# does the shape of an antenna ground plane matter?

I would like to test a tranceiver setup I created before I mount it in my car. In my backyard, there's a length strip of metal; 10m in length, 10cm wide. Now I wonder if I can use that as a ground plane for an antenna.

Is it important for an antenna ground plane that it is round? Or will an irregular shape (or even long strip) work as well? (...as long as the size of the surface is big enough?).

We use irregularly-shaped ground planes all the time — car roofs, and metal boxes, and all sorts of things.

However, the difference between a ground plane and an antenna with two elements is in the fact that a ground plane (or set of radials) allows the current to spread out. As it has been explained to me, this is the key property which means the size of a ground plane does not much matter — the more the current can spread out, the lower the impedance presented to the feed point. (Ideally it would be 0 Ω, a perfect ground that accepts any amount of current and stays at 0 V, as measured relative to your vehicle chassis or whatever.)

• If you mount your antenna on one end of the long strip, you are not using a ground plane antenna, but an “L” antenna.
• If you mount your antenna in the middle of the long strip, it is a little bit more like a ground plane antenna, or an antenna with 2 "radials".
• If you cut your long strip into two pieces, cross them, and put the antenna in the middle, that's even more like a full ground plane (4 radials).

Now, this doesn't mean that your antenna will work better this way. Rather, it will work more like it would with a full ground plane — your test will be more realistic. But in the end, there is no substitute for testing and tuning the antenna in its final installation location.

It depends entirely on what you are testing.

If you just want to see if it works, there is a minimum size to the ground plane. What you are describing sounds way too small, but that depends on the frequency, so I can't really tell with the information supplied.

The "ground plane" is really just the other half of a dipole needed by a monopole. So it needs to be at least a quarter wave long to work, shorter if it's spread out like radials. For just testing if it is going to work, the shape doesn't matter a lot. Rule of thumb is that the ground plane has to be at least as big as the monopole.

However, the shape does have large consequences to the radiation pattern, and the angle between radials and the monopole has a large effect on impedance. So if you are testing effectiveness, radiation pattern and gain, there's no substitute for placing the antenna in its final location.

• It's also common to use planets as ground planes. Oct 30, 2022 at 15:56
• Dirt is a poor conductor typically. Nov 9, 2022 at 12:54

• "The main function of a groundplane is to cancel stray currents on the coax shield". I do not agree with this. The main function is to create a ground so that the electric charge in the monopole will induce a mirror image in the ground plane, effectively doubling the 1/4 wave to electrically look like a 1/2 wave.
– wbg
Oct 16, 2022 at 16:57
• I agree that a current is induced on the groundplane, and because of that it will appear electrically like a half wave dipole; furthermore, as I indicated, in the case of an uneven groundplane, the field will favor the direction of the counterpoise. Oct 17, 2022 at 13:25
• Also, you didn't say it this way, but one thing that I see a lot of though are descriptions that 1/4 wave groundplanes are effectively 1/2 wave dipoles, particularly when using the mirror image analogy, and they're not a dipole since no differential current is applied to the groundplane half of the system, and the current induced is much weaker, and should ultimately be cancelled out by the time it reaches the feedpoint shield. Oct 17, 2022 at 13:25