[TowerTalk] Measuring Coax Length

[Roger (K8RI) on TT]

I used self fusing scotch tape plus a double layer of 88 tape. All of 
the tape for each connector landed on the ground in one piece.  they 
looked like pieces of expanded metal. Very neat removal, except it also 
took all of the silver plating, leaving the connectors looking like sand 
blasted brass.  There was no sign of metal on the tape.

As for method, if you have something that will give you a relatively 
accurate measurement of "something" per unit length...go for it.

I don't have any hand held instruments that have an accuracy that I'd 
trust.  They might or might not work.  I do have a rather expensive 
digital VOM that would measure C00. My old Simpson is usually a bit 
closer.  I hadn't used my 250 in years, so gave it to my son who uses it 
regularly.  Yup. Two weeks later I needed it.  I purchased a brand new 
250 on e-bay at a very reasonable price.  I was afraid of batteries 
being left in, or switched shot.  I did have to clean and lube the 
switch, but it appeared to have never had a battery installed. Got lucky.

   As a calibration tech (one of my many jobs in industry)  The 
instruments were all checked against standards before their weakly use.  
The measurements might be quite temperature sensitive.  I haven't tried, 
but will if my memory lasts long enough.

   On 10/29/2014 12:12 AM, Mike Fahmie via TowerTalk wrote:
> 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)