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Re: Helix feed details



I always make a shim to go between the N-connector and the mounting surface.

73,

John
KD6OZH

----- Original Message ----- 
From: "Greg D." <ko6th_greg@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, 14 June 2001 20:16 UTC
Subject: [amsat-bb] Helix feed details


> Hi folks,
> 
> Ok, the pictures of the first turn's worth of a Helix feed, the words
> that I read here, and the parts on my bench aren't quite matching
> up...
> 
> The N connectors that I can find locally have a considerable "bump"
> on their back side, a little smaller in diameter than the front-
> side barrel, and about 1+ mm high.  They are definitely not flat.
> 
> If I mount the N connector on a the end of my coffee can so that the
> surface of the back of the connector is flush against the can, then
> the bump sticks into the can far enough that the helix feed wire ends
> up much farther away from the can than it's supposed to be.  I put it
> all together this way, even if it's wrong, and I was barely able to
> hear the beacon at apogee last weekend.  But, I expected better.  It
> seems to perform about as well as the original dipole feed on the
> original grill (which is now covered with a metal screen).
> 
> Should I bend the helix wire so that it follows the contour of
> the bump down towards the can, and then angle it up slowly over
> the first 1/2 turn?  Or, could I do something with the foil strip
> that is soldered to the first 1/4 turn to compensate for the larger
> distance?  (Easier, since the wire's rather stiff...)  Does this
> even matter in practice?
> 
> The preamp is attached directly to the back of the can with a short
> N-N adapter, then directly to the Drake.  It's not a great preamp,
> (1+db noise, 10db gain), but it seems to help.  I want to get a
> good baseline for what this setup can do before ordering a DEM unit.
> 
> Thanks,
> 
> Greg.
> 
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