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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