[Antennas] coax 'sweet lenght'

Sun Oct 10 22:12:32 EDT 2004

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.
73,
Bob N7XY