# S11 for practical antennas

1. ## S11 for practical antennas

What value of S11 is required for practical antennas . Is it less than -10dB or -6dB ?

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2. ## Re: S11 for practical antennas

Don't depend on S11 as much as the antenna gain. If you have the gain you want, then S11 really doesn't matter all that much. That said, try to keep your VSWR under 3:1. Narrow band antennas are fairly easy to get the performance you want, wider band antennas need a lot of "wiggle room".

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3. ## Re: S11 for practical antennas

This cavity-backed slot antenna has a first resonance at about 2.45 GHz. At this frequency, the cavity backed slot antenna is roughly 0.474 wavelengths long - which is roughly the length of a resonant dipole antenna. S11 drops to below -20 dB at this frequency, indicating that most of the power is radiated away. The bandwidth, measured (somewhat arbitrarily) as the frequency span that S11 is less than -6 dB is roughly from 2.35 GHz to 2.55 GHz, giving a fractional bandwidth of slightly over 8%.

Hope it helps :)

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4. ## Re: S11 for practical antennas

Thanks for the reply .
Any tips on improving the gain (Currently I am working on an Inverted F PCB antenna and a gain of about 4.3dBi). Also any points to consider while designing a dual band antenna considering that we require high bandwidths for the two resonance frequencies.

5. ## Re: S11 for practical antennas

The previous post is a quote from the internet tutorial http://www.antenna-theory.com/antenn...ture/slot2.php which doesn't actually answer your question. I suggest to ignore it.

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6. ## Re: S11 for practical antennas

VSWR at TX is a bit as efficiency for car engines. If you have a lot of fuel and an engine that is uncritical in its function, you will get needed result at cost of the fuel (with an less good VSWR).
It is mainly for small battery powered transmitters best efficiency are of importance, but also some very powerful transmitters need to keep losses extremely low as they else not can keep temperature within safe range for the electronic circuit.
VSWR at RX is a similar situation which effects signal/noise ratio. Is every dB counted to be able to receive a signal, must VSWR be low.
High VSWR due to poor matching at antenna will result in that for example outside of a coaxial braid will be a part of the antenna. Can increase problem with near-field noise or reduce effective overall gain.

VSWR is a ratio and often expressed as a such.
Expressed as dB is it usually called "return loss" which is a positive value. Negative loss is an amplification.

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