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gain and efficiency of antennas

Question asked by DUSSOPT on Mar 31, 2003
Latest reply on Mar 31, 2003 by ALEPEREZ

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I have a problem with the gain and efficiency of an antenna calculated by Momentum.
First, I simulate a microstrip dipole on a substrate fed by differential ports. The directivity is 2.65 dB, the gain is 1.42 dB and therefore the efficiency is 75% (some losses occur because of surface wave modes in the substrate).

Now, I simulate a similar dipole, but slightly assymetric. The impedance is almost identical since the dipoles are similar, but the efficiency is now exactly 100%, with the directivity and gain equal to 2.35 dB!

anybody can help?
thanks!

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janvh Mar 31, 2003 11:04 PM
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Can you send your project to technical support.

Thanks
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ALEPEREZ Mar 31, 2003 11:20 PM
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This is not exactly an answer to your question but may help you to find a mistake somewhere in the setup or UI usage (otherwise you need to contact Tech Support as already mentioned). It's a short example how Gain/Directivity computed by the UI can be compared with a manual computation.

The input power used by Momentum is not 1 W but is derived from the
thevenin source specified... e.g.:

At 7.685185185 GHz:

S_50 = 0.12288 + j 0.2533705 = -11 dB
Zin = 50*(1+S_50)(1-S_50) = 55.229 + j 30.397

The circuit was exited by a thevinin source (1 Volt + 50 Ohm series).

V_antenna = Zin/(50 + Zin) = (0.561 + j 0.127) Volt

I_antenna = 1/(50+Zin) = (0.00877 - j 0.002533) Ampere

P_injected = 0.5 * V_antenna * conjg(I_antenna) = (0.002301 + j*0.001267) Watts

is injected into the antenna

P_radiated = 0.00214 Watts is radiated (space waves)

No dielectric losses, no conductor losses =>

0.00230 - 0.00214 = 0.00016 Watts in lost in the surface wave

The efficiency = 2.14/2.30 =>93%

This should match with G/D:

G = 10**(7.298511896/10) = 5.368
D = 10**(7.555658014/10) = 5.696

G/D = 94%

This is a fairly close match.
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