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2.4ghz Feed design for minimum 70cm pickup

Hi folks,

So, the new Kuhne 2.4ghz preamp is on order.  SSB now carries the
MKU 232A2 TM ("Tower Mount" = weather enclosure).  Should be here
any day now.

I am concerned that my brand new $180 preamp will be sitting not
very far from about 100watts of 70cm uplink power, and we have seen
postings here about the maximum 1 mw input tolerance by these preamps.
The Confier preamp that is currently in service seems to not be
bothered by the proximity to the uplink antenna, but I'd rather not
take chances with the new one.

So, I want to redesign the 2.4 ghz feed to keep 70cm pickup to a
minimum.  The current design is a 3 turn helix, in a 4" x 2" can.
The helix is connected directly to the coax center conductor, after
the typical 1/4 turn brass shim matching section.

I have also seen a matching section design where the helix is attached
part way along a short strap (1/4 wave long?) that goes between the
coax center pin and a grounded post attached to the can.  At 70cm, I
expect this strap would look like ground, keeping the preamp safe.

Does anyone have any experience with this sort of matching section
construction?  How critical are the dimensions (I expect they are),
and how well does the resulting feed perform?  Am I right about this
design offering a measure of RF protection?  Will it be enough?


Greg  KO6TH

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