An invisible pet fence is a loop of wire barely underground, reportedly broadcasting at 600kHz, although other reports say 10.65kHz. There's a popular method for finding a break in the wire loop that involves placing this 100µH RF choke onto the transmitter, in parallel with the broken loop. Some write-ups suggest you additionally wrap the ends of the broken loop wire around the leads of the choke before reconnecting to the transmitter, instead of just putting them in parallel. You then use a portable AM radio tuned to ~600kHz to scan for the lack of a throbbing signal, indicating the location of the break. Many folks have had luck with this, so I'm sure it works at least for some systems.
I'd like to understand why this works, and whether we can improve on it.
I understand that one role of the choke is simply to provide continuity. That allows the transmitter to stop beeping and produce the signal. If that was all it was doing though, why not use a resistor? Because the choke won't allow AC to pass and therefore keeps the RF signal stronger into the boundary wire loop?
Another suggestion is that wrapping the boundary wire leads around the choke leads provides a crude isolation transformer. Pulses through the leads of the choke are picked up by the wrapped wire. Perhaps this increases the signal you would get without wrapping?
Any suggestions or insights?
EDIT Is there a way to identify a similar choke in my parts heap? I've probably got a bunch of them, but I've never used one and don't know what I'm looking at.
EDIT2 Well, I lucked out: An old solar light had a 100µH inductor on it (brown, black, brown, silver, measured resistance at ~3Ω, so it's not a resistor). It's smaller than the "choke" available at RS but has the same impedance rating. Any reason to think it won't work? It's roughly 1/4" long and 3/16" wide and green, fwiw.
I'd like to understand why this works, and whether we can improve on it.
I understand that one role of the choke is simply to provide continuity. That allows the transmitter to stop beeping and produce the signal. If that was all it was doing though, why not use a resistor? Because the choke won't allow AC to pass and therefore keeps the RF signal stronger into the boundary wire loop?
Another suggestion is that wrapping the boundary wire leads around the choke leads provides a crude isolation transformer. Pulses through the leads of the choke are picked up by the wrapped wire. Perhaps this increases the signal you would get without wrapping?
Any suggestions or insights?
EDIT Is there a way to identify a similar choke in my parts heap? I've probably got a bunch of them, but I've never used one and don't know what I'm looking at.
EDIT2 Well, I lucked out: An old solar light had a 100µH inductor on it (brown, black, brown, silver, measured resistance at ~3Ω, so it's not a resistor). It's smaller than the "choke" available at RS but has the same impedance rating. Any reason to think it won't work? It's roughly 1/4" long and 3/16" wide and green, fwiw.
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