Re: [amsat-bb] Uplink & Downlink on the same dish

[Charles Suckling <charles.suckling@xxxxxxxxxxxx>]

Mike

It is certainly possible to use one dish for Mode L/S on AO-40.  I have
been experimenting with this over the last few weeks (using a 1.5m dish).
I am using a concentric feed (L-Band helix wound around an S-Band helix)
similar to that designed by Freddy ON6UG and published in Oscar News some
years ago.  The two helices were tested on their own and in the presence of
each other using sun noise checks, and there was no discernible degradation
in the performance of either helix in the presence of the other.  I have
also tried side-by side helices but there is too much off-pointing loss for
my liking with this arrangement.

I have been using the arrangement with  about 10-15W or so at the feed on
1269, which produced a very good downlink signal at the times I tried it
(ie -10dB MB).  As  noted in other postings, good filtering is essential to
avoid the desense which would otherwise occur.  I initially tried a
single-section stub filter (with about 45dB attenuation at 1269) and this
reduced the desense to the point where I could hear my downlink signal,
but I had to put another one in series with this to get enough rejection so
that there was no discernible degradation.  I am in the process of laying
out a pcb for a two section notch filter which I hope to try in a few days.
 This should have in the region of 80-90dB rejection at 1269.

The notch filters are not very lossy.  The single notch has only about
0.1dB loss at 2401, and the double notch one should not have more than
about 0.25dB. 

You may also need a notch filter at 2401 in the TX line, in case your tx
puts out any noise at 2401 which would otherwise increase the 2401 rx noise
level.  I have built a filter for this also, which proved to be necessary
in my system to remove residual interference from the uplink signal.

I am posting details of this project from time to time at
www.g3wdg.free-online.co.uk.

73

Charlie G3WDG


 
----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org