Groundplane

[Roadtrekker]

How large and what shape should a groundplane be the n a fiberglass roof? I will be using the stock Midland MXT275 antenna.

 

73,

Tim

nh0h / WRKX428

[Roadtrekker]

P.S. The only info I have found is:

 

1. 6 inches. Huh? A 6 inch counterspoise? 6 square inches?

2. The same radius as the antenna' s height. This seems to make sense, but would not be feasible on my roof.

[Harlanwillis]

A 6 inch Square piece of stainless steel or aluminum for UHF works great. 18 inch Square for VHF.

[gortex2]

Stock antenna is magnetic so stainless and aluminum is no good for his use. As said 6" minimum but larger the better. 

[SteveC7010]

How large and what shape should a groundplane be the n a fiberglass roof? I will be using the stock Midland MXT275 antenna.

 

73,

Tim

nh0h / WRKX428

How are you planning on fastening a mag mount to a fiberglass roof?

 

Typically, for fiberglass roofs like RVs and some commercial vehicles and ambulances, etc. we use a thick surface NMO drilled through the roof with an appropriate ground plane mounted on the underside. I like the 3/8” NMOs for this. For UHF, a minimum of 12” diameter is good. You can use most any conductive material such as sheet aluminum, metal window screen material, and more. The ground plane needs a good electrical connection to the ground side of the antenna mount to be of any use.

[Roadtrekker]

Thanks for the replies.

 

Yes, I am planning on using a mag mount antenna on my fiberglass roof. I do not wish to drill any holes having in mind resale value. I was thinking of using double sided tape to secure a coated steel ground lane to the roof. Semi-permanent, and no damage to the roof.

 

What I needed to know was the size of the steel. Six square inches might just work, but I thought no I will go a bit larger. Just measured the area available for the antenna and found it to be 8 1/2 inches by 22 inches, so I will be looking for a piece of steel about 8 inches square.

 

Thanks again for all your input.

 

73,

Tim

[SteveC7010]

Ground plane needs to be 1/4 wave length in all directions. For GMRS, that is a 6” radius around the base of the antenna. You can have just east/west and north/south radials like a base antenna but for a vehicle it’s more practical to use a disc of metal.

 

I won’t get into the resale argument over the drilling the hole since your mind is clearly made up. But I do wonder where you’re going to find double-sided tape that is both weatherproof and 70+ mph rated.

[axorlov]

Alternative is to use 1/2 wave antenna, that does not need ground plane.

https://www.theantennafarm.com/catalog/index.php?main_page=index&cPath=191_192_196_215

[Extreme]

I'm using 8 x 8" square of 1/4" aluminum for a ground plane on my SxS, grounded to the frame.  Works great.

Something like this might work.  Should be stout enough for your needs, easy to drill and/or bend if needed.

 

https://www.amazon.com/gp/product/B00HRHB3A0/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

[jas]

Ground plane needs to be 1/4 wave length in all directions. For GMRS, that is a 6” radius around the base of the antenna. You can have just east/west and north/south radials like a base antenna but for a vehicle it’s more practical to use a disc of metal.

 

I won’t get into the resale argument over the drilling the hole since your mind is clearly made up. But I do wonder where you’re going to find double-sided tape that is both weatherproof and 70+ mph rated.

Best answer. That is my understanding as well. The GMRS band is about 64-cm or 25-inches.

 

Best,

 

JAS

WRKP245

[Citizen]

The ground plane needs a good electrical connection to the ground side of the antenna mount to be of any use.

 

I believe this to be true, but also this is something that has confused me for a while now.  I've asked before, but no one has answered directly yet.  Seems appropriate to ask again here, so here goes:

 

Most mag-mounts are for vehicle roof tops, which start as bare metal, but then are primed, painted and then clear-coated (sometimes more than one layer of each).  And in fact, sometimes the magnet itself is painted or sealed.  If the mag-mount antenna needs a good electrical connection to the ground-plane, then doesn't all this primer/paint/clear-coat/sealant prevent a good electrical connection?

 

I realize of course that RF transcends the layers of paint et.al., but still, there is no direct electrical connection unless the vehicle roof-top is bare metal and the magnet is not coated.

 

Where am I going wrong?

 

For an NMO mount where a hole in the roof-top must be drilled, then I can see that bare metal is exposed (as a result of the drilling process) and a direct electrical connection can be achieved, as long as no rubber grommets or washers are used.

 

Thanks.

 

...

[BoxCar]

Inductive coupling. The magnetic field from the base couples the field from the antenna to the roof.

[SteveC7010]

And, the ground side of the antenna’s coax is grounded to the radio itself.

[Lscott]

Inductive coupling. The magnetic field from the base couples the field from the antenna to the roof.

Actually it’s capacitive coupling. The base of the magnet mount is one side of the capacitor and the sheet metal under the paint is the other plate.

 

At times to much of a gap between the magnet mount and the sheet metal results in poor coupling and high SWR. As the frequency goes down, UHF -> VHF -> HF, the problem of poor coupling get worse. Also very thick paint coatings and adding extra padding under the magnet mount base also decreases coupling.

[Citizen]

Inductive coupling. The magnetic field from the base couples the field from the antenna to the roof.

 

Actually it’s capacitive coupling.

 

At times to much of a gap between the magnet mount and the sheet metal results in poor coupling and high SWR.

 

Then, it seems safe to say:

 

1) A good electrical connection is not required for a UHF mag-mount.

 

2) NMO will always be better than a mag-mount (at least on a typical vehicle).

 

...

[Lscott]

Then, it seems safe to say:

 

1) A good electrical connection is not required for a UHF mag-mount.

 

2) NMO will always be better than a mag-mount (at least on a typical vehicle).

 

...

1) Correct. You do need a good metal ground plane immediately under the magnet mount. It doesn't have to be magnetic, just conductive. If it isn't magnetic then of course it's a bit more of a challenge figuring out how to keep the mount in place. 8-)

 

2) Not necessarily. A well coupled ground plane on UHF can perform similarly to an NMO type. A crappy NMO install, didn't scrap away the paint etc., may perform worse than a magnet mount.

 

The issue with magnet mounts is they are not recommended for huge mobile antennas. For example I have a dual band on a luggage rack mount on my 04 Jeep. The antenna is nearly 60 inches tall. The wind resistance at 70 MPH is rather high. If I used a magnet mount I would have needed one of those tri-magnet or quad-magnet mounts to keep it on the roof. Also if you read the fine print some antenna manufactures state "Not recommended for magnet mounts" on their large vertical antennas.

 

If you don't want the hassle of worrying about a ground plane for your antenna then look for one that is a 1/2 wave, also the spec's may say "ground independent" too. Those you can stick just about anywhere, they don't need a ground plane, so long as you keep the vertical radiating section away from things like roof pillars etc.

[axorlov]

I think everything is covered (capacitive coupling), but I can add that 1/4 wave antenna works like half of the 1/2 wave dipole, where the other half (other 1/4 wave piece) is an imaginary part created by the reflection from the flat groundplane. So, DC ground is not needed, but the closer the groundplane to the ideal RF ground the better. Capacitive coupling between magnet and metal roof provides low impedance on UHF frequencies, good enough to work as an RF groundplane.