Mobile Antennas:
The Compromises of Placement vs. Performance
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The most important element of a good radio installation, is the antenna. Just like a stereo system will only sound as good as it's speakers, a radio system is only as good as it’s antenna's performance. Therefore antenna selection and placement are important factors to consider if you are trying to achieve the best performance.
If you are mounting your antenna for looks and don't really care about performance, don't bother reading any further, there is nothing in this article you will want to read. For the rest who are still with me, please understand that in my business, lives may depend on the radios I install, so maximum performance is essential, and my number one goal.
Since
I am writing about mobile antennas, please assume that everything I say
primarily applies to mobiles only (I won't draw distinction between bases
and mobiles even though many points here apply to both). There is a lot
of controversy over mobile antenna types and placement especially when it comes to CBs, opinions abound (I've
heard 'em all) and everyone seems to be an "expert",
hopefully I can provide some basic facts about this subject, in simple terms,
so you can be aware of what you are up against, and make intelligent informed
decisions on your installation.
~o~
A ground plane antenna (which all land mobile CB antennas are), needs to have 2 things, an adequate radiating element (radiator), and an adequate counterpoise or ground plane (GP), usually the body. Since an antenna must be at least 1/4 of its radio's wavelength, nearly all CB antennas have to be a compromise of either the radiator, or the GP. The only one that is not, would be an 8 ft 1/4 wave (108") stainless steel whip mounted on a spring in the middle of the roof. This is not a very practical setup, not to mention ugly. VHF, UHF, and Cellular have a much shorter radio wavelength, and therefore the antennas are relatively compact, and mounting is less of a problem than for CB antennas, so let's explore the compromises for the CB.
"Loaded" antennas for a CB, (everything shorter than 108", and with a base coil) are a compromise in the radiator, but don't compromise the GP, and allow you to use the middle of the roof, which is the beast GP. Bare in mind, the radiation pattern will always travel over the mass of the ground plane, so in the middle of the roof, the pattern will be the most uniform in all directions. If you place the antenna on the left front fender, you will have a directional pattern, it will radiate stronger to the right rear of the vehicle. It will still radiate in all other directions, just not as well (VHF & UHF are less affected by this because they use a smaller GP). The best loaded antennas for CB, VHF & UHF, are made by Antenex. I my business, I have yet to replace one that has failed. Maxrad is a very good second.
If you use the roof for more than one antenna, or you have a rack or light bar, try to observe a minimum a separation distance of 36" for the CB (you can cheat down to 24"), 2 meter needs 18", and as little as 10" for 440 mhz & cellular. You need the same distance from a rack, or light bar. A loaded CB can be as little as 18" from these things as long as the whip itself is clear of the obstruction.
Since the GP should also be 1/4 wavelength in diameter, rack mounting does not work well for VHF & UHF, because they are too far from the roof, and try to use the rack as a GP, which isn't as efficient as the roof. Even if the rack is grounded to the body with a wire or strap, it isn't RF coupled to the roof, and the bars will try to act like radiators, scattering the signal in an ineffective and unpredictable pattern for all frequency bands. Even a CB can't effectively get a good RF coupling through a rack. The ground plane needs to appear solid to the RF radiation in order to effectively radiate its energy out in a usable pattern. So a roof rack would have to have a metal (or wire mesh) floor in order to be a good GP.
Think
of the ground plane as a launching pad for the radio waves.
Now think of yourself as a radio
wave, ready to bounce into the air.
The roof of a vehicle is like a
trampoline...
A roof rack is like clotheslines.....
Now which would work better to bounce
from?
It would take a lot of clothes lines
to make a good trampoline.
It would take a metal floor roof rack
to make a good ground plane.
If the roof is not a practical location (because of a rack, or you're not willing to drill a hole), then the next best spot (besides the middle of the trunk for a car), is on a 90º angle bracket attached to the inner panel of the front fender. This location will provide decent performance for a CB even though it will be somewhat directional. For higher frequencies like 2 meter, 440 mhz, and cellular this location is quite acceptable. This is because as the frequency increases, less GP is needed, therefore placement is more forgiving. You should never use the bumper for an antenna mount, unless you really don't care how you radio performs, and "through the glass" antennas for any frequency, are the last choice (cellular is the only one that works fairly well, but still not the best). Magnet mounts are not really grounded to the GP, and so their performance is poor, and gets worse in the rain. They’re okay for temporary use on the highway, but are an un-acceptable choice for use on any vehicle that is used off-road, especially in wooded areas. 27 mhz is very unforgiving, and a bad (SWR) match can blow the transmitter in an instant. I have repaired a lot of CBs that were using a mag mount, 'till it got knocked over.
