Ground independent mobile antennas

[n1chu]

I use 3 Diamond motorized fold over antenna mounts. While they do the job at the push of a button, my not having to exit the vehicle to fold down each antenna before entering my garage, they are listed as requiring a “Ground Independent” antenna. Two of my antennas are amateur radio and one is a multi-band scanner antenna. Diamond makes ground independent antennas for the three amateur radio bands I use (220MHz and a dual band 2 meter 440MHz antenna), so I chose Diamond amateur band ground independent antennas for the amateur bands. That leaves my multiband scanner antenna and this antenna is the reason I write this. Are the use of ground independent antennas only a concern when transmitting? Or does the requirement also apply to receive only antennas? Since I’ve not seen mobile scanner antenna's advertised as ground independent, can I assume it’s not a concern? (I have been working with the knowledge an antenna is comprised of two parts, the vertical radiator and its ground plane, supplied by the metal body of the vehicle the antenna is mounted on, both equally important. Hence the reason for its ideal mounting location being the center of the vehicle which would be the center of the roof because the metal vehicle roof, hood and trunk act as ground radials electrically. So, if I mount a ground DEPENDENT antenna on a ground INDEPENDENT antenna mount, such as my Diamond fold over mounts, do I loose ground dependent performance? And if so, would a ground strap from the antenna to the vehicle’s metal body satisfy the need for a ground connection?

The foldover mounts have two wires for power, one for 12 volt and the other for ground. This leads me to believe the motors are isolated from the body of the vehicle and a ground strap from the antenna to the body of the vehicle would not adversely affect their operation while, at the same time, supplying the other half (ground) for the antennas needs.

Thoughts?

 

[ScannerSK]

The ground plane would apply to transmitting not receiving from my understanding. This sounds more like an HF related question.

I assume the antenna coax is grounded at the receiver which would also provide a ground at the antenna. So, is it really 'ground independent'? Are coils used in all of these 'ground independent' antennas? If so, possibly the term 'ground independent' is just a fancy term for the coils being used to reduce the wavelength of the antennas.

As for whether a ground strap is required for ground dependent antennas, this may depend upon whether the receiver is grounded. The only reason I could see a ground being required at these frequencies would be to drive a coil. At these frequencies, I would measure the SWR and if it is good, then I would say it is good to go.

Are all handheld antennas considered ground independent or ground dependent?

 

[mmckenna]

The ground plane would apply to transmitting not receiving from my understanding. This sounds more like an HF related question.

The presence or lack of ground plane would impact the antennas radiation pattern, which would come into play with both transmitting and receiving.

Are all handheld antennas considered ground independent or ground dependent?

Depends on the design. Usually the radio chassis and the capacitive coupling to the users hand acts as a counterpoise for it.

 

[JustinWHT]

Are all handheld antennas considered ground independent or ground dependent?

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[vagrant]

I use that motorized mount and Diamond antenna. It‘s mounted to the rack in the middle of the roof. A ground plane indeed helps on TX and RX. You can experiment by driving into a parking lot with cars around you. It made a noticeable difference on RX. I didn’t ask about my TX, but it probably helped as well.

 

[n1chu]

It sounds reasonable and I am inclined to believe the ground plane effect is present on both transmit and receive. (My needs are strictly receive.) But there’s so much antenna theory involved, it seems some theory’s contradict others. In short, it’s a confusing subject. Diamond requires ground independent antennas be used with their remote motorized fold over antenna mount for vehicles. They offer a few for use with their remote antenna mounts. But I can’t remember seeing other brand names touting “ground independent” on their mobile antennas.

The parking lot idea sounds intriguing. I’m thinking up scenarios where I could park in the middle of a lot and measure reception parameters, first with the lot full just before quitting time, and then after quitting time when the lot empties. The difference would be the metallic covering of the parking lot and then removing that same metallic covering, which acts as a ground plane. Listening to one of the weaker weather frequencies while the lot is full of cars should only get weaker as the cars leave. But by how much and to what degree does this change on other bands? I guess the only way to learn this is to try it. But if I had to guess I’d say the receive only difference will be minimal with either a ground independent antenna or a non-ground independent antenna… and I’m not inclined to try antenna preamps such as those advertised for scanners. They do amplify the signal, but they also amplify the ground floor, which requires increasing the squelch level setting. I employeed one that was powered through the coax for my home outside antenna setup. It worked but the increased noise floor was not appreciated… I swapped out my feedline coax to LMR400 instead.

 

[mmckenna]

But I can’t remember seeing other brand names touting “ground independent” on their mobile antennas.

Just about all the commercial LMR antenna manufacturers make "ground independent" antennas. Look for 1/2 wave antennas, they do not require a ground plane, but they work better with one.

But if I had to guess I’d say the receive only difference will be minimal with either a ground independent antenna or a non-ground independent antenna

The gain specifications of an antenna work the same in transmit and receive.

Getting an antenna with some gain and mounting it on the ground plane should improve performance. Even mounting a 1/2 wave ground independent antenna ON the ground plane should give you about 2.15dB of gain over the same antenna not on a ground plane.