antenna-theory.com

[wirelessrudy]

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.

[bigSteve]

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.

[wirelessrudy]