New Antenna...
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A New Antenna

So the next problem came when I upgraded from a QHA in the loft to an outside turnstile (being unable to afford a professional QHA for outside).  The problems came back with a vengeance!  I needed help, and at that point I was able to borrow a spectrum analyser ("This one takes hours to warm up and the frequency display is off, so you can borrow that for as long as you want!" [sorry, Dave]) to diagnose the problem.  What I found horrified me, not one signal, not two signals, but three page signals all within about 500KHz of the satellite band!  being a radio amateur, I live in a very good VHF location, but so do all the local pager transmitters!  I've shown that using two different antennas produces different relative levels for the differing interfering signals.  (Remember the frequency calibration is off in these shots, the two large signals are at 137.975 and 138.175 MHz.  There is a list of pager frequencies available on the Internet).

QRM-1-small.jpg (13702 bytes)
Not one interfering signal....

QRM-2-big.jpg (14392 bytes)
nor two other signals...

QRM-3-QHA.jpg (13755 bytes)
but three all at the same time!  The first three shots on the outside turnstile antenna.

QRM-3-QHA.jpg (13755 bytes)
You get different amplitudes with a loft-mounted QHA than with the outdoor turnstile

Can we reject the interference?

A filter to reject interference that's only 500KHz away from 137MHz has to have a very good performance, which results in large and expensive cavity filters.  Even then, you can usually get just a narrow passband and a narrow stop-band, at least that's what I've seen with cavity filters for amateur radio repeaters.  Here we want both a good flat passband response, with a quite wide stopband to kill all three pager signals.   As hinted above, we should actually perhaps be trying to solve a different problem, i.e. to make the receiver perform well in the presence of these huge signals.  To put this into perspective, the satellite signals are expected to be around -100 to -105 dBm (anyone agree or disagree with this?), but the interference is around -30 dBm (two signals at -32 dBm).  So the interference is 10,000,000 times the power (3,000 times the voltage) of the wanted signal.  The clue to this was how well the 2m filter did in improving the situation.  It hardly rejected the interference by more than a dB or so, but it worked because the problems were  coming from receiver non-linearities caused by the great strength of the interference.

Next - modifying the RX2 to handle these levels

Copyright © David Taylor, Edinburgh   Last modified: 2015 Jan 18 at 09:32