I'm on a quest to find the proper termination for my Beverage on Ground antenna (BOG) My MFJ-259 is just not accurate up near 500 ohms. I see two different styles of TDR a pulse and a square wave. The square wave is neat in that you can watch the square wave get square as you adjust the termination and the pulse about the same you watch the reflection disappear when you get the proper termination. Is there any other reason to pick one over the other? Also, Is the output impedance important? I see many set to 50 ohms, using four drivers with series 220 ohm resistors. Then I see other circuits that add additional series resistance for 75 ohm, 100ohm, and 120 ohm, etc. So if my antenna is actually 250 ohms, do I need to raise the output impedance of the pulse driver? Some use a 74AC14, which would be fine, but if I build it, is there a fast edge chip that will run at 4.5 volts or even 3 volts. I understand the layout is important to the fast edges. Thanks, Mikek Here's a square wave generator, Here's a pulse generator,

Either is fine. Just don't eat the whle can of Altoids in one session. If your pulse driver doesn't match the cable impedance you'll get reflections there as well. If both ends are mismatched it'll run back and forth. Your cable is AFAIU many tens of feet long so it doesn't have to be super fast logic. 74HC would be ok, whatever you have. Those should work fine but adapt the resistors to your cable impedance. -- Regards, Joerg

TDR pulse or squarewave ?

Jun 14, 2018 41 Replies

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Jeff Liebermann 8 years ago

What can go wrong? Inside is probably a cadmium sulfide photoresistor. If it's like these, the conductive path follows a serpentine pattern, which kinda looks like an inductor. The inductance is probably fairly small (I couldn't find a number). At

1MHz, the inductance probably has little effect, especially with the padding. However, if things act strangely in your testing, you might want to measure the inductance with a common LRC meter.

Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558

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amdx 8 years ago

I had good success when I used the Vactrols on my Flag antenna, and many have used them for variable terminations, I don't suspect much inductance. I didn't have a good night for finding any front to back. My antennas acted about the same. Sometimes atmospherics will do that, I'll try another night or two before I revert back to a fixed resistor. I went to use the Vactrol ass'y I took off my Flag antenna during testing, I found the 300ohm series resistor was measuring 10Mohm and the Vactrol resistance section was open, I suspect lightning may have damaged it. I installed a new Vactrol and resistors. I have one Vactrol on the shelf, I can measure it when I get my Rigexpert. I have an LCR meter, but the highest frequency it goes to is

100kHz. Mikek Mikek

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