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Date Posted: 11:12:38 04/16/02 Tue
Author: Steve Herschbach
Subject: Re: Regarding "multi-frequencies"
In reply to: Jim McCulloch 's message, "Regarding "multi-frequencies"" on 09:27:31 04/16/02 Tue

Hi Jim,

There seems to be little advantage to using more than a very few frequencies to make the actual comparisons needed for optimum depth or target id. And as you point out a good spread is essential.

Now I may be wrong here, but I've heard the Fisher units transmit a single frequency but receive back that same frequency and a harmonic - 5 kHz and 15 kHz.

Info on the Minelab 17 and 28 frequency units is hard to come by. I have some question as to whether they actually transmit 17 or 28 discrete frequencies or if they are using a mix of transmitted frequencies and harmonics. As near as I can tell they actually compare 3 or 4 frequencies, for exactly the reason you note. What changes is the 3 or 4 frequencies the machine decides to use. If it is only ground conditions or if user input helps decide which frequencies it chooses I do not know. Anyone that can elaborate on the Minelabs please jump in!

The DFX appears to be unique in actually having two completely separate transmitter/receiver circuits. This is why you can use either 3 kHz or 15 kHz, or both together.

Is there any reason why transmitting actual separate frequencies would be superior to using the harmonics of a single transmitted frequency?

I have one big question on multi-frequency units. For years I told people machines with different frequencies need different coils. So all the White's coin detectors can swap coils, but the Goldmaster needed separate coils. Or the Gold Bug and Gold Bug 2 coils are not interchangeable. Examples go on forever.

Now I'm being told one coil can run at lots of different frequencies. I smell a rat here. Is it that the coil is perfect for one frequency, and is not working optimally at the other frequencies? Or do the machines have a way of electronically retuning the coils for each frequency? Or are the coils somehow physically wound differently for multiple frequencies?

I'll use the Minelab XT18000 here as an example. The machine seems to work best at 20 kHz. When you go to 60 kHz it does not seem as ''hot'' on small gold as it should, considering the high frequency. It does ok but something is missing. I've always blamed it on the coil, as I suspect the coil simply is not really running right at 60 kHz. I'd like to try the XT18000 with a coil tuned specifically for the 60 kHz mode.

Anyone got any info on coils and multiple frequencies?

Steve Herschbach

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