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Re: [Discuss-gnuradio] Mode S and ADS-B

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Mode S and ADS-B
Date: Thu, 05 Jul 2007 20:37:42 -0700
User-agent: Thunderbird (X11/20070326)

Eric A. Cottrell wrote
> One thing to try is to use the scope program to set the threshold.  The
> default is low for alot of front ends including the DBSRX.  The DBSRX
> tends to be deaf.  Also I find a gain setting of 32 to 36 seems to work
> best for the DBSRX.  

The DBSRX typically has a noise figure around 3-5dB.  I wouldn't call
that deaf.  You are connecting a discone to a wide open front end
without a filter, so it is far more likely that you are experiencing
desense from the thousands of other signals you are picking up.  Also,
raising the gain above 32 would reduce the noise figure you see.
> Getting the antenna up in the clear is important.  A discone will work
> if you have a good receiver but I found building a simple 1/4 wave
> ground plane on an N connector beats out the discone.

Probably because it is more frequency selective than the discone, thus
reducing the strong interferers.

When you set up a discone on the DBSRX you need to understand that you
are picking up multiple cellphone bands, the pager bands, 2 ISM bands,
microwave ovens, radars, etc, in addition to the nice weak signals you
are looking for.


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