Dipole over perfect ground, directivity discrepancy

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johan
Posts: 3
Joined: Tue 28 Dec 2021, 18:15

Dipole over perfect ground, directivity discrepancy

Post by johan » Tue 28 Dec 2021, 19:50

Hi,

I was playing with openems and put together a simulation of a dipole over a perfect ground plane. The dipole is placed 0.5 wavelength above the ground plane, which should result in a maximum directivity of about 8 dBi. However, the result I'm getting is 5.3 dBi.
I do not understand this discrepancy, am I missing something in the simulation?

/Johan
Attachments
dipole_perfect_ground.m
(4.65 KiB) Downloaded 164 times
Screenshot_20211228_183918.png
Screenshot_20211228_183918.png (29.9 KiB) Viewed 2063 times

jonsen
Posts: 1
Joined: Thu 02 Dec 2021, 04:11

Re: Dipole over perfect ground, directivity discrepancy

Post by jonsen » Sat 01 Jan 2022, 23:20

Why are you expecting 8 dBi? I'm new to this but that seems high for a dipole.

johan
Posts: 3
Joined: Tue 28 Dec 2021, 18:15

Re: Dipole over perfect ground, directivity discrepancy

Post by johan » Sun 02 Jan 2022, 11:34

8 to 9 dBi seem to be the generally accepted gain of a 1/4 wave dipole at 0.5 to 1 wavelength above perfect ground. A quick NEC simulation shows:
Screenshot_20220102_101423.png
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Now I don't expect this simulation to reach that high, because NEC is using an infinite perfect ground. The PEC in this simulation will only be as big as the simulation is. But I expected the result to come closer than it did, and extending the simulation in X and Y to 10 wavelengths did not give a higher directivity, so I don't think it is the limited ground size that is the problem here.

One thing I noticed is that the value I get is almost exactly 3 dB less than expected (5.34 dBi, compared to NECs 8.38 dBi).
I know that when using a PEC mirror in CalcNF2FF I have to manually divide Prad by 2, because CalcNF2FF does not internally compensate for the apparent doubling of the power the PEC mirror is causing. Could this also cause the directivity to be normalized to a value 3 dB too high?

Removing the PEC and simulating the same model in free space result in a maximum directivity of 2.18 dBi, which is close to the theoretical 2.15 dBi.

johan
Posts: 3
Joined: Tue 28 Dec 2021, 18:15

Re: Dipole over perfect ground, directivity discrepancy

Post by johan » Thu 06 Jan 2022, 18:05

I modified the simulation to use a perfectly conducting sheet instead of the -Z PEC mirror. This results in a directivity of 8.2, very close to what NEC shows for the same antenna. So it doesn't look like the NF2FF calculation take the PEC into account when it calculates the directivity.
Is this intentional?
Screenshot_20220106_151934.png
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Attachments
dipole_perfect_ground.m
(4.95 KiB) Downloaded 159 times

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