Looking for the best way to check actual velocity factors, continuity and coax conditions I’ve turned to Time Domain Reflectometry. With the advise of Steve Uhrig, WA3SWS (SK) I narrowed my search down to one of the RiserBond “Yellow Box” TDRs and was able to purchase a surplused unit from an Arizona Contractor who was upgrading to fully computerized versions.
A TDR is sort of like a Sonar Unit for wire. There are several great on-line write-ups so I’ll simply link to the The Free Dictionary Wiki for your reference.
In use a TDR is simply electrically hooked to the cable, terminated or unterminated, set-up with velocity factors & cable type if known, configured for a couple other adjustements, and then the user interprets the screen.
Actual velocity factor can be worked out by testing from each end using a set procedure.
Interpreting the Screen seems to be the biggest trick. An excellent on-line teaching reference
can was found at the RiserBond on-line Training website. PDF Versions of some are now at: http://www.radiodetection.com/search.aspx?search=tdr
In use you can see connectors/splices, faults, cable changes and even what is electrically going on at the antenna.
At the W9EVT Washington Island QTH we’ve used the TDR to check dozens of cables burried in conduits.
For K9EME’s (was KB9EME) REACT efforts lots of spools of coax and hardline were checked & measured using the TDR. In theory checking spooled cable could require a correction factor, but we found the difference between coiled and uncoiled length readings to be small enough (inch or two over 100-200 ft) to ignore.
At field day with W9DK Mancorad the TDR was used in tandom with antenna analysers to check out and debug the field day antenna arrays.
Not limited to coax, the TDR can be used to check for problems with paired wiring as well.
A TDR is an occassional use item that is expensive enough that perhaps a club or group might chose to buy one rather an individual amateur. I’m told there are some Amateurs who rent out their TDRs to local contractors and other Amateurs, which would partially recover the investment over time.
Much like Alice’s Looking Glass, looking at cable performance and the inspection of inaccessably wires & coax with a TDR is simply magic!