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Old July 4th 08, 01:06 PM posted to rec.radio.amateur.antenna
Bob Bob Bob Bob is offline
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First recorded activity by RadioBanter: Nov 2006
Posts: 85
Default Question about Measuring and cutting coax length

Hi Andy

I think you'll will get into a large discussion over this!

I am no means an expert on this topic and everytime I need to be, I have
to relearn a few things. In my experience though the almost only time I
want to cut coax to length is if I am using it for Z transformation
between antenna and TX. Like if the antenna has a Z of 100r I will end
up using odd quarter wave multiples of 75r coax to "match" the system. I
also use it to cut stubs for notch/band pass filtering and tuning . For
one project I remember using a quarter wave length across the 5V
terminals of an old SMPS to reduce radiation on about 75MHz.

What I am getting at is if the TX source Z is 50r and the antenna load Z
is 50r any length of coax will do. The effect of tuning the coax length
will be most evident if the system is reactive at either end. How much
that difference it really makes I dont have experience in. I tend to shy
away from coax systems in this case anyway as high VSWR on the line
itself is a recipe for a large loss of system efficiency. (ie it has
become part of the system tuning and can be regarded as a lossy load)

I normally use resonance/dip oscillator to determine length. If you only
have a VSWR device you can figure the length by actually leaving the far
end open or short and actually going for a maximum rather than dip.

I apologize for not giving you a definitive answer.

Cheers Bob VK2YQA

Andy wrote:
I want to measure and cut some coax for 14.200 MHz. It is to be connected
to my Cushcraft MA5B