[TowerTalk] Measuring Coax Length

[Roger (K8RI) on TT]

On 10/28/2014 8:05 PM, Jim Brown wrote:
> On 10/28/2014 2:39 PM, Jim Lux wrote:
>> For TDR (or looking for integer half wavelengths as you sweep the 
>> frequency on an analyzer) you have to assume a particular propagation 
>> factor,
>

Not only that, there are often small variations in a single length. 
There are likely small variations in 4 lengths that long cut from a 
single spool.  So they may be different electrical lengths.

With a good TDR you can measure the characteristics and find the 
electrical lengths at a given frequency.

73

Roger (K8RI)

> Mike,
>
> Measuring the length of a long piece of transmission line is tricky 
> for several reasons, most important because Vf (the velocity factor) 
> varies a bit with frequency.  Study k9yc.com/Coax-Stubs.pdf
>
> An easy way to get close to the actual length is to put a connector on 
> one end, leave the other end open, and measure the impedance with an 
> antenna analyzer. At every frequency where it's some odd multiple of 
> quarter waves long, it will look like a short. So sweep it with 
> whatever analyzer you own, write down those frequencies, and do the 
> arithmetic.
>
> 73, Jim K9YC
>
>
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-- 

73

Roger (K8RI)