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feed horn versus patch antenna in parabole focal point

 
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wirelessrudy
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Joined: 05 Sep 2011
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Location: Spain

PostPosted: Mon Sep 05, 2011 10:19 am    Post subject: feed horn versus patch antenna in parabole focal point Reply with quote

I've seen people placing patch antennas in front (or off-set) of parabolic antennas to increase the gain of said patch antenna.

My theory is that by placing a patch antenna in the focal point of the parabolic dish you will increase the gain but at the same time also increases the ingress of unwanted signals or noise.

This because a patch antenna usually is bigger in size than the focal point that falls in the opening of a feed horn.
Radio waves transmitted from the micro-strip antennas just outside the focal point are not reflected parallel to the parabola's axe.
For receipt than counts the opposite argument. The patch array will pick up signals not entering the dish parallel of its axe.

This would mean that where a feed horn would actually increase the S/N ratio of the incoming wanted signal from higher signal gain and less receipt of off-set signal, this last argument is not valid for a patch array antenna placed in the focal point of the parabola.

Is this theory true and yes or no, can somebody build the theory around this effect and maybe make some drawings or build even a flash example to show the effect of moving focal point on the vertical axe.

I need to explain this to guys that think placing a patch antenna in front of a dish is only creating benefits where I think we have negative side effect to count with as well.
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bigSteve
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Joined: 14 Mar 2009
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PostPosted: Tue Sep 06, 2011 5:27 am    Post subject: Reply with quote

That doesn't make sense. You wouldn't use a patch antenna to increase the gain. The gain comes from the reflector. Also, patch antennas are extraordinarily narrowband:
http://www.antenna-theory.com/basics/bandwidth.php

So I don't think it would be a wise decision to use those with a dish antenna. Maybe it is a spiral antenna. Most of the time I see reflector antennas fed with horn antennas.
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wirelessrudy
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Joined: 05 Sep 2011
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Location: Spain

PostPosted: Tue Sep 06, 2011 7:32 am    Post subject: Reply with quote

bigSteve wrote:
That doesn't make sense. You wouldn't use a patch antenna to increase the gain. The gain comes from the reflector. Also, patch antennas are extraordinarily narrowband:
http://www.antenna-theory.com/basics/bandwidth.php

So I don't think it would be a wise decision to use those with a dish antenna. Maybe it is a spiral antenna. Most of the time I see reflector antennas fed with horn antennas.
It is actually the other way around. Guys have an patch antenna and put it in front (or off-set) of a parabolic dish to increase the gain. It happens regularly amongst Wifi (amateur)WISPs. see: http://forum.mikrotik.com/viewtopic.php?p=269513#p269513

They think this is a very smart solution where I try to defend it is not the most smart solution you can think off. It has negative side effects too.

By the way, what the bandwidth has to do with it is not clear to me?
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