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Re: To helix or Not to helix



What Doug proposed would minimize losses and would not be that expensive to
implement. If you do build the feed separately you have to adjust it to
provide the proper impedance at the input to the LNA. This requires a signal
generator and a directional coupler.  Either method requires test equipment.

73,

John
KD6OZH

----- Original Message -----
From: "i8cvs" <domenico.i8cvs@tin.it>
To: "John Stephensen, KD6OZH" <kd6ozh@AMSAT.Org>; "AMSAT-BB"
<amsat-bb@AMSAT.Org>
Sent: Thursday, 31 May 2001 23:43 UTC
Subject: R: [amsat-bb] 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.
> > > >
> > > > ----
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> > > > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> > > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > ----
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> >
> > ----
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>
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