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Parabolic antenna - reflection lost

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mauloftin

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reflected antenna

Hi,

A have feed and dish. Can you tell me, how can I eliminate reflection lost between feed and horn? What are the types of solution ?
 

neazoi

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mauloftin

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parabolic dish transmitting 2.4ghz

neazoi said:
You have basigally lost the focal point of the dish, either because the dish is bended or because the focal point is misalligned. see the next articles to help you determine it again.

https://www.microwave.gr/index.php?option=com_content&task=view&id=26&Itemid=43

https://www.satsig.net/focal-length-parabolic-dish.htm

Also check polarization of the feed (if it is linear). You may have accidentally rotated it and not necessarily loosing the focal point.

Hi,

I am not sure, if I understand, but I think, that is no problem with focus focal point of the dish....
 

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focal point in a parabolic antenna

How do you define "reflection lost", in order to help you?
 

mauloftin

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microwave parabolic antenna

neazoi said:
How do you define "reflection lost", in order to help you?


OK, I will try it explain again:

You have feed. The feed produce EM wave. This wave is reflected by dish. But the reflected wave from center of dish returns to the feed - it is problem. When you measure antenna, you see this as S11 - reflection lost. I hope, that you understand me.

for example>
the feed have S11 < -18 dB, but with dish is about < -12 dB.
 

neazoi

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antenna reflection

Yes that is clear now,
It mostly happens on non-short focal-length dishes as the flat (non-curved) area at the center of the dish, that the feed "sees", is bigger.

A solution is to use a penny-feeder like the one I have shown you in the first link because the feed radiates from the two sides and there is no such "flat center point".

Another solution that I am thinking if you do not want to replace your feeder, may be to insert a small piece of metal at the center of the dish like the attached picture. Apologise apot the picture I have drawn it in paint :)
 

    mauloftin

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mauloftin

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back and side lobe of a parabolic dish

neazoi said:
Yes that is clear now,
It mostly happens on non-short focal-length dishes as the flat (non-curved) area at the center of the dish, that the feed "sees", is bigger.

A solution is to use a penny-feeder like the one I have shown you in the first link because the feed radiates from the two sides and there is no such "flat center point".

Another solution that I am thinking if you do not want to replace your feeder, may be to insert a small piece of metal at the center of the dish like the attached picture. Apologise apot the picture I have drawn it in paint :)

Picture is nice :)

Yes, I can not change feed. "It mostly happens on non-short focal-length dishes" I think, that you do not think non-short, but non-lenght, OK? Short focal lenght is worse (for the reflection lost), yes?

Yes, your solution is first thing I thought, of course, but this solution have bad influence on side lobes of parabolic antenna, and in my case is it problem. - attachment.
 

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antenna feeder cz

Yes it broadens the angle of the lobe but these extra lobes are of low power. You can insert a smaller parabolic metal piece so that not so much power is lost. But this again depends on how flat the dish is at the center (usually applies to big dishes) and it may need some experimentation (I do not know any formula for it). The low power extra lobes is the price you have to pay using this method.


>> I think, that you do not think non-short, but non-lenght, OK?
Exactly. How rapidly the dish curve "closes" (sorry about my English).
For transmitting ONLY, the size of the dish does not matter, but the angle of the lobe. But as the size of the dish gets smaller, the tolerance in reflector errors gets lower, especially as the frequency goes up. A small glitch in a reflector at 10GHz has much more influence in the lobe than at 2.4GHz for example. In a small dish the impact of the glitch is even bigger.

>>Short focal lenght is worse (for the reflection lost), yes?
Not necessarily. For transmitting ONLY, they can be better as far as concern the reflection loss. The reflection loss gets greater as the center point of the dish flattens. This flatness is based on the curveness of the dish. So I would say for transmitting only, a small dish (30cm?) with a high curveness has a much lower reflection loss.

For receiving the situation is totally different.

For your case, I do not know if I can think of any other feasible solution right now...

Added after 5 minutes:

There is another thing you can do. If you use the cone metal piece you have shown then you can add a strip of metal around the dish edge, to minimize the side lobes. but the cone metal has to have low angle, in order to minimize the amount of metal to use and not so wide, in order to avoid radiation comming back to the feeder.
A little bit of experimentation must be done here.
See the mobile telephony "closed" dishes that are using a metal around them.
 

    mauloftin

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mauloftin

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parabolic metal antenna

Ok, I will try to experiment.

The sidelobes is my second problem :) - (If know ETSI EN 302 217-4-2 V1.4.1 (2009-03) - Class 2 antennas RPE 3 GHz - 14 GHz)

OK, thank you very much! I continue to think.....

Thank, have a nice day.
 

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