Time Domain Reflectometery at The Palm HQ ala W2AEW

Recently the feed-line to my 144 MHz antenna started to go bad – at first it just had elevated SWR but was still useable, but eventually the SWR rose to the point that the transmitter started to reduce power.  Just before the September ARRL VHF contest I pulled it down and replaced it with a temporary line for the contest.

In the meantime, I asked the folks on the Badger Contesters and Northern Lights Radio Society email reflectors how one should go about troubleshooting a bum feed-line. A number of helpful ideas were offered, but the one that most intrigued me was using a time domain reflectometer (TDR) to "ping" the line and find out where physically on the line the problem is located.  I assumed that such a device would be prohibitively expensive (and the commercial devices are!)  But then I found the YouTube videos by W2AEW documenting how to build one for just a few dollars (presuming one has access to a good, fast oscilloscope, which I do), so I thought I'd give it a try.

Here are all his videos (so far) on this specific topic:

#37: Use a scope to measure the length and impedance of coax

#87: Schmitt Trigger Oscillator / Tutorial / 74AC14 Inverter / squarewave generator

#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter

#90: Measure Capacitors and Inductors with an Oscilloscope and some basic parts

#203: Schmitt Trigger Oscillator revisited | TDR | Measure Capacitors and Inductors

Not as neat as W2AEW's, but it works.

I built a circuit just like his, except that mine has its own Li-ion battery for power.  My rise-time isn't as fast as Alan's, but it seems to be fast enough.  I created a little spreadsheet that lets me calculate the length of the coax based on the measured reflection time and the coax's velocity factor.  And then I gave it a try on a known-good piece of coax measuring 160 inches long, which I thought was RG58/U, with a velocity factor of 0.66. 

Running the pulses from my oscillator circuit in that and measuring as W2AEW described gave me just about 36 ns for the pulse's round trip.  Plugging that into my spreadsheet gave me a disappointingly inaccurate 141 inches – hmmm, that's pretty far off.  I thought maybe I wasn't measuring things correctly on the scope, but I placed the markers several times and each time came up with just about exactly 36 ns (I plugged 36.23 into my equation.)  Then, just on a hunch, I checked the marking on the coax and found out that, lo and behold, it's actually Belden 8219, not RG58/U.  8219 has a velocity factor of 0.73 and plugging that into the equation gave me 156 inches.  Add in the 3.5 inches for the BNC T and gender adapters and voila! I get 159.5 inches or within 1/2 inch of the measured length!  Now that is cool.

Next step is to measure the actual length of that bum feed-line, then ping it from both ends to see if it has an identifiable bad spot somewhere.  Stay tuned.....