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Re: To helix or Not to helix

Sorry to hear your disenchantment Pieter-
Let's see...King and Wong got 16.8dBic out of a 22T helix (with "cup")
The DEMI/Directive Systems 21ele looper is rated at 17.5dBi
With the CP mismatch the yagi "should" be 2dB+ less effective.

Why the difference?
	0. Very rarely is 1 S-unit=6dB...to get a quanitative measure you'll need
a calibrated receiver w/o AGC
	1. Did you use a cup on the helix?  Your pix I've seen before don't have a
cup.  That's allegedly worth 2dB right there.
	2. Impedance mismatch?  I feel compelled to stress again that narrowband
LNAs are not very tolerant of input mismatch.  You can easily burn several
tenths of a dB NF in the 2.0:1 
VSWR circle, depending on the exact device/design used.  And phase matters
too...two antennas that are both 1.5:1 are not *necessarily* guaranteed to
behave identically with a given LNA.

For the UHF/microwave antenna homebrewer, it would be *really* nice if
LNA/downconverter manufacturers could supply a Smith chart plot of NF and
available gain vs. input impedance.  Of course that requires the homebrewer
to have vector impedance data which I imagine is very rare.

At 02:26 PM 5/30/01 -0400, Pieter Ibelings wrote:
>Hi Guys,
>I performed some antenna comparisons yesterday between 3 different helix
>antennas and a loop yagi. My results show that the loop yagis have a LOT
>more gain for a given length than a helix antenna. The longest helix that I
>have is about 23 turns and is about 6 inches shorter than the 21 turn
>looper. The loop yagi was consistently about 1 S unit (6dB) better. The
>linear to circular loss appeared to be inexistent.
>I was going to stack four helix antennas and now I have decided I am going
>to stack four loop yagis. I think I now need to get rid of all my 7 helix
>antennas for a good price.
>Here is the picture of the Loop Yagi sitting on a small vise on top of my
>Pieter N4IP
>----- Original Message -----
>From: "C. Martins" <martins@vega.net.dhis.org>
>To: <amsat-bb@AMSAT.Org>
>Sent: Wednesday, May 30, 2001 1:40 PM
>Subject: [amsat-bb] Helix lenght limit

Scott Townley		
Gilbert, AZ  DM43
HF/VHF/UHF/Satellite MOBILE station

"When the Going gets Weird, the Weird turn Pro"
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