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Re: Feeding time....



The feed needs to be LHCP so there are only two types that I have actually
used -- a helix or dual-mode horn with a circularizer.

Volume 3 of the RSGB Microwave Handbook has information on the dual-mode
horn but it only works well for a F/D of 0.5-0.6. I built one of these for a
4-foot dish several years ago and it works well but takes time to build and
adjust. You need a signal source and two helical antennas to adjust the
circularizer.

A helix is ajustable and much easier to build. Kraus gives the following
formula for the number of turns required:

    N = 8400/(B^2 * S)

B is the -10 dB beamwidth and S is the turns spacing in wavelengths. Kraus
also indicates that a conical shield attached to the ground plane with a
diameter at the open end of sqrt() will reduce sidelobe levels. I added the
shield to a helical feed for a 0.3 F/D dish that I used for AO-13 mode S and
it did seem to reduce pickup of microwave oven noise. I noticed no
difference in antenna gain.

The W1GHZ Microwave Antenna Handbook (www.qsl.net/n1bwt/contents) has a
section on offset feeds so it should provide a way to calculate the required
beamwidth.

There are 2 types that I have not tried myself. A VE4MA style feed (circular
waveguide with choke plate) should work if a circularizer section is added.
A diagonal horn also supports circular polarization with a circularizer
section. Both appear to be about the same level of complexity as the
dual-mode horn.

73,

John
KD6ZOH

----- Original Message -----
From: "Jens H. Jensen" <topcat@hardware.dk>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, 17 May 2001 18:25 UTC
Subject: [amsat-bb] Feeding time....


> So, it's feeding time..
>
> I have just gotten a small dish intended for AO-40 S-band RX.
> I must decide the best way to feed it, and need your advice.
>
> Which feed is the most efficient?
>
> Which is the easiest to make?
>
> Which is the easiest to adjust for preper illumination?
>
> The dish is the offset-variety, between 35-40 cm diameter.
>
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>

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