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

[Michael Tope]

For fairly long cable rolls (> 1/2 WL at the highest frequency where the 
analyzer provides accurate impedance data), it's pretty straightforward, 
Don:

    1) Connect your analyzer to one end of the cable roll. You can leave
    the far end either shorted or opened.

    2) Look for the frequencies where the input reactance of the cable
    roll has a zero crossing (or at least a deep minimum) and the input
    resistance is either very low (near effective short) or very high
    (near effective open).

    3) Take the difference in frequency between two adjacent frequencies
    where the input impedance of the cable roll looks like a short
    (reactance and resistance minimum) or two adjacent frequencies where
    the input impedance of the cable roll looks like an open (reactance
    minimum and resistance maximum).

    4) The approximate electrical length of the cable roll in feet is:

        Le = 492/delta_F,    where delta_F is in Megahertz.

    5) The approximate physical length of the cable is just the
    electrical length multiplied by the velocity factor of the cable 
    (i.e. Lp = Le * vf)


I had to do this recently when buying a large spool of cable. The seller 
indicated that it was 2835 ft. I measured ~140 KHz between low Z 
readings (reactance and resistance minimum) on my MFJ-269.

    Le = 492/0.14 = 3514 ft


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.

Hope this helps.

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

On 1/12/2020 10:31 AM, Don wrote:
> I'd appreciate your sharing that with those of us casual users with 
> antenna analyzers but who profess to be novices in their use. Have 
> used it for some of the applications but still a learner. Mine happens 
> to be a 269C and I find the instructions to be somewhat lacking.
>
> Don W7WLL
>
> On 1/12/2020 10:24 AM, David Gilbert wrote:
>>
>> If you have an antenna analyzer that can measure complex impedance 
>> (as in R +/- jX) and if you are willing to put a connector on one end 
>> of the cable, I can tell you how to get an accurate length 
>> measurement using TLW, the transmission line application that comes 
>> free with the ARRL Antenna Book.  You don't need an instrument with 
>> TDR capability (Time Delay Reflectometer) to do it.
>>
>> 73,
>> Dave   AB7E
>>
>>
>>
>> On 1/12/2020 3:44 AM, Phil wrote:
>>> I am involved in selling items to benefit the estate of a SK.
>>> One of the items is a unused roll of 400MAX cable that is still
>>> in the plastic cling wrap it was shipped in.  I know there is
>>> at least 100 feet of cable there but I think there is possibly
>>> 50 or even 100 feet more.  I don't want to unroll the cable to
>>> measure it and get it all kinked or dirty.  The roll weighs
>>> 17 pounds.  I've checked the DXE web site but can't find anything
>>> that gives the weight per foot.  The roll is approximately 8 inches
>>> inside diameter, 16 inches outside diameter, and contains 55 to 60
>>> turns.  The packing label on the roll says DXE400MAXDCU200.
>>> Can anyone help me with a good estimate of the length?
>>>
>>> thanks, 73, Phil   W5BVB
>>
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