relevance of "ground independent" re UHF antenna

Short and untechnical answer - it doesn't need to be mounted on a metal body for a good earth. It can - for instance - be mounted on a fibreglass body such as a boat.

Not sure if the last reply is correct
Let me quote from a GME brochure that I have

"An antenna needs a large uniform metal surface beneath the radiating elements to perform correctly. This is referred to as 'ground plane'. Therefore the best position to install an antenna is in the centre of a metal roof, however, this is not always possible ie installation on a bull bar or mirror mount. In this case 'ground independent' antenna should be used to give the antenna its desired radiating pattern without metal beneath"

A ground dependant aerial usually likes to be situated on a roof or similar, to give that large area of metal "ground plane" under it... ground indepepndant don't need that large area so can be fitted to a bar or what not....

This is stretching my memory of antenna theory (already 24 years old!!) - feel free to shoot down..

Every frequency has a wavelength, which varies according to that frequency - it is a known constant. If you were to make your own antenna, you would calculate the wavelength, and then cut the antenna to size.

Often, (though depending on frequency), rather than use a full wavelength 'size', it is almost as efficient, but much easier to use either a half wavelength size, or a quarter wavelength size..

If I remember correctly, the formula is:

length in meters equals the frequency in MHz over 71.25.

There is also a difference between physical length and electrical length - which is why many antennas have the radiating element wound around and around - to fit a much larger electrical length on a much smaller physical length.

Now to start working on answering your question.

Every antenna consists of a positive side and a negative side - you would have seen this in a centre fed dipole which hangs between two posts, and coax into the middle - one side is positive and the other side is negative.

The standard UHF antenna uses the vehicle as the earth, or negative - what is it's electrical length and does it match the positive side - well, sorta good enough.

In the ground independant version, both the negative and the positive are electrically the same length and therefore much better matched and better performing.

As I say, it's been a long time since I did all that sort of stuff at school, and feel free to shoot any of it down..

Having said that, I have seen many times where two distant parties with ground independent antennas are communicating, and those with a standard antenna are only hearing one side of the conversation. (the closest)

I'm sold on the ground independant jobs!!

You can design two types of antenna (at ANY frequency) - ground independant (e.g. dipole) or ground dependant (e.g. quarter wave= 15cm at UHF.)

A ground indepenedant you can mount anywhere - up a pole (elevated feed), on fibreglass etc.

A ground dependant will only work well if mounted on a metal ground plane at least a 1/2 wavelength in diameter = 30cm at UHF.

So if you are going to mount it at the bonnet edge, roofrack or drill a hole in the roof the way taxis USED to do, use ground dependant -otherwise ground independant.

But remember! Although a ground independent antenna doesn't need a nice ground plane to work properly, metal around the antenna will affect the radiation pattern. Don't put a ground independent antenna on your bullbar and expect the same coverage behind you as you get in front. The metal work of the vehicle WILL affect performance.
People often tell me they want the best antenna when they buy a UHF, but they don't like my response when I tell them to get a stainless steel 4.5dB whip and mount a base in the middle of the roof! I tell them the 4.5dB whip is the best compromise of gain over radiation pattern (plus being quite flexible when hitting trees etc.) and the roof mount will make it sing like pavarotti! (-:

I'll give you my very laymans 2c. Ground Independant you can mount on the bullbar. Ground depenadant up on the edge of the bonnet or preferably middle of the roof. In reality there is a million factors that will influence what goes on.