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RE: No Stupid Questions, right? (focal point)



Years ago when I visited the hardware integration lab in Orlando.  Stan Wood
WA4NFY said from his observations, that a quarter wavelength in front of the
ground plane was the best place to put the focal point on a small helical
antenna.  This weekend I'm planning on doing some experimenting placing a
three turn helical antenna in front of a 32" dish with a f/D of 0.4.  Well
see what happens.


'73,

John
N2HMM

	----------
	From:  Edward R. Cole [SMTP:al7eb@ptialaska.net]
	Sent:  Thursday, May 10, 2001 10:49 AM
	To:  amsat-bb@AMSAT.Org
	Subject:  Re: [amsat-bb] No Stupid Questions, right? (focal point)

	Kevin and...

	Good questions and not sure I have a definitive answer.  I have
heard the
	"first-turn" position.  For a dipole with reflector the dipole is
most
	often quoted.  I think you will find that the dipole in a yagi is
also
	given, though not often used for a dish feed.  For horns the
position is
	just inside the opening or "throat", so does a helix resemble a
dipole or a
	horn..??  

	So best advise is to mount it so one can adjust its position
in-or-out
	while observing a reference signal [whatever that is]...well a
signal
	source is nice, or a neighbor's carrier, or even sun noise if the
dish is
	large enough.  Adjusting on the satellite signal is probably a
recipe for
	frustration due to varying signal.

	In the final analysis, my experience is that the feed will work even
when
	not at the optimum focus of the dish.  Get it close and mount it
securely
	aimed at the correct point on the dish [this varies a little whether
its a
	center-fed or offset fed dish].  Then..."kill the engineer"...and
have fun
	using it!  At some point its time to stop fooling with it and just
operate
	;-)  {giving myself advice, here}

	Ed


	>From: "Kevin J. Smith" <kjsmith@home.com>
	>I asked what part of a 3.25 turn helix should be at the focal point
should
	>be at the focus of a parabolic dish a few months ago.  It was
suggested that
	>the focus should be at the end of turn 1 of the helix.  But in the
next
	>sentence, it was suggested to experiment by moving the helix
through the
	>focus while listening to a signal generator.
	>
	>Kevin Smith
	>N3HKQ
	 

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