[TowerTalk] coax 'sweet lenght'

[Don Havlicek]

corneliuspaul at gmx.net wrote:
> 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

shouldn't that be  4 * 1/2 wavelength on 15m ???

Don N8DE

> - 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
> 
> 
> 
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