Mobile communications antennas come in all different sizes and shapes. They also are designed for operation on various frequencies usually divided into various communications services. Some the communications services include the business band, maritime mobile, citizens band (CB radio), etc.. Regardless of the frequency or band of the antenna there are several common considerations to think through when considering both the type and location of mobile radio antenna mounts. This article will review the major issues and discuss options to consider.
Each type of antenna and mount needs to have a good ground and a good ground plane. While the need for this will vary by frequency, having a good ground and ground plane is desired for every frequency in practical terms. Many base station antennas use the physical ground, enhanced through the use of ground radial wires or not, to help form a take-off point for radio waves being transmitted. On a car the flat portion of the top of a car forms a ground plane for a mobile antenna. Location of the antenna on that ground plane becomes important. A center location produces the most nearly round pattern. A location towards one direction produces an elongated pattern in the opposite direction with less effectiveness in the direction towards the antenna. While the center of the roof of a car can be a great location for a mobile antenna mount it may be difficult to work the coaxial cable back to the radio. An easier solution from the coaxial cable’s perspective may be the trunk. Many people opt for a trunk lip mount. A mount in the center of the roof requires drilling a hole in the vehicle while a trunk lip mount uses set screws to hold the mount to the lip of the trunk.
Radiation of as much radio frequency energy as possible is both a good thing and a problem. If the antenna is mounted in the center of the roof, the roof acts as a shield and prevents your body from being radiated by the antenna to the extent practical. If the antenna is mounted on the trunk lip then it is spraying radio frequency energy right at the passengers of the car right through the back window of the car. The amount of radio frequency energy and potential risk varies with frequency and amount of power used as well as amount of time used.
Other options exist and a review of catalogs and blogs on the internet can provide good quality pictures and review of the advantages and disadvantages of many of them. Expect to find antennas mounted on the gutters of the roof, various sides and center of the trunk, side panels of a vehicle, front bumpers, back bumpers and even the engine hood. Remember that there is no 100% perfect solution and the best solution for a particular person will depend on the type of antenna, type of vehicle, budget, technical ability, time, patience, etc..
Regardless of location and shape of the ground plane, a good radio frequency (RF) ground is needed. Think of this example – if negative radio frequency energy can’t flow to ground positive radio frequency can’t flow off of the radiating element of the antenna. I often tell new ham radio operators there are three things you need to know about mobile antenna mounts – grounds, grounds and grounds!
Once you determine the location of the antenna mount and how to achieve a good radio frequency ground (usually with a wide strap or mesh – radio frequency flows along the OUTSIDE of metal unlike electricity) then consider the TYPE of mobile antenna mount. Remember that search engines and blogs are wonderful things for researching types and locations of antenna mounts. Any one catalog or website likely will not have every imaginable type of antenna mount. Also remember that every antenna mount will have both advantages and disadvantages. I will review an example of two or three mounts and you will likely get the idea.
Consider the classic ball and large spring mount. For a large whip type of antenna this can be a good but perhaps not perfect choice depending on your preferences. Also consider a less sturdy but less “springy” fixed trunk lip mount. Now – if you hit a large tree branch with that whip style antenna while driving there will likely be different outcomes with these two mounts. The large spring on the ball and spring mount will have a lot of “give” and will allow the whip to stay attached to the mount with the mount staying attached to the vehicle while the whip slides under the tree branch. A fixed mount attached to the lip of the trunk with a few set screws likely gets ripped off of the trunk and maybe the trunk lip gets bent.
There are disadvantages to the sturdy ball and spring mount which requires four good-sized holes in the body of the vehicle to mount it. One disadvantage other than potential rust and reduction in trade-in value is the motion of the whip while driving. A mount without a spring will result in a steadier whip than one with a spring. Remember – the heavier the antenna the heavier the mount. Larger ham radio antennas are taken by amateur radio operators to metal fabricating shops to be customized. Also ask your local auto dealer for ideas as well as anyone that works with trailer manufacturing, etc..