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Re: Adding Preamp to 2.4 Gig



The loss in a good pair of N-connectors with Teflon insulation should be
less than 0.1 dB and contribute less than 7 K of thermal noise. When a low
noise preamp is being used, there are probably other sources of noise that
have a larger influence and should be eliminated first.

A small helix has sidelobes that can pick up ground noise. -10 dB sidelobes
can generate up to 29 K of noise while -15 dB sidelobes lower this to 9 K.
Kraus indicates that sidelobes can be minimized by mounting a conical shield
to the ground plane with a diameter (in wavelengths) of sqrt(2.5 * number of
turns * turns spacing) at the open end.

Unless the dish is offset fed, it may also be useful to increase the number
of turns in the helix to under-illuminate the dish and lower the amount of
noise that radiates from behind the dish directly into the feed. This lowers
the gain, but some combination will maximize the signal to noise ratio.

73,

John
KD6OZH

----- Original Message -----
From: "Cliff Buttschardt" <cbuttsch@slonet.org>
To: "Jesse Morris" <w4mvb@juno.com>
Cc: <n5gpc@earthlink.net>; <amsat-bb@AMSAT.Org>
Sent: Monday, 21 May 2001 21:27 UTC
Subject: Re: [amsat-bb] Adding Preamp to 2.4 Gig


> Hi all.  There have been many reports of superior LHCP helix
> feeds for off center dishes that I wonder if it would be
> useful to mount a pre-amp, (DEM kit?) directly on the rear
> of the helix reflector WITHOUT the use of any connector?  The
> reflector usually is about 5 inches square which is plenty of
> room for a pre-amp.  Ultimately, a single PC board with the
> pre
> amp on one side and the helix termination on the other would
> be
> compact and light.  Difficulty would be finding a match  for
> minimum noise figure as well as weather proofing.   Cliff K7RR


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