Ground plane question

[ntsqd]

I'll admit to not having searched this, but I've kept up on threads dealing with this topic for a while and I've never seen this addressed. What happens when the base of the antenna is extended above whatever is going to be the ground plane? Are the performance results band-dependent?

Carried to an extreme example: Say a non ground plane CB antenna is mounted to a fiberglass body, but a 10 gauge ground wire is run the ~24" span to the flat metal floor of the vehicle. Or a less extreme example, a 2M or CB antenna is placed on a sheet metal bracket that raises up through the gap between the hood and the fender. Discounting the actual shape of whatever is intended to be the ground plane and the antenna's location relative to it's borders, how does it being elevated above the ground plane change the way the antenna behaves?

 

[Tennmogger]

Good questions, and there are no definite answers. With a ground wire replacing the ideal ground plane normally provided at the antenna base, the feedpoint impedance will change, getting higher. It's still possible to tune the antenna for a reasonable match. How much it changes is unpredictable. Still, raising an antenna base on a pedestal has been done lots of times and is still a workable solution. Getting an antenna into a better location, even with a poor ground plane, can offset problems of a poor location (like low on a bumper).

The radiation pattern of the antenna will also change as you said. An ideal ground plane is symmetrical around the base of the antenna and the antenna radiates equally well in all directions (theoretically!). As the ground plane changes, the pattern also changes, generally radiating best in the direction of the best mass of ground plane. Example: Vertical antenna mounted on left rear corner, the pattern will be skewed to favor the direction off the right front.

The pattern will definitely change with frequency and band changes. You may find some bands that simply will not tune up without a proper ground plane.

You used the term "non ground plane CB antenna". Is that in fact what you have? Some antennas really do not need a ground plane and that would be the ideal solution to your problem. Examples are half wave antennas, bazooka matched quarter wave type that uses an isolating sleeve below the feed point, a J-pole antenna, etc.

Antennas like this typically are for one band only though so if multi-band operation is needed then you should strive for a better ground plane. Multiple wires run to multiple grounded locations helps. A fiberglass top could have a disc of thin aluminum glued to the underside to act as a ground plane....even aluminum foil has been used, but it's a challenge to make a good connection to aluminum foil. Copper tape with a sticky back can be used to make a ground plane under a fiberglass roof.

Hope that helps,

Bob

I'll admit to not having searched this, but I've kept up on threads dealing with this topic for a while and I've never seen this addressed. What happens when the base of the antenna is extended above whatever is going to be the ground plane? Are the performance results band-dependent?

Carried to an extreme example: Say a non ground plane CB antenna is mounted to a fiberglass body, but a 10 gauge ground wire is run the ~24" span to the flat metal floor of the vehicle. Or a less extreme example, a 2M or CB antenna is placed on a sheet metal bracket that raises up through the gap between the hood and the fender. Discounting the actual shape of whatever is intended to be the ground plane and the antenna's location relative to it's borders, how does it being elevated above the ground plane change the way the antenna behaves?

 

[dzzz]

It won't change the way a non-ground wave antenna behaves. Better a non gp antenna in a good location than a gp antenna in a bad place. With a non-gp antenna the wire should be purchased with the antenna.
The higher the frequency, the smaller the effective ground plane needed. I have no existing gp, so I'm making a small gp for the cell phone amplifier antenna (pcs) and using a non-gp antenna for cb. Short wave gets the second best location after the cell phone antenna.
I don't like the fragile nature of non-gp antennas. But performance is within 10-20% of gp if tuned.
Aluminum is not a gp, correct.? It has to be ferrous? I was planning on adding stainless steel. But I've read of people using copper. Copper isn't ferrous unless I missed something in fourth grade.

 

[Pskhaat]

J-pole at 27MHz would be like ... head maths ... 26 feet or so?

 

[ntsqd]

"the best of intents.... "
What I intended to say was "a no non-ground-plane antenna", in other words an antenna the requires a ground plane. Which is what I should have said. doh!
Theoretically I can see where a separation of the mast from the ground plane would hinder performance and that it would be proportionate to the separation distance (be it linear, squared, etc.), but I wondered about the reality.

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My infantile antenna understanding is that a ground plane need not be ferrous, only that it be conductive. It need not even be a disc or a sheet, it can be a series of radial elements not unlike the mast and that 1/4 wave is nearly the ideal std. radial length regardless of the band. Very manageable for 2M, not so much for CB.

 

[CanuckMariner/Nomad]

Perhaps getting a no ground plane antenna will solve your problem. I had the same issue with my CB as I have a FRP top and the Wilson Trucker 2000 antenna was not giving me the range (<<< 1 mile) I should have gotten (3-5 miles).Turns out it was not getting sufficient ground plane due to the coil being lower than the roof line and I could not mount it on the roof as it was FRP.

With the no ground plane antenna from Wilson (sold the other antenna for what I paid for it) , it resolved the range issue and improved way more than I thought it would. Now I can get up to 9 miles when on a line of sight road/plane...NV basin/range, Death Valley, Black Rock/Alvord Deserts, etc.

 

[PhulesAU]

There are a couple vendors that sell a "Radial" kit, that screws into the mount base , and then the antenna element screws into it. this will help create a GP for you, but isn't going to ideal. I we had an Idea of where/how you had it mounted , maybe we could guess you a fix.

 

[ntsqd]

I'm not looking to fix anything, I guess that I wasn't clear about that. I'm mostly trying to understand how future decisions will affect future antenna performance. What spurred this question is that for an up-coming trip I need to install a CB antenna (ick!) temporarily.

My extreme example above was the case with the vehicle in my avatar, but I no longer own the buggy. My less extreme example was one possibility under consideration until last night when it was concluded to be a no-go. The easy options available are all compromises, the best option, replace my existing NMO mount antenna with a CB NMO mount antenna can't happen in time or at a cost that I'm willing to bear.

 

[CanuckMariner/Nomad]

I'm not looking to fix anything, I guess that I wasn't clear about that. I'm mostly trying to understand how future decisions will affect future antenna performance. What spurred this question is that for an up-coming trip I need to install a CB antenna (ick!) temporarily.

My extreme example above was the case with the vehicle in my avatar, but I no longer own the buggy. My less extreme example was one possibility under consideration until last night when it was concluded to be a no-go. The easy options available are all compromises, the best option, replace my existing NMO mount antenna with a CB NMO mount antenna can't happen in time or at a cost that I'm willing to bear.

NGAs are only $20-$40.

 

[ntsqd]

"NGA"?

FWIW I'm done with mag mounts. Aside from being a different topic altogether they're simply no longer considerable.

 

[CanuckMariner/Nomad]

"NGA"?

FWIW I'm done with mag mounts. Aside from being a different topic altogether they're simply no longer considerable.

NGA = no ground antenna!