[TowerTalk] Measuring Coax Length

[Mike Fahmie via TowerTalk]

My post triggered an unexpected flurry of replies!  Yes, there are various ways to measure coax lengths, both electrical lengths and physical lengths.  The purpose of my post was to describe a way that a rough measurement of length could be done using the commonly available 'Capacitance' measuring capability of many DVM's (network analyzer not required!).  I was surprised at how accurate it was, but noted that I might have just been lucky.

9913 coax has greater susceptibility to propagation of water than solid or foam coax, no argument there.  My installation uses 9913F7 foam on the tower and 9913 on the run from the tower to the lightening panel 70' away.  Most of that run is sheltered from the weather and I'm using Type N connectors throughout the system and sealed with self fusing tape so I'm not too worried about water intrusion.
Solid/foam coax is not immune to water propagation. Back in my novice days (well back in the last century), I had a coax fed dipole which I constructed without any attempt to seal the antenna end of the RG-59.  I learned about capillary action when I found a small puddle at the radio end of that 80' run.  I doubt that this thin layer of water affected the attenuation much unless the polyethylene absorbed it, but in the long run it would probably corrode the shield wires.  The issue is different with 9913 since a large gap exists between the center conductor and the insulating tube surrounding it.  If water collects there, it's all over!
-Mike-WA6ZTY

From: Roger (K8RI) on TT <K8RI-on-TowerTalk at tm.net>
 To: towertalk at contesting.com 
 Sent: Tuesday, October 28, 2014 8:41 PM
 Subject: Re: [TowerTalk] Measuring Coax Length
   
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)




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