RE: [amsat-bb] dual band helix feed for dish?

[Charles Suckling <csuckling@xxxxxxxxxxxxxxxxx>]

Kevin

Yes, I have been considering this for some time, since I would like to use the 
10ft dish here for both L-Band uplink and S-Band downlink.  Currently I am 
using a 2.25t G3RUH helix as the feed for S-Band, and this seems to work fine.

I have already experimented with putting two feeds in the dish, but with my 
low f/D (0.33) the offpointing angle is too much when aligned for one of the 
feeds and I therefore want a feed in the same physical position for both 
bands.

The feed I am working on  has an L-Band helix wound around the existing RUH 
one for 2.4GHz.  I will measure the effect of the L-Band helix on the 
performance of the S-Band one and vice-versa (using VSWR and sun noise checks) 
to see what the "compromise" looks like.  I think this is likely to yield 
results quicker than modelling.  The other crucial question of course is 
cross-coupling between the two antennas.  If this is too high, then the combo 
is unlikely to work well, but if it is reasonable (say -15dB or better) then 
the idea is probably worth persuing.  I am sure that some extra filtering will 
be needed in both the  feed to the S-Band LNA/LNC and in the L-Band TX line (I 
expect there will be some wideband noise coming out of the TX which would 
otherwise raise the S-Band noise floor).  The filtering may be accomplished by 
notches, LPFs or HPFs or maybe bandpass.  All part of the fun!

BTW, if the antenna works, I have a proposal for the name - the DNA (Dualband 
New Antenna.

I'll post any results to my website at <www.g3wdg.free-online.co.uk>.

73  Charlie G3WDG

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