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Re: To helix or Not to helix



You could connect an amplified noise source to an antenna located many
wavelengths from the feed and adjust the feed/preamplifier combination for
best NF.

73,

John
KD6OZH

----- Original Message -----
From: "i8cvs" <domenico.i8cvs@tin.it>
To: "Scott Townley" <nx7u@arrl.net>; "AMSAT-BB" <amsat-bb@AMSAT.Org>;
"Douglas Braun & Nadia Papakonstantinou" <doug@dougbraun.com>
Sent: Thursday, 31 May 2001 15:55 UTC
Subject: R: [amsat-bb] 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.
> >
> > ----
> > 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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