[TowerTalk] Re: [Antennas] coax 'sweet lenght'

[Bob Nielsen]

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. 

73,
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: antennas-bounces at mailman.qth.net 
> > [mailto:antennas-bounces at mailman.qth.net] 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.
> 
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-- 
Bob Nielsen, N7XY                          n7xy (at) n7xy.net
Bainbridge Island, WA                      http://www.n7xy.net