Underground dog fence: shield with coax?

Thread Starter

bdj1212

Joined May 9, 2022
7
Hello,

I want to install an underground electric fence, but we have a small yard with neighbors on all sides. I need the electric fence to work on the front half of our property, as we have a physical fence along the back of our property. I want to shield the electric fence along the back of the property so that the dog has full freedom in the back yard. See diagram. (The sample layouts with existing fences recommend looping around the fence or double looping back, but both of those options would eat into our limited yard space.)

Has anyone ever tried connecting an electric fence to coax to shield sections of the fence? I imagine doing so without an impedance matching network would result in reflections, but it might still work.

Brian
 

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sagor

Joined Mar 10, 2019
696
Is there a reason why you can't just double back the wire along the original route? That leaves the "coax" area totally without any wires.
 

Thread Starter

bdj1212

Joined May 9, 2022
7
Is there a reason why you can't just double back the wire along the original route? That leaves the "coax" area totally without any wires.
Our front yard is small. The wire generates a ~6' "stimulation zone", and the wire will be on the edge of the front yard. Doubling back requires leaving 5' of space between the two wires to avoid cancelling the signal. Doubling back along the front yard would either turn the entire front yard into a stimulus zone, or we'd have to put the return wire on the other side of the sidewalk, which probably wouldn't survive very well. The shielded coax allows the signal to return without a second stimulation zone.

Brian
 

Thread Starter

bdj1212

Joined May 9, 2022
7
Tony -- both ends of the wire are supposed to end up at the Fence Transmitter. I drew in the resistive load just to represent what I imagine is on one end of the wire, but I doubt it's just a resistor. I could try that though! I have done no experimentation with this yet. The collar is a bit snug. :)
 

SamR

Joined Mar 19, 2019
4,197
I use the "wireless" electric fence and it has adjustable setting to determine the perimeter. It is a circular area however. Emits a constant RF signal and when the collar loses signal first emits a beep and then on total signal loss administers an adjustable shock level. So once the signal is lost and when the dog returns and regains the signal it will not be shocked. We do remove the collar in the house as any electrical outage or blocked signal will administer the shock. Far, far, easier to install. :cool: I've only had one very bullheaded dog that would, on occasion, "jump" the fence to go a'roaming.
 
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crutschow

Joined Mar 14, 2008
29,810
I believe if you don't have a signal, then the collar will activate, so not having the wire at the fence would likely not work.
Can you put the wire on the other side of the fence or high on the fence, which would allow the dog to get closer to the fence?
 

ronsimpson

Joined Oct 7, 2019
2,078
The dog fence we have must be with in 2 feet of the dog to cause an effect. Returning the wire back to the start on top of a tall wooden fence will work, to make a dead zone.
We had to make a circle with the wire, (return back to the start).
Ours works by sending current in the long wire. It is not really "RF". A break in the wire stopped every thing.

It has been a long time ago, but I think ours "transmitted" at 100khz or 120khz.
 

Thread Starter

bdj1212

Joined May 9, 2022
7
I set up the fence loosely, and it is the system Ron described. There's a Warning Zone and a Stimulation Zone, but if you escape then you're free.

crutschow, the real issue is that there's a gate in the back yard fence. Stringing up the wire above the gate isn't a great solution. The instructions recommend twisting the wires (10 twists/foot) to cancel out signals, but that requires doing the out-and-back loop I'm trying to avoid.

Reading the instructions more carefully, it says "splicing shielded cable to the Boundary Wire will also not cancel the signal." I'm thinking....if it's well grounded....why not?
 

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Thread Starter

bdj1212

Joined May 9, 2022
7
@Yaakov The instructions say no: "The signal cannot be cancelled by running the wire through plastic (duh) or metal piping. Splicing shielded cable to the Boundary Wire will also not cancel the signal."

Are the manufacturers just trying to avoid yahoos like myself messing around with conduit/coax and complaining to them when the system doesn't work? I could see the reflections from the impedance mismatch making a mess, but presumably a matching network could take care of that.

I think the next step is to bust out the coax from somewhere in the basement.
 

SamR

Joined Mar 19, 2019
4,197
Most coax is designed for RF and higher frequencies. Collars, at least the ones I have, operate at lower freq. My wireless transmits ~10kHz.
 

MrSalts

Joined Apr 2, 2020
1,762
Is your yard so small that the back 2' next to the fence have to be used? Perhaps this wireless fence will prevent your dog from digging under the fence and escaping?
I hope you put the fence wire right next to your sidewalk to make sure your dog gets right up next to pedestrians taking a stroll. That's how it's done in my neighborhood - also, a real barking contest going on as all the dogs are essentially untrained.
 

Tonyr1084

Joined Sep 24, 2015
6,891
Tony -- both ends of the wire are supposed to end up at the Fence Transmitter. I drew in the resistive load just to represent what I imagine is on one end of the wire, but I doubt it's just a resistor. I could try that though! I have done no experimentation with this yet. The collar is a bit snug. :)
I'm wondering if you ground the end I showed where I showed and grounded the fence unit where that wire is supposed to return to. Wondering if that would work. Use the ground as part of the circuit. Maybe not. Might turn the whole yard into an electronic fence. Just wondering. I have no experience or knowledge of such things.
 

Thread Starter

bdj1212

Joined May 9, 2022
7
So using alligator clips, I hooked up ~30' of coax to the Boundary Wire, plugged the other end into the terminal of the Transmitter, and connected the coax shield to the ground terminal of the Transmitter. I turned on the transmitter, which beeps if the loop is not intact, and the loop was intact.

The moment of truth: the collar continues to buzz even when placed by the coax. I grounded both ends of the coax, and it still continues to buzz.

So the fence company (so far) is correct: shielding the signal with coax does not block the signal. I'm not clear why, but I suspect that @SamR is on the right idea that this signal is low frequency, and coax shields are effective at high frequency. I've been trying to find a plot of coax egress vs frequency but haven't been able to dig one up. Shy of going through the calculations, I imagine that increasing the diameter and reducing the resistance would both help. But I tried threading the coax through a 4' stretch of 1.5" diameter iron pipe (R=1mΩ), and the collar still buzzes with the pipe (and coax) grounded.

I measured the signal with my multimeter (I don't have a scope at home), and the measurements bounce all over. The AC voltage across the terminals is ~100-150mV, and the frequency measurement is all over the place, 100kHz to ~190kHz. I'm wondering if it's some kind of a chirp signal.
 
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