Ground plane types on vehicles

[WSCG897]

I was going to purchase a 1/2 wave NGP antenna, but then came across some info suggesting a ground plane does not need to be flat and continuous around the antenna (i.e. does not need to be a flat sheet of metal).

Will mounting a 1/4 wave antenna via L-bracket to a long (48") piece of aluminum be enough to act as a ground plane? If so, what considerations must I take into account ensure it is properly done?

I am thinking of the mounting on one end of the aluminum front "fairing" on my iKamper Skycamp roof top tent (RTT). See red arrow in photo below. The material of the tent cover is gel-coated fiberglass.

[BoxCar]

Should be good, but I wonder about the antenna cable routing if the part moves.

[dosw]

I've done some research into ground planes, and it's entirely possible I've got some of this wrong, but here's what I understand:

 

A good ground plane will provide a radius that is at least a quarter wavelength. So for GMRS, that would be at least 6.4" radius. Ground planes should extend in all directions along the horizontal plane, or at a downward diagonal, depending on the antenna / design. If the ground plane isn't a contiguous surface it is often set up as a series of rods. Two opposed rods would have better propagation lobes along the plane of those two rods, and worse at 90 degrees. Three rods is fairly common. Four is also very common. There are diminishing returns for ground plane rods beyond four, though there's always a non-zero positive impact to adding more symmetrical rods. There's also a diminishing return for making the rods longer than a quarter wavelength. But at less than a quarter, the lobe tries to curve around the end of the rods. Asymmetrical ground planes may have a somewhat truncated propagation lobe in the shorter directions. In your case, the propagation lobes may try to curve around the rack's pipe, for example.

 

The point being, I don't think a roof rack's beam is ideal as a ground plane. But ideal may be the enemy of good enough. However, given that there are pretty good "no ground plane" antennas out there, you may be able to at least select one that doesn't force as much compromise.

 

For my RV I use a Laird TE B4505CN ("N" means no ground plane). I had to trim its bottom whip element for GMRS, but product literature helps by suggesting where that sweet spot is. And as I mentioned in another post here recently, it's very easy if you have a VNA instrument (NanoVNA).

 

Someone who doesn't want to buy measurement equipment could probably find someone on a local GMRS repeater willing to spend a half hour helping measure and cut. But for the B4505CN the manual recommends the appropriate length for GMRS. It's a 5.1dbi antenna.

 

[AdmiralCochrane]

Propagation tends to be across the ground plane.  Other than that, there isn't much to consider.  In general the outer shielded part of the coax and radio are generally electrically connected to the entire metal part of the vehicle body.  An unconnected piece of metal near the antenna may affect it in a minor way, but not likely to be measurable or significantly measurable.

It will be generally more practical to make sure the tent rack has strong electrical continuity to the body of the Jeep and each section likewise carries continuity to where the antenna is mounted.

Don't over think this, it's easy and simple.

[ladysmithgwl]

There are some GMRS antennas out there that are advertised/spec'd/marked as "no ground plane".  Have you looked at those?  The Browning BR2445 "ghost" antenna I am running is a no ground plane antenna.

[MaxHeadroom]

All good replies so far, but I will add a clarification:

"Ground plane" is not a DC ground like your radio connection to your vehicle frame or battery - but a term for the RF ground which influences the antenna radiation pattern.

A ground plane antenna will expect a flat metallic surface below it to give its rated pattern and performance - a "Non Ground Plane" NGP antenna is essentially a dipole antenna with a center feed point and your ground is part of the antenna element(s).

Just wanted to clarify because in the post above a basket rack with an NGP antenna is what would be needed, since a ground plane wouldn't exist realistically on a Jeep hard top. NGP antennas are longer since you need your radiating and ground element versus a 1/4 wave, but is the proper answer. If you want a ground plane antenna - then the other answers are correct and I have seen it on fiberglass roofs where the shop will insert (as large as they can) a piece of sheet metal between the headliner and roof and then drill the NMO - still not perfect but as good as you'll get.

Hope that helps clear up some of the "mystery" between antenna types/locations/methods!