OT. Antenna tester

[Dean Hoffman]

I fix center pivot irrigation systems and am looking for an easier way to diagnose potential problems. The pivots are wire guided corner systems. A picture here:

OR:

The corner arm follows a buried wire and swings out in the corners of a field picking up additional acres that a regular circle pivot will miss. The antennas I want to test are mounted like this below the black tower box.

OR

The far left one angled down steers in forward, the far right one angled down in reverse. The horizontally mounted one is the safety/reference. It's helps with the steering and shuts the system down if the machine gets off the wire. We have to remove them to test them now. We energize the transmitter, aka oscillator, for the buried wire then set a new antenna directly over and perpendicular to the guide wire. We remove the suspect antenna and put it in the exact place as the new one and compare readings. Typical readings using our Fluke meters with a dummy load are 20 mv or so. Link to picture of antenna: I'd like to be able to test the antennas in place and without having to energize the buried wire transmitter. The transmitters are powered by 120 vac. Energizing the transmitters sometimes requires starting a diesel engine that powers a three phase generator. That can be a problem in cold weather and just getting to them in snow can be an issue. My idea was to use 12 volts from an accessory outlet on an atv. I'd make some sort of a wand to hold against the antennas that would energize the antenna so I could take a reading similar to what I see normally. The antennas run at 833, 1000, or 1200 hz. I found this gizmo and thought it might be useful: My idea is to put three of these gizzies in some sort of box and set each one to one of the three frequencies. Put three outlets on the box then plug the wand into whichever one matches the antennas I want to test. Is this at all workable? Is there something better, maybe something ready made for the job?

Thanks

[rbowman]

A grid dip meter might do the job but good luck finding those anymore. Basically they are an adjustable oscillator and when they're held near a resonant circuit the meter will dip. The name comes from the vacuum tube grids but they went to solid state.

A quick poke around the web didn't turn up anything outside of eBay. They've become 'antenna analyzers' that are fairly pricey even for MFJ's offering. (aka Mighty Fine Junk in ham circles).

[trader_4]

How about whatever normally drives the buried wire? I'm guessing that's probably powered by AC, but you could use an inverter to power it. I suppose there may be three flavors of those too though, which would then mean you'd need three of the transmitters. Any of that could make that impractical.

But your idea sounds excellent. It would put out a square wave, you'd probably want an amp on the end of it to get the power up. The square wave will not be a pure freq, it will have harmonics, the amp will help filter some of that. The antenna widget probably won't care about harmonics, but if it does, a simple low pass filter could be added. You could easily rig all that up in a small box.

[trader_4]

Thinking about this, what you really want is a battery powered, handheld, frequency generator. Here's one:

Or this Chinese one for just $90:

You'd have to look at whether they produce enough power, but if the antenna responds to a buried antenna, you'd think it would not take much power from something held close to it. If you buy one on Ebay you can try it out and if it doesn't work out, sell it on there.

[Dean Hoffman]

Cut a bunch.

Yes, the oscillators/transmitters are powered by 120 vac. They're about 4x4x6" or so not counting the plug in. Also, they're not cheap. I'll have to do some snooping around the shop to see if we have more used ones. The one I've found is the middle frequency. I don't know if that's adjustable. There was a time, long ago, when one could adjust the output voltage. Three of the oscillators plus an inverter wouldn't be the end of things but probably wouldn't be quite as handy. The money would be the big thing. It would be nice from a safety stand point to keep the voltage down also. I've done plenty of dumb things over the years and figure others will too.

[trader_4]

Did you see my post on the handheld signal generators?

[Dean Hoffman]

Yes. I looked around a little and you hit close to the bottom price on the BK Precision. I'm sorely tempted to just order it and not mess around with the other stuff. That isn't really expensive given how much we'd use it. Thank you

[makolber]

I'd

'op ask your question at sci.electronics.design

m

[Davej]

I really doubt that. In fact I wonder why you would have any trouble with antennas? Do you find "defective" antennas? How are they defective?

[Dean Hoffman]

Doubt ?? Surely not on usenet. Here's another picture:

The antennas just get weak and don't pick up the signal from the buried wire I mentioned earlier. I don't know what causes it. The factory has experimented with different cords and sealants. Their last idea is to put the cord part way down on the side of the antenna. The cord used to come out of the end where it might be harder to seal things and water might be more likely to get in. Replacing an antenna, and only an antenna, has fixed a lot of corner systems over the years. A defective steering antenna lets the corner arm wander off the wire it's supposed to follow. Then the safety antenna shuts it down due to lack of signal. A bad safety antenna won't pick up the signal and shuts the machine down even if the corner arm is over the wire like it should be.

[trader_4]

I think what he may mean is that we usually think of antennas as passive devices, with no electronics. I'm guessing that these are more of a antenna/receiver? What comes out? I'm assuming some kind of control signal, not an analog waveform of the received carrier frequency. In the former case there is plenty to go wrong.

[Davej]

Okay, I would not think of that unit as an "antenna" but rather a "sensor" since it probably contains all sorts of electronics. In order to test it in place you would need to be able to reproduce the signal that is in the buried wire. What signal is applied to that buried wire? If you can simulate the signal from the buried wire then you could test the sensitivity of this "sensor," but you would need to experiment with known "good" and known "bad" sensors to establish a testing procedure.

[trader_4]

Wow, I think you're on to something there, Pilgrim.

[Dean Hoffman]

Right now we have to energize the wire and compare the questionable one to a new one. I guess these antennas are basically just coils. They work off the magnetic flux emanating from the buried wire. Being off to one side or the other generates a current which is different depending on what side of the wire the antenna is on. Turning one upside down makes the system turn the opposite way it should. I ordered the signal generator T4 suggested so I'll see what I can learn. I could use the oscillators we have as repair parts but those things are expensive and it would be a bit clumsy to use them.

[Davej]

This is still very vague. What are the three audio frequencies? Are there three buried wires each driven by a different frequency or does each sensor output a specific frequency if a signal is detected? He reads 20mV of what? Why not simply use a spare transmitter connected to a short test wire? The transmitter runs on 120VAC so power it from an inverter on the 12VDC ATV battery.

[Davej]

You mentioned three AUDIO frequencies. The signal generator mentioned earlier produces RADIO frequencies.

[Davej]

Oh okay, it does audio also. 0.1Hz to 9.99MHz.

[DecadentLinuxUserNumeroUno]

rbowman [e-mail] wrote in news:grrtamF1ctbU1 @mid.individual.net:

Pretty smart little presentation sheet. Cool job.

[rbowman]

Yup. Even the Mighty Fine Junk analyzer was a little pricey the last time I looked. The whole SDR thing has changed the game.

[Ralph Mowery]

china has them already built for less.