TDR Logic, mental lapse

Hi Mike, I'm not a tdr guy. But what was the impedance on the far end? (open circuit, short, terminated)

575 ohms sounds like a lot for the characteristic impedance.. But maybe OK... Do you have any other long wires you could test your ideas with... a long piece of twin lead?

George H.

An wire near a good ground plane will be a lot lower than 575 ohms, closer to 100 or so. A wire alone in free space, no ground plane, is

377 ohms.

What is your TDR grounded to?

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

George, I couldn't see your response in my newsreader, (my problem)

The end was open.

The wire is 260 ft. I split the wire in the middle and used an antenna analyzer to measure it as a dipole (on the ground) find the resonant frequency. 1.190MHz. This makes the Velocity factor 63%. If that is of any use.

Mikek

I need to just do this because, if I made notes I can't find them. Normally these antennas are terminated with about 250 ohms to get a flat resistance over frequency I did a 3 wire ground resistance test using the method on pg 24.

I started at 117 ohms, after some soil treatment I got it under 60 ohms.

That's a good question, I had 3 ground rods in a triangle on the final product, but for this initial testing, it may have just been 1 four ft rod.

Before anymore questions, I'm going to get my ground system installed.

Thanks, Mikek

Yes but can't one buy 300, 450 and 600 ohm open wire feeder line?

piglet

It's balanced, though, so by the image method it has twice the impedance of an unbalanced line with half the spacing to ground. IIRC in order to get an impedance higher than 377 ohms with mu = epsilon = 1 requires a slow-wave structure such as a helix or a metal waveguide.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Huh, is that 377 number right? (impedance of free space) Can I have an impedance greater than 377 ohms? Starring at a table on impedances it looks like it would be easy to make twin lead > 377. Z = 138 * ln (b/a) where a is radius and b wire separation

George H.

Hi piglet. Is open wire feeder line something like twin lead? Thx, GH

If each wire is 377 ohms free-space, the differential impedance is

754. They do see one another in a flat pair with finite spacing, and there is a bit of plastic, so that brings it down a tad. There is a zero-volt plane midway between a symmetrically-driven pair, which is the moral equivalent of each wire being d/2 above a ground plane. Times two for differential impedance.

Telephone pairs are considered to be 600 ohms, but that's a fudge for typical ohmic losses in long runs. They are closer to 100 ohms differential in real life.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Calculators like Appcad and Saturn can produce wire and microstrip impedances over 377. I assume they are just wrong.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

This is not a feed line it is the antenna (Beverage on the Ground) The Velocity Factor measures 63%

Mikek

That makes sense, thanks for the explanation!

piglet

Huh.. tricky image method ehh? :^) So for twin lead a max impedance of ~750 ohms?

George H.

Basically, with a little mod, width and holes for reduced capacitance.

450 ohm, but often varies from that when measured.

You can make 600 ohm or buy kits. Mikek

Yes, but for the higher Zo the plastic gets notched away (so it can be nicknamed ladder line) and the spacing gets wider.

piglet

Ya, it takes a wide departure to get far from 100 ohms on a twisted pair. Mikek

Yes it makes sense but I don't believe it will work as you describe. The TDR waveform seems normal: but without timing numbers and line lengths, I can't tell what you're looking at.

The problem is that you have only one wire and seem to be using the ground (earth) to provide the other half of the antenna or transmission line. The earth, when used as a conductor in this manner, is VERY lossy. There will be enough attenuation in the return earth path that you shouldn't see a reflected waveform. Check the timing on the reflections to make sure you're actually seeing a reflection, and not the next pulse.

Do you have a buried wire ground system under the antenna/transmission wire? If so, there might be enough capacitance between the buried wires and the wire conductor to see a reflection.

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

Thanks, I like the symmetry, ground plane argument... I was having a hard time picturing the difference between balanced and un-balanced.

George H.

So any balanced pair has a some ground surface (V=0) between them. Silly idea... does anyone make pcbs with diff pairs top and bottom and equal distance from the internal ground plane?

I'm not sure what the advantage would be? Higher impedance and no ground currents?

George H.