Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
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Below are all of the forum threads, including all
the responses to the original posts.
Post subject: Noise floor
Posted: Wed Sep 30, 2009 2:31 am
Joined: Wed Sep 30, 2009 2:26 am
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
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
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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