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# Antenna realised gain S11 link

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#### yefj

Hello, i got and antenna which was built is CST.
At 6.52Ghz
The directivity is 8.8dBi
the realised gain is 8.3dBi
S11 of the antenna -10.22dB.
How do i link these three parameters.
Power and efficiency plots shown bellow.
How do i link the S11 so i get the realised gain number?
Thanks.

Realized Gain = Gain*(1-S11)

dB= 20log(V1/V2)= 10log(P1/P2)

directivity:
8.8dB=>7.59

-10.22dB =>0.0951
1-0.0951=0.9049
realised gain:
8.3dB=>6.76
7.59*0.9049=6.868191

how to link the radiation efficiency and totaly effiency to the power terms described in the plot?
Thanks.

Realized Gain = Gain*(1-S11)
Pardon me, yefj, that should have been Realized Gain = Gain*(1-(S11^2)). We can calculate the term (1-(S11^2)) to be 0.905, or roughly -0.434 dB.

It's pretty awkward to handle efficiency in dB, but it looks like the radiation efficiency is 0.986. This gives a gain of roughly 8.738 dBi. Subtracting the above term to include the return loss gives 8.304 dBi for the realized gain.

Total efficiency, then, should be the combination of radiation efficiency and mismatch loss -- i.e., Total Efficiency = radiation_efficiency*(1-(S11^2)) = 0.986*0.905 = 0.892 or -0.496 dB. This is somewhat close to the -0.499 dB shown in your data.

Let's assume the total power stimulated is 0.5W. The amount of power accepted by the antenna is 0.5W*(1-(S11^2)) = 0.4525W, which is pretty close to the shown accepted power. Multiplying the accepted power by the radiation efficiency gives 0.4525W*0.986 = 0.446W, which again is pretty close to what is shown. "Power outgoing at all ports" seems to be the reflected power, at 0.5W - 0.4525W = 0.0475W this seems to be correct.

Hello PlanarMetamaterials, the frequency is at 6.52Ghz the S11 is -10.18 as shown bellow which is 0.309 in absolute value.
S11_abs=10^(-10.18/10)=0.0959401
absolute value squared is 0.0954 , so 1-0.0954=0.9046 like you said.
directivity is 8.8dBi realised gain is 8.3 dBi.

directivity in Abs 10^(8.8/10)=7.58578
10*log10(7.58578*0.9046)=8.36457
radiation effiency is -0.06 so 8.36457-0.06=8.3
How do we plan a horn antenna to have radiation efficiency as close to 0dB as possible?
is there some good article i could read?
Thanks.

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Radiation efficiency is drive by almost entirely by losses in the antenna materials. Metals with high-conductivity platings and dielectrics with low loss tangents should be what you're aiming for.

That being said, 98.6% is already quite high efficiency. I doubt you'll be able to get it much higher.

Hello PlanarMetamaterials,losses in the antenna can cause bad S11,
so i am confused why these losses are any different thebn losses that could cause bad S11?
Thanks.

Losses in the antenna can contribute to a reduction in impedance matching, sure -- but these are rarely dominant, by a large margin. The largest contributors to poor impedance matching (i.e., "bad S11") are, in my opinion, improper mode matching and undesired parasitic reactances.

An example of improper mode matching could be a patch antenna that is too large and supports multiple modes that your feed can couple to -- if you don't take all the modal impedances into consideration, this can cause S11 degradation. An example of undesired parasitic impedances could be a feed line too close to a radiating element -- this parasitic reactance can be significantly larger than the input impedance, leading to poor S11 as well. If you show us what you are trying to do, maybe we can offer some suggestions.

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