I found this URL (among others) with a Yahoo search for Smith Chart:
http://www.web-ee.com/primers/files/SmithCharts/smith_charts.htm . Look at
Fig. 8 to see an example of a constant VSWR circle of 3:1. 2:1 would be a
circle of smaller diameter and 1:1 would be the point at the origin. The
diameter stays constant if the coax were lossless. If the coax had loss,
the circle would spiral in toward the center as you went clockwise around
the chart several times, each full revolution representing a half WL at the
selected frequency. (So if your coax were extremely long, you'd have
circled the Chart a large number of times, thereby guaranteeing you'd spiral
in close to the center, 1:1 point. This shows how a long coax can fool the
transmitter into thinking the antenna load is nearly perfectly matched.)
BTW - here is the parent URL of this site: http://sss-mag.com/smith.html
. Lots of good info on Smith Chart applications, if you're into that sort
Gene Smar AD3F
----- Original Message -----
From: "Bob Nielsen" <email@example.com>
To: <firstname.lastname@example.org>; "'TowerTalk List'"
Sent: Sunday, October 10, 2004 1:12 PM
Subject: [TowerTalk] Re: [Antennas] coax 'sweet lenght'
> WRONG! Ignoring cable losses, the VSWR is independent the length of the
> cable. What changes with length are the complex components of the
> impedance, while the absolute value (VSWR) remains constant.
> If you plot the impedance on a Smith chart, you get a circle whose
> origin is at the center of the chart where the impedance vector rotates
> as the cable length changes, but the length of the vector is constant
> while the resistive and reactive components of the impedance vary.
> However, because a matching circuit may not be able to compensate for
> all possible resistance and reactance values for a given VSWR
> (particularly in the case of a very high VSWR, such as an end-fed
> half-wavelength antenna), a different cable length may transform the
> complex components of the impedance, as seen at the transmitter, to a
> value which can be more easily matched.
> Bob Nielsen, N7XY
> On Sun, Oct 10, 2004 at 11:03:34AM -0500, Chris Boone wrote:
> > ANY feedline repeats the Load Z (Impedance) every
> > 1/2wavelength..remember that must take into account the velocity factor
> > of the feedline...for normal coax its about 66% so 0.66 x 468/freq in
> > MHZ = length of 1/2 wl coax at that freq...the only thing about this is
> > you can use ANY feedline bewteen radio and antenna as long as its 1/2 wl
> > and the radio will match the antenna (if the antenna is 50ohm +0j
> > already)....SWR at the radio will be accurate.....if the antenna is NOT
> > 50ohms or has reactance, the feedline will always cause a WRONG SWR
> > reading at the xmtr unless it is a 1/2 wl (in coax)..
> > Hope that helps.
> > Chris
> > WB5ITT
> > > -----Original Message-----
> > > From: email@example.com
> > > [mailto:firstname.lastname@example.org] On Behalf Of FireBrick
> > > Sent: Sunday, October 10, 2004 8:49 AM
> > > To: TowerTalk List; Antenna List
> > > Subject: [Antennas] coax 'sweet lenght'
> > >
> > >
> > > I remember discussion that there was a 'sweet length' for
> > > rg8, rg13 coax lengths. The length was a multiple that gave
> > > the best match, etc.
> > > Or is this the Ham version of 'urban legend'???
> > >
> > > But senility won't allow me to remember what it was or where
> > > I stored this info.
> > ______________________________________________________________
> > Antennas mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/antennas
> > Help: http://mailman.qth.net/mmfaq.html
> > Post: mailto:Antennas@mailman.qth.net
> Bob Nielsen, N7XY n7xy (at) n7xy.net
> Bainbridge Island, WA http://www.n7xy.net
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
> TowerTalk mailing list
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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