Detector Operating Frequency

I always thought that the search coil had to be matched closely to the detector's operating frequency to get the best efficiency. It just dawned on me that the Teknetics Omega 8000 (7.8 kHz) and G2 (19 kHz) have a big difference in kHz but they still use the same DD search coils. Minelab uses different search coils to change frequencies on some of its detectors. Is the stock 11 inch DD search coil better matched to the Omega or G2?

tabman

Search-coils don't have to be matched to a particular machine / frequency.
There are two common things that make a coil 'matched':

* If a tuning component is built into the coil. Normally this means a capacitor, connected to the transmit winding. This makes an L-C circuit, with a tuned resonance determined by maths. Some of the 6.6KHz Bounty Hunter coils are like this, for example. However, you can still run them at lower frequencies, by adding more capacitors. This is done in the Fisher F2/F4, the added capacitor (inside the control box) reduces the coil down to 6kHz.

* The coil is made so that in air, very little signal passes directly from the transmit side to the receive side. This is called 'nulling' the coil. With little residual signal, it's easier to handle the 'real' signal caused by the ground, and the targets buried in it. There are a number of ways of doing this, and some of them are frequency-sensitive. An example is the 13 kHz Fisher F75 coil, which can be seen to go "out of null" even at 12 or 14kHz. However other techniques can result in a null that is quite broad in its frequency range, the Greek series/F5/ G2/F19 coils are an example. I would guess that multi-freq machines would also benefit from having a decent wide null, though I have no practical experience. The Deus coil may be an exception, some think that they handle the change in null by electronic means, rather than having a fancy coil design.

There is a thread on here where Keith S. had a G2 opened up, and it did look like ther may have been some electronics fiddles to make the coil work better, possibly because it was originally designed to be a 7.8kHz coil.

Thanks for the info.

tabman