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RE: Need recommendations on 435 yagi feed



FWIW

In my experience too the 2 ohms won't matter. VSWR between 95 & 93 ohms
would be about 1.02:1.

What's much much more important in my experience of switching and connecting
phased antennas is to get the individual antenna feedpoints to have as pure
a resistive impedance as possible at their individual feeds with the correct
impedance, remembering that the two antenna planes will have some mutual
coupling... ie, tune up each antenna individually first, but with both
antennas in situ.

Whereas you might be able to tune in both antennas with a satisfactory match
in one polarization (or phasing), when you switch the polarization it will
likely go badly wrong if you had some reactive components which were
previously simply cancelling each other out.

This is about the only time I've found that having a really low VSWR at each
feedpoint is a positive benefit.

I placed quite an interesting but little documented discussion on this
subject at http://www.g6lvb.com/images/stacking.gif which was taken from the
RSGB VHF/UHF Manual 4th Edition.

BTW, in the UK you can get RG-62 from your local Maplin store.

Howard G6LVB

-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Gary "Joe" Mayfield
Sent: 14 September 2001 12:28
To: Floyd Sense; amsat-bb@AMSAT.Org; Douglas Braun & Nadia
Papakonstantinou
Subject: Re: [amsat-bb] Need recommendations on 435 yagi feed


The two Ohms won't matter.  If you want to spend more money you can go to

http://www.cablexperts.com

Click on 75 Ohm cable.  Near the bottom of the page they have RG180/U which
is 95  Ohm.

73,
Joe
ka0yos@amsat.org

----- Original Message -----
From: "Floyd Sense" <sentek@sprintmail.com>
To: <amsat-bb@AMSAT.Org>; "Douglas Braun & Nadia Papakonstantinou"
<doug@dougbraun.com>
Sent: Thursday, September 13, 2001 6:48 PM
Subject: Re: [amsat-bb] Need recommendations on 435 yagi feed


> Close, but no cigar!  RG-62 is indeed quite common, but it's 93 ohms not
95
> as is RG-133.  I'm certainly open to anyone who can speak with authority
to
> tell me that the 2 ohm difference won't matter.  I'm not capable of making
> that judgement myself.  By the way, the RG-62 pre-dates LANs by a decade
or
> so.  It was used in 3270 display systems as the signal carrying cable
> connecting the keyboard/display terminals to the control unit.  If done
> properly, you could run the cable up to 5000 feet, but I think the
> recommended limit was 2000 feet or so.  The frequency used was rather
low -
> less than 5 MHz as I remember.
>
> K8AC
> Floyd Sense - Angier, NC
> ----- Original Message -----
> From: "Douglas Braun & Nadia Papakonstantinou" <doug@dougbraun.com>
> To: <amsat-bb@AMSAT.Org>
> Sent: Thursday, September 13, 2001 17:33 PM
> Subject: Re: [amsat-bb] Need recommendations on 435 yagi feed
>
>
> > In the USA at lest, all the ham cable/connector vendors seem to
> > carry it.  And it is pretty cheap.  It was originally meant for
> > some sort of IBM LAN technology (token-ring?), so lots of it was sold.
> >
> > Doug
> > NA1DB
> >
> > At 03:59 PM 9/13/01 , John Wright wrote:
> >
> > >The 95 ohm coax is very hard to get. BUT if you take the appropriate
> length of 50 Helix cable ( like Pope H100 ), pull out the center conductor
> and replace it with a thinner center conductor, Not sure how thin, but
that
> can be calculated, you can make yourself a piece of what should really be
> 100 ohm cable. N 'T' pieces are about £10 stirling in the UK ( about
$15 ),
> so you should be able to get them for less than the price you quoted. I
used
> this system many moons ago in AO13 days. so it works. Incidentally the 1"
> offset of the feed point makes very little difference to the phasing.
> >
> > ----
> > 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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