quarter Wave)
Size: About 6 inches long.
Performance: Requires a metal ground plane to function.
Pattern: Radiates signal upward and outward in a wide dome.
Best For: Wooded areas, valleys, and mountainous terrain.
1/2 Wave (Half Wave)
Size: About 12 inches long.
Performance: Ground-plane independent (works without a metal base).
Pattern: Concentrates signal lower toward the horizon than a 1/4 wave.
Best For: Fiberglass vehicles, UTVs, and general handheld radio upgrades.
5/8 Wave (Five-Eighths Wave)
Size: About 15 to 16 inches long.
Performance: Requires a solid metal ground plane.
Pattern: Flattens the signal signal toward the horizon for maximum distance.
Best For: Flat terrain, open highways, and base stations.
Collinear (Multi-Wave / Gain Antennas)
Size: Usually 30+ inches long.
Performance: Combines multiple phased wave segments (e.g., dual 5/8 wave).
Pattern: Extensively squashes the signal into a very flat, tight disk.
Best For: Permanent home base stations or maximum-range highway use.

Mounting Options

There are several different ways one can mount an antenna to a vehicle, depending on the application. There is one option that is considered to be ideal but is also often avoided because it requires a permanent modification to your vehicle. If the situation allows for it and you are confident enough in making this modification, you will be able to maximize the performance of your radio, minimize antenna system complications and you will have a professional looking, permanent installation. There are special drill bits available to make it easy to drill the right sized hole for a NMO mount antenna to fit into perfectly and make a water tight seal with an O-ring. A high gain antenna mounted this way and near the center of your vehicle will serve you very well for many years.

However, if you are not willing or able to go this route for any reason, you still have some decent options. Just remember that all other options will leave some coax exposed to the elements, possibly flopping around in the wind as you drive and maybe pinched in your door jam. AND, your new GMRS installation may not gain the approval of your wife or significant other. That being said, the next best option for many is a magnet mount. This kind of mount can be placed at the center of the vehicle as well and can provide you with very good radio performance. If you have a steel roof that is available for an antenna to be mounted, (and you don't want to drill the hole) then a magnet mount may be the wisest choice for you.

Now, if you are a Jeeper, or you own a vehicle with an aluminum body, you may not be able to use a magnet mount at all. In these cases, one must consider brackets which are made to be mounted on either a roof rack, mirror, hood or trunk lip, bumper, hitch or otherwise. In most of these cases, you will not be able to mount your antenna in the center of the vehicle but it will be offset somewhat. That does not mean that your radio will not perform well, but it does mean that your antenna radiation pattern will be somewhat directional. In other words, your signal will be stronger in certain directions than others rather than being fully omni-directional. Although this could lead to a bit of an inconvenience to the operator at times, it's not really a problem once you learn the radiation pattern through range testing. You will eventually find yourself 'pointing' your vehicle at the target station in order to maximize your range.

The most important thing about this whole effort of mounting your antenna is that the antenna be mounted outside of your vehicle (of course). So, no matter how you do it, it will be a big improvement over that rubber duckie antenna on your handheld and inside your vehicle. Just remember that there are 2 parts to any successfull antenna: the radiating element and the counterpoise. In mobile installations, your vehicle is used as the counterpoise. If you fail to provide your antenna with a decent counterpoise/ground plane, the radiating element will not be able to do its job properly. In fact, a ground plane kit should be used in cases where the vehicle is not providing a good ground plane. And the ground side of the antenna should somehow be connected to the body/frame of the vehicle. Doing these 2 things will help your antenna to perform well by compensating for what your less than optimal mounting situation is lacking.

Antenna Feed Line (Coaxial)

The coax connecting your radio to your antenna is considered part of the antenna system as it carries the RF signal to and from the radio. And, in most installations, the feed line will be culpable for the largest percentage of loss in the system. Especially in the UHF band (and higher frequencies) signal loss is a big consideration. In other words, much of the power that your radio produces will be leaking out of your coax before it ever reaches the antenna. This natural phenomenon can be minimized by minimizing the length of the coax and by using high quality, low loss coax.

In mobile installations, this is usually not much a problem at all because the required length of coax is very minimal already. As long as your coax is no longer than 15 feet, you will probably not have much loss even on standard coax (as supplied by manufacturer with most antennas or mounts). Keep in mind that any additional connectors, adaptors, splices or in-line components will add to the overall system RF power loss. The goal should be to minimize loss and maximize performance and range, while using only as much TX power as you need to get the job done. Below is a chart showing the difference in loss per 100 feet over various coaxial types and bands for your reference. Use the 440 MHz readings for the GMRS band as it is close enough to it for these purposes.

To be continued...

Page updated

Google Sites

Report abuse