[TowerTalk] Need help estimating cable length of DXE 400 MAX

[Michael Tope]

Jim,

Thanks for this info. Good stuff.

FWIW, the lowest frequency I used was about 2 MHz. The 140 KHz was the 
frequency difference (not the absolute frequency) between adjacent 
points where the impedance came back around to the same zero reactance 
crossing point on the Smith chart. According to the graph in your 
paper,  the difference between the Vf at 2 MHz and the asymptote would 
be about 2%. That was plenty close for my purposes (I just wanted to 
make sure the length of the subject reel was close to what was advertised).

73, Mike W4EF..............

On 1/12/2020 12:54 PM, Jim Brown wrote:
> On 1/12/2020 11:07 AM, Michael Tope wrote:
>> The published velocity factor for this type of cable is 0.81 so,
>>
>>     Lp = 3514 ft * 0.81 = 2847 ft
>>
>>
>> This is very close to what the guy was advertising, so I knew I 
>> wasn't being short changed.
>
> BUT -- VF for ALL transmission lines varies with frequency, and is 
> typically about 5 percent slower in the range where you measured. The 
> published value is that to which it converges at VHF, and is what most 
> TDR instruments measure. The difference will make the line "measure" 
> that percentage longer than it really is using the technique of 
> looking for the first quarter-wave null.
>
> See page 8 of http://k9yc.com/Coax-Stubs.pdf for an example of this 
> measurement made by finding nulls at harmonic frequencies and plotting 
> VF from those data. This can be done with instruments as simple as an 
> MFJ-259 -- you turn the dial to find the null(s), then write down the 
> frequency(ies) and compute VF.
>
> 73, Jim K9YC
>
>
>
>
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