[TowerTalk] coax 'sweet lenght'

[corneliuspaul at gmx.net]

At 03:12 13.10.2004, you wrote:
>On the other hand, if the load is not matched to the transmission line, 
>which happens in the real world a lot, a transmission line that is a 
>multiple of 1/2 wavelength will at least make sure that the antenna's 
>feedpoint impedance is not transformed by the transmission line, and that 
>the apparent resonance does not change.  This may make no difference (it 
>doesn't here) but may help to explain the idea of the "sweet spot."
>
>73, Pete N4ZR

Yep - and it is the reason why I like to use a 3/2 wavelength coax cable
for 14 MHz when doing tribander yagi (20/15/10) measurements in the field.

such a cable is

- 3 * 1/2 wavelength on 20m
- 2 * 1/2 wavelength on 15m
- 6 * 1/2 wavelength on 10m

so on all 3 bands it does not introduce any unneccesary impedance 
transformation,
but reflects the feedpoint impedance to the end of the cable. (losses 
neglected!)

When using RG213 coax with a VF factor of 0.66 this cable is roughly 21.20m 
long.
(Can be trimmed to exact length by the method of your choice)
21m is long enough for most portable measurements, yet short enough not to
introduce big losses.


73 Con DF4SA