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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?).

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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.

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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.

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  • $\begingroup$ It's also common to use planets as ground planes. $\endgroup$
    – Someone
    Oct 30, 2022 at 15:56
  • $\begingroup$ Dirt is a poor conductor typically. $\endgroup$
    – user10489
    Nov 9, 2022 at 12:54
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I tried adding a comment, but it looks like I need a "50 reputation", so I'll add my comment as a post. You really don't give enough information about your setup, is this antenna for 2M, 6M 10M, 12M, 20M, etc.? The main function of a groundplane is to cancel stray currents on the coax shield, and provide an impedance to the the system that would not otherwise be reliable if your coax shield terminated and you simply had a 1/4λ length of conductor sticking out as your antenna; with that said, an uneven groundplane can cause directionality and cause your radiation pattern to be lopsided, as the electromagnetic field of the antenna radiator, wants to couple with the "ground". Anecdotally, when I ran 10M in my car with a bumper mounted antenna, the gain of the antenna noticeably favored the front of the car, in other words, in the direction of the groundplane. However, you can model a goundplane antenna that has 2- 1/4λ counterpoise elements,each 180 degrees apart from each other, you will see a radiation pattern that is nearly a perfect circle; add a 3rd element (each element then being 120 degrees apart) and you've got as perfect a circle in radiation as can be expected. Of course in a car, with the sheet metal being limited by the dimensions of the car, you may be forced to put up with a pattern that favors one vehicle direction over another; however, if you are in the UHF region, the affect on the pattern may be indecernable even if the antenna is mounted on a corner of the car.

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    $\begingroup$ "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. $\endgroup$
    – wbg
    Oct 16, 2022 at 16:57
  • $\begingroup$ 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. $\endgroup$
    – Lou-in-USA
    Oct 17, 2022 at 13:25
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    $\begingroup$ 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. $\endgroup$
    – Lou-in-USA
    Oct 17, 2022 at 13:25

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