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In general, how to interpret the negative gain values on antenna radiation diagram plot? I have attached an example.

My guess is that they represent some value relative to some reference value. If so, which reference value is used in such cases?

enter image description here

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  • \$\begingroup\$ 0 dB means the same signal level as the reference, negative value means less than that. \$\endgroup\$ Commented Dec 1, 2021 at 19:25
  • \$\begingroup\$ Keep in mind the dB (by itself) is just a relative measure of the difference in two power (or gain levels). The absolute value is not as important in so far as antenna patterns go, but rather the relative difference in the gain at different angles is. \$\endgroup\$
    – SteveSh
    Commented Dec 1, 2021 at 22:22

2 Answers 2

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in general, how to interpret the negative gain values on antenna radiation diagram plot?

and

My guess is that they represent some value relative to some reference value? If yes, which reference value is used in such cases?

Possibly, the "dB reference" will be the theoretical device know as an isotropic radiator: -

enter image description here

Image from here and, note that in the diagram above the 0 dB reference point happens to be the edge of the circle. It's an arbitrary reference point of course and, indeed the dB numbers in your diagram may be arbitrary as well.

But, the negative numbers don't represent loss; they probably represent gain rather than loss. So, close to the antenna, the gain is 20 dB and as you move from the device, gain or transmitted power falls (or thins-out) as it also does with an isotropic antenna.

But without further details, that's the best guess I can make.

To confirm all of this you should link the data sheet for the device in your question. That data sheet will, almost certainly, permit a more definitive answer.

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  • \$\begingroup\$ The gain pattern of an antenna does not change as you move further away from the antenna, assuming all measurements are made in the far field. If the gain at a given angle is 20 dB lower than another angle, that difference holds no matter what the distance from the antenna. \$\endgroup\$
    – SteveSh
    Commented Dec 1, 2021 at 22:12
  • \$\begingroup\$ @SteveSH yes, I covered that as a possibility in my answer. I've tried to cover the possible options to give enough input to the OP to release a link to the data sheet. Without further detail from new information, this remains a mystery. \$\endgroup\$
    – Andy aka
    Commented Dec 1, 2021 at 22:59
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The scale is probably in dB so the gain isn't really negative, the gain is smaller or larger. 0dB is 1

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