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R: To helix or Not to helix



I aegree
It is possible to connect an amplified noise source to an antenna located
many wavelengths from the feed and adjust the feed/preamplifier
combination for best NF but things became more complicaded than
they already are.

1)You need a noise head amplifier

2)You need to run a cable between the noise amplifier and the antenna
    radiating noise many wavelenght from the feed/preamplifier

3)Only few amateurs are in condition to permorm an adjustement like that

 4)I cannot adjust my system without the above 1-2-3 points

  5) why complicate the life ?

   73 de i8CVS Domenico

----- Original Message -----
From: John Stephensen, KD6OZH <kd6ozh@gte.net>
To: i8cvs <domenico.i8cvs@tin.it>; AMSAT-BB <amsat-bb@AMSAT.Org>
Sent: Thursday, May 31, 2001 10:11 PM
Subject: Re: [amsat-bb] 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
> > >
> >
> >
> >
> >
> >
> >
> >
> > ----
> > 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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