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Re: Positioning a Helix feed

The signal source needs to be 100 wavelengths or more from the antenna and
the antenna range needs to be clear of objects that may cause reflections
(including the ground) to accurately position the feed. Its easier to use
the sun as the signal generator. The sun should be more than 20 degrees
above the horizon and will provide 1 or 2 dB more noise than the surrounding
sky if it is unobstructed. Use the noise level with the antenna pointed
stright up as a reference and aim at the sun by centering the shadow of the
feed on the center of the dish to make measurements. You'll need an AC
voltmeter to measure receiver output.

You'll have to repeat the process for each measurement as the antanna gain
will vary but maximizing the difference between sun and sky will maximize
the sensitivity of the system. Adjust the feed position in 1/2" increments
as you take measurements and pick the point with the highest sun noise.

This works because the atmosphere is transparent at 2400 MHz so the antenna
sees a temperature of about 3 K. The sun is 100-200 K.



----- Original Message -----
From: "Greg D." <ko6th_greg@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, 12 June 2001 06:27 UTC
Subject: [amsat-bb] Positioning a Helix feed

> Hello all,
> I'm trying to position a new helical feed (with the coffee
> "cup"), and tried using a diode noise generator attached to
> my HT.  It worked, but there was no repeatability to the
> positioning.  Very small movements of the HT (held by my
> daughter), dish, etc. caused very big changes in my S-meter.
> How should I position the Helix?  My thoughts:
> 1) go with the best, but imprecise, position using the
> noise generator.  Use a vice if needed to keep things
> steady.
> 2) Position by measurement.  I saw a reference on this
> BB that the typical MMDS dipole feed was at 12" from
> the grill.  If correct, position the end of the first
> turn at 12" (assuming the Phase Focus discussion
> concluded that correctly).  Note that this isn't where
> the first process put it (a few inches different)
> 3) Cover the dish temporarily with tin foil, and shine
> a light on it (or use the sun).  Position helix at
> maximum brightness.  Can I assume the focus for light
> and 2.4 ghz will be in the same place?
> What do you think?
> Thanks,
> Greg  KO6TH
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