If you desire maximum range and performance from your CB, then you do not want to compromise the radiator, and you will need to consider mounting a full ¼ wave antenna somewhere... but your choices are few. The bumper is probably the poorest location for an antenna you can find, because the body blocks too much of the radiator, and usually gives a bad SWR (you might as well have a shorter loaded antenna where it will do some good). Mounting it on a tire carrier or on the front of a brush guard will work okay, but isn't a good coupling to the GP. So for the best performance that only leaves the roof (best performance, but not strong enough for a 1/4 wave whip), or high on the rear quarter panel. Here the sheet metal is strengthened by the bends that form the rear pillar, the window opening, long crease of the fender, and the fuel filler door. All these add up and make this location strong enough to withstand the stresses of a whip, and it will use the whole body as a GP, and it is mounted high enough so the obstruction from the roof will be minimized.
All of these locations (except the roof), are a compromises of the ground plane location, because your pattern will become very directional. However, due to the increased performance of an 8 ft antenna, your weakest direction will be nearly as good as a loaded antenna in the middle of the roof, and in it's strongest direction, you can have up to 3 times the range. This could be valuable in a situation where you can use your vehicle's directional pattern to your advantage by pointing it in the direction you are trying to transmit.
For the CB, the 8 ft stainless whip is the ultimate radiating element... bar none. The next best would be a long fiberglass, (the longer the better). Fiberglass antennas are wound, so they can have 108" (or more) of a wire in an antenna less than 108" long. However, 27 mhz is a "temperamental" frequency, and even though you might have 108" of wire in a much shorter antenna, 27 mhz will see the 108" of wire as a good match, but will just see the overall length as it's useable radiator, so it will not perform as well as a 108" SS whip. Wound fiberglass antennas are no better than their length, they are like big resistors, designed to fool the transmitter into thinking it's hooked up to a big antenna. The same applies for a 8 ft wound fiberglass that boasts as much as a 1/2 wave or even 5/8 wave of wire in an 8 ft overall whip, 27 mhz will treat it as a 1/4 wave antenna. I have performed exhaustive side by side tests with a 1/4 wave stainless steel, and all types of fiberglass antennas of varying lengths (antennas have been my hobby for many years), and the only one that equals the 8 ft stainless steel whip, is a fiberglass that is 108" long.
So here are the pros and cons of stainless steel & fiberglass. An 8 ft fiberglass antenna will stay more vertical than a stainless steel while you are driving down the road (which is good for maximum range), and not "whip" around as much as a stainless steel. However a big disadvantage of fiberglass is that it's not as flexible as stainless steel, and is much harder on the mount and mounting surface, and a fiberglass antenna can split if bent too far, and it will also shatter when it strikes an object hard enough (like a branch), ...and any antenna 8' long WILL hit something hard eventually! A stainless steel whip will not break, it will only bend, and can easily be straightened. NEVER transmit with a damaged fiberglass antenna, so If you choose to use fiberglass, ...carry a spare.
I hope I have provided you with enough information to answer the most common questions, and didn't loose you in all that. Remember, a CB can work "okay" for short distance with a less that optimum antenna, and a less that optimum placement, but like a stereo sounds only as good as its speakers, a radio will only perform as good as it's antenna.
You are sure to find lots of folks who'll tell you what a good setup for a CB is, but here's the bottom line... The best radio setup is the one that is carefully thought out, properly installed, with a SWR of less that 1.5:1, and most importantly, .....works well.
The point of all this is, a poorly
installed antenna, even though very ineffective, can still work good enough for
close communication like a trail ride, or a caravan on the interstate. But it
is going to be worthless for anything else. In my business, lives might depend
on how good a radio works, so I have to be sure that the setup is as good as it
can be. On a trail ride, well, lives still may depend on how good a radio
works....... so why settle for good enough?
Feedback is welcome:
dch@olypen.com
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