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R: To helix or Not to helix



I aegree with the idea of the single PC board but the problem  is to connect
 the head of a PANFI to  preamplifier input in order to adjust it for the
best NF because it requires two female N or SMA connectors fitted in the
elix reflector plus two male N or SMA connectors and a coax jumper with
 all associated losses at best of  0,5 dB added to preamplifier noise
figure.

73 de i8CVS Domenico


----- Original Message -----
From: Douglas Braun & Nadia Papakonstantinou <doug@dougbraun.com>
To: Scott Townley <nx7u@arrl.net>; <amsat-bb@AMSAT.Org>
Sent: Thursday, May 31, 2001 3:07 AM
Subject: Re: [amsat-bb] To helix or Not to helix


> I think a really cool idea would be a single PC board
> that has etched on it all of:
>
> The helix reflector.
> The helix matching network.
> The entire preamp circuit.
>
> Then you could be guaranteed that the antenna and amplifier
> are all well-matched to each other.
>
> Doug
> NA1DB
>
> At 05:28 PM 5/30/01 , you wrote:
> >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.
>
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