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

[Chris Boone]

Ok...what I should have said was "apparent" SWR changes according to the
metering.....
If the cable length changes and the antenna is NOT a resistive only
component, the coax will cause an apparent change in the reactance, thus
making the SWR meter change...

Any argument with that??

Yes, TRUE SWR itself does not change....but the metering is not perfect
:)

> -----Original Message-----
> From: antennas-bounces at mailman.qth.net 
> [mailto:antennas-bounces at mailman.qth.net] On Behalf Of Bob Nielsen
> Sent: Sunday, October 10, 2004 12:12 PM
> To: antennas at mailman.qth.net; 'TowerTalk List'
> Subject: 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.