TDR pulse or squarewave ?

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 

http://www.analogconsultants.com/

I wasn't very clear on that, the 74AC14 is fine, but if I'm building, I might as well get the fastest edge chip as long as it will work on 3V or 4.5v.

Ok, seems a little odd I need to measure my cable then adjust the measuring device to properly measure my cable. But, I expect I will see the end reflection and the driving device mismatch reflection, I can adjust to get rid of the device mismatch reflection. Thanks, Mikek

74AC is pretty fast. You could shave off a few percent with other logic and live with 2.7V or less but then you might as well use ECL for the real speed.

You don't have to. However, you do need to know exactly how a pulse behaves upon encountering a mismatch. Whether it'll invert or not, et cetera. Then you can use that knowledge to gauge impedances and matching without bothering to have the source well terminated.

Bingo. Sometimes on a long cable I actually use only the driver source impedance. I change that until I only just one reflection left (the one at the end of the cable) and then I know the cable impedance. Mostly for cases where there is no spec on the cable but we need to use it for RF stuff.

BTW, now it dawned on me what your handle "amdx" means :-)

I still have to build some sort of outdoor loop for the Drake SSR-1 downstairs in my brew house. Inside there is too much noise. I'd like to keep it passive though, maybe 20ft or so of wire looped around under the deck.

In the evening I can pick up radio stations in the tropical band around

5MHz.

--
Regards, Joerg 

http://www.analogconsultants.com/

74lvc1g* will work from 1.5V (and lower) to 5V

If you drive a 50ohm cable with three parallel (74lvc1g* + 143ohms in series) then the risetime is

I'm lucky to have come across at Tektronix 2465 CTS, 300MHz scope.

I removed the termination from my just measured at 253ft long BOG. (I thought it was 300ft) I measured it at 500kHz, 1000Khz, 1500kHz and 2000khz using my MFJ 259 with wire brought out to drive a DVM. I recorded the readings, then went back to the shack and found a resistance to match what I read on the DVM at the antenna. The reading was very close to 500 ohms. This is much higher than the expected 250 ohms most of the BOG URLs mention. Not sure why, Maybe I need a better ground, it's only 5ft long in very wet dark soil. Maybe because it's a wet area I have more losses on the wire. I made up a termination of 500 ohms and installed it. I'll check performance tonight.

Question, I measured the wire open, and got 500 ohms. Now I terminated it with 500 ohms, will I still measure 500 ohms as the impedance? Grabbing my equipment to go measure, I think it will.

Mikek

Those are nice scopes but still barely justifying anything faster than 74AC.

The Tek 2465 is "the" scope, IMHO one of the best ever built. Except for its delayed trigger clutch which they shouldn't have made out of plastic, but there is always something.

I don't quite understand, not sure what a BOG is. Regarding the various AM frequencies it shouldn't make a difference, the cable is the same impedance at all of them. Though you probably meant something different here.

Yes, it's the same except that you won't see a reflection coming back from the end if terminated.

--
Regards, Joerg 

http://www.analogconsultants.com/

Say what???

I assume that any antenna system has a terminal impedance that is a STRONG function of frequency. Sounds like you're trying to use a time domain measurement to solve a frequency domain problem. You haven't mentioned anything like high resolution data acquisition and FFT-like domain conversions. So, just how do you plan to use the time domain information when you get it?

TDR is a great way to adjust frequency-independent termination resistances. For antennas, not so much.

As for pulse vs square wave, there ain't much difference as long as the pulse is long enough to get the info you want. Frequency hasn't been mentioned. I'm assuming it's LOW.

Impulse response is another issue. I you get the pulse short enough, you can approximate a wide range of frequencies and get something useful from a FFT, if you have super acquisition capabilities.

I have 2 minutes of google experience with BOG antennas ;-) Looks like you feed a matching transformer with low impedance coax. Sounds like you need to match the wire at BOTH ends.

First thing I'd try is to listen to a signal and tweak the termination for (whatever your best criteria are). If you can't tell the difference, it don't matter.

I submit that a fast-rise pulse generator won't help you at all. I was going to suggest a frequency burst, but you probably can't get any useful information if the period of the burst frequency isn't much shorter than the length of the wire.

Arrgh, Inconclusive, it did drop about 30 ohms, but we also had a little rain storm since my last measurement. I'll try this on another day. Mikek

Use a 4:1 transformer or balun on the MFJ-259 to connect to your zip cord transmission line. Make sure the balun covers well beyond your operating frequency range so that the balun doesn't contribute extra inductance or capacitance to the measurement.

Is neither one a choice? One doesn't use a TDR to measure antenna impedance. One uses a TDR to measure transmission line impedance.

As Mike ( snipped-for-privacy@netzero.net) mentioned, the load impedance is frequency sensitive. Since the impedance of the antenna changes with frequency, the frequencies near the characteristic impedance of the transmission line will not reflect, while the one's that are some complex impedance, will produce some rather other ugly looking reflections. When combined, they really look ugly on the scope. The only termination that will look reasonable is a purely resistive load. You have that in the load, but combined with the BOG antenna, it's no longer resistive.

