PCARES - Vehicle Antenna Placement

PCARES

No antenna works well inside a car. For vehicle operations, you will need to mount an antenna on the outside of the vehicle and run a cable inside to your radio. For these types of antennas, the mount and the antenna may be sold separately or may come together as a bundle.

Antenna Types

Vehicle antennas for VHF/UHF operations tend to be flexible vertical antennas between 1 and 4 feet long. Longer antennas usually provide higher gain. The antenna you choose must support the frequency bands that you will use. For safety, it should support all frequency bands your radio can transmit on, whether or not you plan to use them.

Most vehicle antennas require a ground plane. If your vehicle and antenna mount don’t provide a good ground plane, you will have to find an antenna that doesn’t need one. (See “Antenna Placement and Grounding” below.)

Make sure your vehicle antenna and mount have the same kind of connector so that the antenna can attach to the mount.

Antenna Placement and Grounding

For best transmission signal, the optimal placement of your antenna is as high and as central as you can get it, such as the center of the vehicle roof. (But see the “RF Exposure” section below, which gives a conflicting recommendation.) Having the antenna lower, or off center, means that parts of the car above the antenna will attenuate part of the radiated signal. These effects are usually not extreme, so don’t worry too much about being in an optimal location.

Most vehicle antennas require a ground plane, which basically means they need to be mounted to a conductive metal surface (and again, ideally in the middle of it). If you are attaching an antenna to a vehicle that doesn’t have such a surface, like a car with a fiberglass body, then you need to use an antenna that doesn’t require a ground plane. Likewise, if you are using an adhesive mount on a nonconductive surface like glass, you will need a non-ground-plane antenna. (The antenna documentation should state this. If it doesn’t, assume a ground plane is required.)

(In an emergency, some people have been known to duct-tape a cookie sheet to the roof of such a vehicle to provide a temporary ground plane, then stick a magmount to it. It will work in a pinch, but is obviously not preferred.)

The following diagram, taken from the 2021 Larsen Antennas catalog, shows the signal loss of various mounting locations and mounting types, relative a permanent mount at the center of the vehicle roof.

Antenna Mounts

There are four common types of mounts:

Magnetic mounts (“magmounts”) adhere magnetically to the vehicle’s roof or trunk lid. A cable runs from them to the nearest gap between vehicle panels (e.g., door frame or trunk lid), and from there into the vehicle. Magmounts are probably the most common mobile antenna mount. However, some vehicle surfaces (e.g., fiberglass) don’t hold magnets.

Lip mounts. These are mounts that clamp onto a lip of the vehicle, i.e., the edge of the trunk lid or edge of a door. Some variants can be clamped between window glass and the surrounding window frame. These work on nearly any vehicle, but they often make it harder to use whatever part of the vehicle they’re attached to (e.g., a trunk lid mount can make it harder to open the trunk). They also have to be mounted at the edge of the vehicle, which can result in some directional signal attenuation (see below).

Adhesive mounts. These are two-piece mounts. The piece with the antenna mount is glued to the outside of the vehicle; the piece that the cable connects to is glued to the inside of the same location. Very often these are mounted on either side of a windshield or window, so they are often called on-glass mounts. These are less permanent than drilling a hole; the adhesive is usually breakable. But they are not suitable for operating in someone else’s vehicle, so they are uncommon in ARES/RACES operations.

Permanent mounts, which require a hole drilled through the surface of the car. In ARES/RACES operations, permanent mounts are rarely used, because of the possibility of needing to operate in someone else’s vehicle on short notice.

RF Exposure

When you transmit through a vehicle antenna, you are exposing anyone near the antenna to potentially hazardous RF radiation. This includes people both inside and outside of the vehicle. It is particularly significant for anyone who remains near the antenna for a length of time. Each setup is different and you will need to do your own RF exposure calculations for your setup.

Depending on where (and if) the vehicle is parked, people outside it may not be hams or may not be aware of when you’re transmitting. If so, the RF exposure limits for an “uncontrolled environment” apply. The safe distance could be as much as 6 feet away from the antenna. Even if your antenna is in the middle of your car’s roof, someone standing next to the car would not be at a safe distance; you might need caution tape or other barriers to keep people further away from it.

Inside the car, the slightly less strict “controlled environment” limits apply, but it’s still nearly impossible to be as far from the antenna as the guidelines require. If you are a solo driver, mounting the antenna at the rear of the vehicle should suffice, but that won’t protect people in the back seat. Failing that, your best bet is to use the lowest power setting that will suffice, and transmit as little as possible, to minimize exposure.

Clearly, the optimal placement for RF exposure conflicts with the optimal placement for best signal. You will have to balance these competing goals according to your own judgement.

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