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

2003年7月01日 12:16:34 -0500

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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