I've built numerous TDR boards. For me, it's an obsession to NOT buy a dedicated TDR instrument. The last few were based on this 74HC14 design: My only major change was replacing the 4.5 battery with a AA cell to

5.0v regulated step-up inverter. 74AC14 also works.

More (and probably better):

Or, you can build one that uses an avalanche diode: (20:04)

--
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

I'll be careful with mine from now on.

OH, sorry, conversations on several groups. A BOG is a 'Beverage Antenna' On the Ground. So you lay a long piece of wire on the ground. put a transformer at one end and termination resistor at the other.

I want to find the termination resistance that is the same value as the characteristic impedance as the wire. Thus I need to measure or use some technique to find the proper termination resistor. Go to 8:40 of this video below, That looks like it will do exactly what I want use a square wave circuit. >

That's correct.

Did the video help clear up what I'm after? Thanks, Mikek

Just noted something else that makes me think the 500 ohms is pretty close. I measured at 4 frequencies with my MFJ-259, 500kHz-102.3mv

1000kHz-107.4 1500kHz-106.9 and 2000Khz-98.4mv. This is with a 250 ohm termination. The mv readings are for comparison, I'll add a note* about there values. When I had a 500 ohm termination 500kHz-104.5mv, 1000kHz-105.6, 1500kHz--104.5, 2000kHz--104.1 I'm assuming them being about the same means something, could be wrong. *105.9mv = 430ohms 106.9mv = 479ohms 107.9mv = 510ohms

I know, you thought it would vary with frequency, and I don't know why it doesn't.

I not being argumentative, just want to understand, it seems my BOG is very similar to a transmission line impedance, it's L-C-L-C-L-C, and will have a reflection from the end or not if terminated properly.

Please watch the two minutes from 8:40, I think that is what I'm after.

There is a guy in Greece (on EBay a few years ago) that makes new knobs and also this clutch for th 2465.

Sounds like that would make the characteristic impedance of the wire quite rocky. It would change as the properties of the ground change. Wet dirt, dry, rocks, sand.

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/

And it does, The ideal is wet low impedance soil at each end for the grounds and a desert in between. I can only work with what I have, and what I have is a clandestine beverage on someone else's pretty wet property. But I'm happy with what I have so far, I have less noise and cochannel interference, than my Active antenna at 24ft or my long wire at 35ft. Here a story, "The Case of Declining Beverage-on-Ground Antenna Performance"

Mikek

I ran out of steam last night, needed some sleep. I left with comparing the BOG to a transmission line, your argument is that it is frequency dependent. Your probably right, but I'm not see much change over a 4 to 1 frequency range now that I have the termination resistor at 500 ohms. I will build the TDR even if it's only to check my speaker wire feed line impedance.

I'm in the market to buy a better Antenna Analyzer, I've looked at video of the RigExpert AA-30, but I don't see any info about how it does at 500 ohms. I have a suspicion that most analyzers lose accuracy the farther from 50 ohms you get.

Any comment about this ground rod resistance measuring method? See last 1/3rd of page.

Mikek

Did you add the step up for longer battery life or did you need more drive voltage?

====snip====

I have to say that my reaction to reading that article was "Well, Duh!".

:-)

--
Johnny B Good

I can already picture Mike walking onto his neighbors's turf with a watering can, his neighbor coming out with a shotgun "Whaddaya doin' in my yard?!" ... "Oh, just watering the termination of my beverage". Then Brutus the dog comes barging out, running to that very spot, sniffing some, lifting a leg and letting go.

:-)

--
Regards, Joerg 

http://www.analogconsultants.com/

Neither.

1. I put an adjustable power supply on Vcc and ran it up and down to see what range of voltages would work for the 74HC14. 25 ohm resistive load so that it draws some current (about 0.2A). The data sheet claims that the 74HC14 will work from 2-7V and I wanted to see what the output would look like over this range. At the low end, I lost some risetime. At the high end, I some some oscillations or ringing. 5V seemed like a good compromise value.

1. The 74AC14 has a voltage range of 2-6v which should work nicely at

5V, but the 74ACT14 is 4.5-5.5v, which requires a 5v regulated supply. I wanted to try all of them, so 5V regulated was the obvious choice.

1. I wanted to run it off a single cheap alkaline battery to make it small. If I were to do it again today, I probably would use a LiIon or LiPo cell and leave out the regulator. It will probably run ok on

3.6v (except for 74ACT14).

1. I couldn't find a three cell AA battery holder that would fit in the intended box. One AA and an inverter are much smaller.

2. I had some switching regulators left over from another project. There may be something better available. I didn't check.

1. I was thinking of selling the PCB, but never got around to learning KiCad so that I could make the board. Yet another unfinished project.

I'll get back to the antenna stuff later. I'm kinda busy today. Two meetings, lavish lunch, coffee with a friend, drug deal at Costco, exercise walk, some errands, radio club meeting, etc. If any work will be done today, it will be under cover of darkness (with the phone turned off).

--
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