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Antenna Matching on platforms

 
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spirited09
Antenna-Theory.com Newbie


Joined: 08 Jul 2011
Posts: 4

PostPosted: Fri Jul 08, 2011 3:03 am    Post subject: Antenna Matching on platforms Reply with quote

Hi ,
I am still relatively new when it comes to antennas, I would like to ask on antenna matching on a platform. Using a EM simulator FEKO, i could successful matched a monopole antenna using a matching circuit, verified on the Smith Chart.

However the problem arise when I place another monopole with its respective matching circuit into the model in which the distance seperation is less than half a wavelength due to the platform constraint. Both the antenna does not seem to match despite being matched when running alone.

Could anyone explain why this is happening, from what I deduce it seems to be antenna detuning but I can't seem to find any readings on it. Appreciate any suggestions . Thanks
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Sat Jul 09, 2011 10:33 pm    Post subject: Reply with quote

So antennas are isolated objects. If you place an antenna near a dielectric (water, rubber, trees, whatever), that dielectric can "load the antenna". This means that the resonant frequency of an antenna can shift down when a dielectric is near it (the reason being that the wavelength for a given frequency is smaller in a dielectric medium due to the slower propagation of light).

In addition, metal near an antenna will reflect energy back to the antenna and disturb the fields on the antenna producing radiation.

So in general, an antenna designed to work in free space can have very different performance when installed in a real environment - a roof top, the side of a building, etc

Hence, it is possible that if you put another radiating antenna near your antenna, they will "detune" each other. The magnitude of this detuning increases as they move towards each other. I would guess you need to re-evaluate the antenna impedances when both antennas are near, and adjust the match accordingly.
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spirited09
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PostPosted: Mon Jul 11, 2011 2:15 am    Post subject: Reply with quote

thanks Schubert, for the explaination, I am matching the antenna using a LC circuit after obtaining the antenna's input impedance.The antennas are mismatch after the first simulation with the calculated LC values due to the detuning affecting it. I re-adjusted the LC circuit based on the input impedance of that first simulation but the results also show more detuning, hence it doesn't seem to ever match, regardless of how many time I recalculate.

Is there a particular formula that I should be looking at that help or topics/readings that show how we can calculated how to compensate for the detuning or the environment factors?
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Schubert
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PostPosted: Tue Jul 12, 2011 1:02 am    Post subject: Reply with quote

No, the only equations/solutions are for the most basic cases (infinite free space, thin linear wire antennas) and even then it's tough to fit them to a real-world situation where radios and objects are around.

I think you should just simulate the impedance of the antenna with the other antenna near it, and then match it in that state. Iterate.
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spirited09
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PostPosted: Thu Jul 14, 2011 8:04 am    Post subject: Reply with quote

Thanks Schubert , that confirms my suspicion. Thanks
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