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Noise floor - RF Cafe Forums

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 Post subject: Noise floor
Posted: Wed Sep 30, 2009 2:31 am 
 
Captain

Joined: Wed Sep 30, 2009 2:26 am
Posts: 6
I am transmitting carriers on one pol of a circular polarized antenna 9.3m Andrew. I wanted to use the other pol for more traffic, but as soon as the amp starts transmitting the noise floor of the opposit pol increases about 3db. How can I fix this problem? I do have a receive reject filter installed on the transmit.


 
   
 
 Post subject: Re: Noise floor
Posted: Wed Sep 30, 2009 3:03 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 236
Location: London UK
Hi koos
I would suspect a mechanism in the ortho-mode transducer (OMT) that exhibits a combination of a slightly elevated VSWR and some mode/polarization conversion. In addition, the currents flowing on the dish surface will contribute to some of the increased noise level on the opposite polarization, as will backscatter from the sub-reflector (I imagine there is one).
If you analyze the expected levels on the degraded channel, from knowing a set of parameters such as return loss, CP Isolation and the channel bandpass filter characteristics, in terms of dBc, you may be able to see what deterioration in any of those would lead to the increase in noise floor you are seeing. One of the elements may be of particularly high sensitivity, so a very small departure from ideal will change the unwanted noise significantly. Only then can you address the issue of what to do about it.

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