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radiation efficiency, gain and resonant frequency

 
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nurul
Antenna Theory Regular


Joined: 26 Apr 2011
Posts: 23

PostPosted: Mon May 30, 2011 5:34 pm    Post subject: radiation efficiency, gain and resonant frequency Reply with quote

hello,

I simulated an antenna design - simple patch antenna.

according to the simulation, at 3GHz, it has a reflection coefficient of -25. and when I check at the smith chart - is it nearly to 1, which indicate that the antenna has a VSWR of less than -2. which mean the antenna is resonating at 3GHz. correct?

when i check at the radiation properties, the gain at 3GHz is around 1.5dB and the radiation efficiency is around 58%.

however, at 2.5GHz, where the antenna's reflection coeff is just about -2.3dB, the gain is around 1.6dB with a radiation efficiency of 60%.

in both frequency, the radiation pattern looks the same.

does the radiation properties of an antenna relies on its operating frequency? shouldn't the antenna described above have higher radiation efficiency at 3GHz compared to 2.45GHz? or.. my simulation is wrong... hehe
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Mon May 30, 2011 6:08 pm    Post subject: Reply with quote

So in general, you can't conclude that an antenna with a lower VSWR or reflection coefficient will radiate better than one with a higher VSWR.

Remember: low VSWR means the antenna is better matched to the receiver or transmission line. This means less power is reflected. Having a low VSWR is a requirement for an efficient antenna; however it isn't sufficient to prove it will have high vswr.

Remember that a 50 Ohm resistor will have a VSWR=1 (no reflected power), but obviously won't radiate.

For your patch antenna: are there any lossy materials there? Do you use impedance matching (inductors or capacitors)?

Also, I don't think your simulations are correct. A couple problems:
1) A simple patch antenna should have very low bandwidth-i.e. it won't cover from 2.5 GHz - 3GHz in a real design.
2) If the antenna reflection coefficient is -2.3 dB, then more than half the power is reflected from the antenna. How then can you have a radiation efficiency greater than 50%? Something is not right.

Make sure all your numbers make sense.
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Brendan
Antenna-Theory.com Newbie


Joined: 07 Jun 2011
Posts: 2

PostPosted: Tue Jun 07, 2011 12:35 pm    Post subject: Reply with quote

Having a low VSWR is a requirement for an efficient antenna; however it isn't sufficient to prove it will have high vswr.
Shocked
Are you sure?
I would say a low VSWR is a requirement to feed an antenna efficiently,with lossy feedline of significant length.The antenna will radiate the power delivered to it.
Perhaps a better way to word that would have been " a low VSWR helps to reduce feedline loss ,therefore creating a more efficient " antenna system."

Your thoughts?

Brendan
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Brendan
Antenna-Theory.com Newbie


Joined: 07 Jun 2011
Posts: 2

PostPosted: Tue Jun 07, 2011 1:12 pm    Post subject: Reply with quote

when i check at the radiation properties, the gain at 3GHz is around 1.5dB and the radiation efficiency is around 58%.

however, at 2.5GHz, where the antenna's reflection coeff is just about -2.3dB, the gain is around 1.6dB with a radiation efficiency of 60%.

As far as I know "antenna" efficiency is not related to VSWR or return loss.
"Antenna system" efficiency is however.
I did read the efficiency article in the antenna theory site and have the same issues with the wording there .
But Im no expert ,just someone with an interest.

Brendan.
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