Purchasing used cable is a very iffy business because hams tend to
leave cable in place for two decades and more. It should be changed out
regularly, of course.
A suggestion I would offer is to make Herb's observation below, of
course, for irregularities but also to measure the length physically and
then relate that result to what appears in the t.d.r. reflectometry. The
reason is dielectric contaminated by old formulation jackets and by
moisture and even overt water ingress. The author was called out to
investigate the station of a chap who was in a wheel chair because of
spina bifida. Disconnecting his coaxial cable at his transceiver
produced dribbling water! Presumably the installation up the tower at
his Yagi-Uda had been done incompetently or something had come adrift up
there. Anyway, the local ham club was called and the fellows came out
and ran in new cable for him and this time installed it properly up his
tower at the antenna.
Especially with all the estate sales we see these days, we should be
very suspicious of used coax for sale. It may be antique and in
deplorable condition. Physical length and velocity factor determined
length must agree. Meggering the cable would be a fine idea, too. See if
you can can beg, borrow or steal a megger for the day of the flea
market. If the seller won't let you megger the cable, then you know all
that you have to know about that piece of cable!
The cable is a capacitor, short it before connecting your analyzer.
John
at radio station VE7AOV
+++++
On 2019-09-27 2:38 p.m., hwalker wrote:
qrp.ddc,
Thanks for taking the time to reply. Ham's such as myself are generally looking to purchase specific types of 50 ohm cable such as RG-58, RG-223, RG-6 etc. The cables are all marked by the manufacturer according to type and we pretty much know the velocity factors by heart, so entering that info into the nanoVNA is a no-brainer. I don't really use the nanoVNA's TDR function for purchasing specific lengths, but rather to ensure for instance that a 10 meter length of cable being offered for sell doesn't have damage at say 4 meters to its inner conductor. One cable I measured before purchasing had very strange peaks and valleys on the nanoVNA's TDR function (wish I could have saved the screenshot), when I examined it more closely I could see slight equally spaced pits on its outer insulation. I'm guessing the cable was used in a mismatched power transmitting application and the spacing of the insulation pit marks was associated with the transmit frequency. So I use the TDR function as more of a sanity check to help sort the chaff from the wheat.
Sorry, I mistakenly assumed you had access to a professional level TDR to not appreciate how, even with its limits, the TDR function on the nanoVNA is a godsend to hobbyists, students, and radio amateurs as essentially a throw-in to its vector network analyzer capabilities. Let's hope in nanoVNA V2 that Hugen increases the number of measurement points and gives you some of the other things on your wish list.
Herb
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