Differences in High Frequency detection (14-20 khz) and low (3-10 khz)

[Canewrap]

What advantages and disadvantages are there to high freq detectors vs. low frequency? What are each type best for and in what ground? Is there a good book out there that discusses this?

 

[JB(MS)]

George Payne designed the first detectors with ground balance capability, discrimination, ID meters, tone ID and almost all of the other features on modern detectors. Even David Johnson, who is undoubtly a genius when designing detectors is concerned, who designed many of the current detectors including the new Fisher series, several Whites and Tesoro models and and the Shadow X5 says George Payne is still the master. Below is an excerpt from one of the articles on my website George wrote, followed by an article by Gary Finch.

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.

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.

George Payne, Copyright

 

[tabdog]

Thanks JB,

That's helpful.

I had read that before and lost track of it.

I did not have much understanding back then.

Now I've forgotten what I did know.

It's good ta read it again.

HH,

Tabdog

 

[Uncle Willy]

High frequency works best on gold and small objects.

Bill

 

[Mr.Bill]

Way too much mis-information being passed out there about this, users jumping to conclusions because nugget detectors are at a high freq., that's what is needed for certain other targets. The same holds for coins, relics.

They need to pay a bit more attention to what George said.