[Antennas] coax 'sweet lenght'

[Bob Nielsen]

On Sun, Oct 10, 2004 at 11:15:04PM -0400, Robert Lay wrote:
> Dear Bob Nielsen,
> 
> > On Sun, Oct 10, 2004 at 06:26:46PM -0700, Dan Richardson wrote:
> > > At 04:38 PM 10/10/2004, Ron wrote:
> > > >  If you want to measure the match between an antenna and it's feed
> line,
> > > >measure it at the antenna's feed point, to measure it 1/2 wl
> (electrical)
> > > >(or multiples of) away from it is the next best place to measure it.
> > >
> > > That is correct, however, the impedance magnitude is the same at
> > > ½-wavelength (or odd multiples thereof) but, if there is a reactive
> > > component, the sign will be opposite. For it to exactly be the same
> > > impedance with the correct sign (angle) the feed line must be
> > > one-wavelength or multiples thereof.
> >
> > No, it will be the same for 1/2-wavelength or multiples.  The round trip
> > path (transmitter-antenna-transmitter) is one-wavelength, so what you
> > measure (excluding the effect of loss) is the same as if the
> > transmission line had zero length.  The magnitude (absolute value of
> > impedance) is the same for any length of transmission line.
> 
> You were doing all right until your last sentence. The magnitude of the
> impedance seen looking into a transmission line is NOT the same for any
> length of transmission line. For example, take a  transmission line
> terminated in Za (where Za > Zo) and vary the length of line over the range
> from 0 length to one half wavelength. You will find that the magnitude of
> the impedance seen looking into the line will vary from a value of Za  to Zo
> and then to Za/2, then back to Zo and then to Za (at lengths of 1/8
> wavelength, 1/4 wavelength, 3/8 wavelength, 1/2 wavelength), etc.
> 
> If you start with the load, Za, being smaller than Zo, then the results are
> the same except at the 1/4 wavelength point the magnitude of the impedance
> seen looking into the line will be twice Za.
> 
> For the case of the lossless and flat line (Za = Zo) the magnitude of the
> impedance seen looking into the line is independant of the line length.
> 
> And yes, Danny was wrong.

Thanks for the correction, Bob.  What I MEANT to say was the absolute
value of the VSWR (not the transformed impedance) was constant. 

73, Bob N7XY