[Antennas] Varying feedline length (was Mosley TA-33)

2003年7月01日 22:15:30 -0600

The impedances change but the SWR does not with the exception of a 
slight decrease in SWR due to feedline loss.
73
chris, n4vi
James Duffer wrote:
> It was written
>> "you cannot change the vswr of the antenna by changing the feedline
> length...what is happening most likely is you are bringing the feedline
> length close to a multiple of 1/2 wave length at the operating
> frequencey and therefore getting a mirror-image of the vswr at the 
> feedpoint
> showing at the meter in the shack...don't forget the correct place to 
> check the
> vswr is at the the antenna itself and not in the shack but sometimes 
> this is not
> physically possible so the best thing to do is get an exact image in the
> shack end by having the feedline in multiples of 1/2 wavelength to
> enable this...this is an electrical length not physical length length 
> and the
> formula to calculate this is by 468/F xVF where V is the Velocity Factor
> of the cable in use and F is the Frequency in use"
>> Very true. However, as was written by another, he was advised by a 
> manufacturer to change the lenght of his coaxial feed. Now why would 
> he tell him that? And what about the success he obtained when he DID 
> change the feed length? It is knows fact that a certain fractional 
> electrical wavelength lenght of coaxial cable can and does transform 
> impedances, a quarter wave can transform a short at one end to appear 
> as an "open" at the opposite, now what do you think appears at various 
> points in between?
>> My advice is to go ahead and try what the manufacturer recommended and 
> if it don't work try something else.
>> 73, Jim, wd4air
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