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Re: Orbit 250 apogee



Yes, that is an advantage of an offset fed dish. Most feeds are designed so
that power is 10 dB down at the dish edges so eficiency is typically 50%.
The feed will pick up any radiation from the ground that is visible off the
sides of the dish with a 10 dB loss compared to the main lobe. Increasing
the gain of the feed will lower the noise level but decrease gain of the
antenna as a whole because less of the surface of the dish is used
efficiently.

Look at www.qsl.net/n1bwt/preface for a reprint of some articles by W1GHZ
that published by the ARRL. W1GHZ has made comparisons of different types of
feeds.

73,

John
KD6OZH

----- Original Message -----
From: "Jens H. Jensen" <topcat@mail1.stofanet.dk>
To: "John Stephensen, KD6OZH" <kd6ozh@amsat.org>
Cc: <amsat-bb@amsat.org>
Sent: Friday, 18 May 2001 19:21 UTC
Subject: Re: [amsat-bb] Orbit 250 apogee


> At 03:04 18-05-2001, you wrote:
>
> --snip--
> >Amateurs have published designs for 0.4 dB NF (28 K) amplifiers in QEX
and
> >Microwave Update and antennas can easily be made with sidelobes that are
15
> >db down so that they only pick up 9 K of ground noise.
>
> Does this mean that using an offset-dish instead of a primefocus dish is
> actually an advantage, since the feed is usually NOT pointed at the
ground,
> but upwards, so that over-illumination will not "see" the ground?
>
> How big is the ground-noise contribution exactly?
>
> Is there anywhere I can read more about it?
>
> 73 de OZ1LRG
>

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