Attenuating signal from buried dog containment fence

[jon morrow]

I am trying to figure out how to attenuate the signal over a specified length of a perimeter dog containment fence. The antenna wire is a closed loop buried about 3" in the ground. Would like to create a 30 ft. long area where the signal is attenuated so as to not activate the dog's correction collar. The system operates at ≈90MHz (FM band). Obvious solution would seem to be to construct a Faraday cage (grounded copper pipe) around the wire over the desired length, but other posters indicate that this does not work. I presume this is because the RF signal and near-field signal is still present entering and exiting the "cage", and creates and inductive current on the surface of the cage that actually may amplify the signal. So… question for those who know more than I….. does anyone see a way to block the inductive effects of the adjacent antenna on the "cage", so that there is attenuation (not amplification) of the signal over the length of the cage (pipe)?? I also do not know exactly how these containment systems operate… their insensitivity to antenna length or configuration, and their short-range sensing, makes me think we are dealing with a near-field effect, probably more magnetic than electrical. Does anyone know? Help.

 

[MikeMl]

If it is the commercial one, like Pet-Safe, it operates at about 20kiloHz (not at 90MegaHz).

Look at the sample layouts shown on page 6 of this manual. One of those might work... Note the use of the twisted pair...

 

[jon morrow]

The brand is DogWatch…. FM band (90-100 MHz). The double loop is problematic since the lot is small.. the double loop configuration shrinks the available yard by over 6 feet on each side…. that is too much to lose for the lot size.
thanks for the link though.

 

[MikeMl]

The brand is DogWatch…. FM band (90-100 MHz). The double loop is problematic since the lot is small.. the double loop configuration shrinks the available yard by over 6 feet on each side…. that is too much to lose for the lot size.
thanks for the link though.

I visited the DogWatch web site. Nowhere does it say that it is using the FM broadcast band. It says that it is using "FM" to detect the signal. Big difference.

I would be extremely pissed if my neighbor put in a wireless dog fence that transmits on 90MHz and interfered with my FM reception.

I have one of the Pet-Safe ones. It uses a ferrite loop stick to receive the signal at a few 10s of kHz.

Cant you use the twisted pair trick to prevent a section from radiating?

 

[jon morrow]

Hmmm.... after a little more investigating... I found their patent. You are correct... they use FM on a carrier frequency of 8192 Hz. Thanks for that clarification....
That lower frequency makes shielding much harder!

In any event, the problem with twisted pair is that there is no way to continue the fence beyond the twisted pair section... and three wires (twisted + 1 going back) restores the signal.
I was thinking of a length of coaxial (say 5 feet) before the "faraday cage" in series with the antenna wire might reduce the inductive signal near the faraday cage enough to quench it?? Might give it a try. How else could it be done?

 

[dr pepper]

Welcome to the forum mr Morrow.

Presumably this antenna is just a single core leaving the transmitter then returning at the other end of the loop, my initial idea is to lead another wire out from the transmitter connected to its ground, then at the point where you dont want the signal use a piece of co-ax, connect the centre to the signal obviously both ends and the braid to the tx ground wire just at one end.
The coax impedance could be tricky to work out.

 

[jon morrow]

Thanks to all... some good ideas. Yesterday I experimented: 1) antenna in long copper pipe (fadaday cage?), grounded and ungrounded. Result= no attenuation of signal, 2) then put a 20' length of coaxial video cable in series with the antenna, grounded the shield. Result - no attenuation. 3) Put the coaxial cable inside the copper pipe, both grounded. Result = no attenuation of signal. 4) Tried Dr. Pepper's clever idea...
grounded coaxial shield at one end, bifurcated the signal at the other end (and vice-versa), putting the signal down the core and also simultaneously down the coaxial shielding. This should have been equivalent to running two lengths of antenna in counter parallel orientation. Result = no attenuation.
I really don't see why solution #4 does not seem to work, since doubling back on the antenna so they are positioned parallel and close, kills the emitted signal when using the normal single conductor unshielded antenna wire. Perhaps the ground on one end of the shielding was inadequate, I did not confirm the effectiveness of the ground.

Anyway... giving up. Will use a rather contorted loop-back layout that is conventional and similar to some proposed in the manufacturers examples of layouts that work.

Still would like to learn more about how this system works, why any parallel or twisted arrangement of the antenna seems to lead to effective destructive signal cancellation regardless of the lengths of the loop-back (which presumably affects the phasing of the signal, allowing destructive interference). And why it is so hard to attenuate the signal with a fairly generous Faraday Cage ...

j

 

[Les Jones]

Jon,
Shielding one conductor will not work as the will only screen the electrostatic component which would already be shielded by the ground. It is the magnetic component you need to shield. It is not feasible to actually shield the magnetic component. It is best to have the two conductors of the loop running very close together (Preferably twisted.) so the magnetic fields from equal and opposite currents cancel out. So if you have one loop and break it at some point then connect the two ends where it is broken to a twisted pair then at the end of the twisted pair connect another loop then the field should be present inside (And just outside the perimeter.) of both loops but should be attenuated along the run of the twisted pair. A diagram of your current layout and what you are trying to achieve would be a great help.

Les.

 

[dr pepper]

If its not bleed over from adjacent cables which is what I'm thinking it might well be the above, I'd expect a pet control system to only produce very small magnetic content.
Cat 5 computer network cable uses twisted pairs, somehow I suspect that wouldnt work either.

 

[MikeMl]

The receiver in these system responds only to the magnetic field from the loop. Twisted pair with equal-but-opposite loop current is the only way to cancel the magnetic field in a section of the loop. A shield around the loop conductor does nothing, because the shield is not carrying the opposite current.

 

[dr pepper]

Cat5 stp, screened and twisted.

 

[MikeMl]

Cat5 stp, screened and twisted.

But it will not work any better than just twisting the wire that the loop is already made of.

You still have to have equal but-opposite currents in the twisted pair inside the CAT5

 

[dr pepper]

No argument with that.

 

[ronsimpson]

I had 15 acres of dog fence. I did not bury the wire but ran it at dog level in the fence. Actually used barb wire for about one mile. Big dog so the receiver was about 3 feet off the ground so using a wire in the fence at 3 feet high worked well. At one point we ran the wire 10 feet high to get over the road. That left a hole for the dog to pass through. (10 foot x 10 foot hole) There was a 6 foot wide spot in the center of the road that was not active.

Other wise you might have to bury the wire 4 feet deep.

 

[willfrench5]

To all -

I found this thread while looking for a way to do the opposite. I have one of the Pet Safe dog fences and a very large property. The signal is very weak, and doesn't give the dogs enough warning time when they approach.

So, I am looking for help on how to amplify the signal.

I tried the only commercial product I can find, a Channel Master 3410 FM signal booster hoping this would help. I think it may be in the wrong frequency range.

Any suggestions you can make on something I can by or make to take the signal and boost it to the loop?

Thank you.

 

[ronsimpson]

Will French, I had some problems with very large pet fences. Ran my fence at three feed high because my do is three feet high. That gave a much stronger signal. I also used larger wire.

I built a "audio power amp" to boost the signal but quickly learned that the resistance of the small wire was the limiting factor. By using "house wire" I got the resistance down and could get a stronger signal.

Hope that helps.
Ron