You ever wonder what operating frequency has to do with metal detecting. George Payne introduced the VLF metal detector around 1980 while at Bounty Hunter. He also made the more important improvements on the design.
This is what he had to say,
The target signal returned to the receive coil can be thought of as composed of two components, one we call x and one we call r. The polarity of the x signal (its direction) tells us if the target is ferrous or non-ferrous. The r signal has only one polarity. Also, the ratio of the x and r signal tells us the target?s phase. In addition, the signal magnitude (which relates to sensitivity) of both x and r are a function of operating frequency.
A VLF detector by its very nature is only designed to respond to the r signal and ignore the x signal. Since the ground reaction primarily produces a x signal in the receive coil the VLF detector does not pick-up the ground but only responds to the r signal of the target. Therefore, the VLF detector only needs the r signal for proper operation.
However, for discrimination we need to measure the x signal and the r signal to determine what the target is. Since we are using the x signal then we have to contend with the resultant ground signal pick-up.
The x and r target signals are frequency dependent and obey very predictable characteristics when the operating frequency changes. We know that the x component decreases as the operating frequency decreases. Above a certain frequency the x component reaches a maximum. The r component acts differently. It is maximum at one particular frequency and decreases if you go up or down in frequency. We call the special frequency at which the r signal is maximum, the target?s ?-3db? frequency. It also turns out that at the -3db frequency the x signal is one-half of its maximum value. This special frequency is unique to each target and is different for different target.
The higher the conductivity of the target the higher will be the targets -3db frequency. Conversely, the lower the conductivity the lower the -3db frequency. The -3db frequency of the high conductivity target will also make the r signal peak at a high frequency, normally well above the operating frequency of the VLF detector. This will make the high conductivity target have lower sensitivity on the VLF detector because the r signal amplitude drops if we are significantly below the -3db frequency. Simply put, maximum sensitivity on a VLF detector would be if we position the operating frequency directly at the target?s -3db frequency. For example, a dime and penny have a -3db frequency of about 2.7KHz. This is where their r signal peaks and would be the best frequency for picking them up using a VLF detector. However, a silver dollar has a -3db frequency of 800Hz. Nickels, on the other hand, have a -3db frequency, where its r peaks, at about 17KHz. Targets like thin rings and fine gold are higher still. Clearly there is no one frequency that is best for all these targets. The best you can do is have an operating frequency that is a compromise.
All of the discussion so far pertains just to ?r reading? VLF detectors. If you now add in the discrimination requirement if gets really confusing. Remember, to obtain discrimination we need to read both the x and the r signal components. As I said the best response to the x signal is not the same as the r signal. We need to be at an entirely different frequency for x. Generally for best discrimination we need to have an operating frequency well above the targets -3db frequency.
As you can see the ideal frequency for each target is different. In addition, for best performance the operating frequency to read x should be different from the frequency to read r. The best we can do is reach a compromise frequency. Generally we can say that high frequencies are best for low conductivity targets and low frequencies are best for high conductivity targets.
Tabdog thinks that 6 or 7 kHz is a compromise for all around hunting, if you are after coins and other things. But for lower conductive targets, 10 kHz or higher works better. That is where Tabdog likes to be. He likes his small golden goodies.
George Payne suggested that for most hunting that a Frequency of around 14 to 17 kHz would be a good compromise. He must not be that big on silver dimes and so on. But they will still be detected even if the depth suffers a little.
Now days there are metal detectors that operate at more than one frequency. Garrett Whites, Minelab and some others have introduced these type of machines. Some have two frequencies and some have more. This is an attempt to have the best of both worlds. They are also more expensive and some what harder to learn.
Most starter detectors are single frequency.
So, I suggest thinking about this when you make your next detector purchase.
But, I do not discount reports by other users.
If all this seems too much to cope with,
There are some starter detectors that have good reviews. That means that people had fun with them.
That is the deciding factor after all.
So yawl have fun out there, and tell me how you done.
I will be interested to see anyones adventures on the road to treasure.
Happy Huntin,
Tabdog
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« Last Edit: October 21, 2009, 04:01:02 am by tabdog »
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