Messing around with coil frequency

[flgliderpilot]

Just out of curiosity, I decided to measure the resonant frequency of the stock coil on my Bounty Hunter Prospector. I used a voltmeter that can handle up to 50 kHz signals, and set the signal generator for a sine wave at 5 V peak to peak. Without the coil connected, the meter was showing about 3.5 V RMS, which seems about right.

As expected, with the coil connected in parallel, the voltage reading varied with frequency. At 1 kHz, it was only reading around 1 V. The maximum voltage was 3.379 V at 7.5 kHz. I've read that older BH detectors run at 6.6 kHz, so it seems my coil is out of tune. But at 6.6 kHz, the voltage was 3.355 V, a difference of less than 1%. So maybe coil resonance isn't very sensitive as long as it's close. I have a 4" coil on order, so I'll see what that one reads next time I go to the hackerspace.

I also tried measuring the voltage on the output pins of the control box, but I couldn't get a signal. The meter would show around 2 V for a split second, then go back to near zero. Maybe the detector sees that the coil isn't attached and shuts itself down. I might try building a breakout that can go between the coil and the control box.

The actual resonant frequency may be different once you connect any additional circuit capacitance via the controller circuitry. Also, you are probably not getting anything from the control box because there is no tank circuit connected (the search coil), so the oscillator isn't oscillating.

 

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