Mick in Dubbo said:
My confusion is in regards to how the high frequency coils expand the range of low conductive targets and the low frequency coils expand the range of highly conductive targets while compressing the range of lowly conductive targets. It's my understanding that a 3kHz coil put out 3 cycles per give time frame, while in the same time frame an 18kHz coil will put out 18 cycles (six times the resolution but with a reduced length to the signal). To me, I think of them like a high resolution TV/camera screen compared to a low resolution one. The low resolution one will pick up less detail, much like how our coils see targets in the ground (ie less sensitive).
Targets response varies as the frequency of the electrical current sent through them changes. And how a target responds to a given frequency is how engineers determine TID assignment. As such, if we changed coil frequencies and not the target property parameters, it would skew the TID of our detector. What Minelab did was design three different TID programs (one set of parameters for use with each frequency) in an effort to maintain consistency when detecting. Doing so allows the user to identify one set of numbers for each type of target, and carry that information over if and when they change frequency of coils. Don't think of it as being compressed or expanded. Think of any given TID being somewhere between the most ferrrous properties and the most non-ferrous. Knowing those numbers would change for any given target, dependent on the frequency being generated, VFLEX allowed the coil to tell the software to provide the TID applicable to that frequency.
Without boring you to death, here is a bit of how I see it coming about..............
Metals with high magnetic properties are known as ferrous targets. Although we often refer to non-ferrous targets as "conductive", electrical conductivity and the target's electrical inductance are both used to determine non-ferrous values. VLF metal detectors (30kHz and lower) are actually Induction Balance detectors. Induction Balance detectors implement a coil with two transmit windings, each transmitting an electromagnetic signal "out of phase" with each other. As such, without any target under the coil, these two transmit windings nullify each other (balanced), not causing the receive coil to indicate a detected signal. However, when a target interferes with the balance of the two transmit windings, the induction balance is "gone", and eddy currents are generated by the target. These eddy currents are "picked up" by the receive portion of the coil. Opposite in polarity, the received signal is delayed, (referred to as the target's "phase shift") due to resistance to the flow of current and inductance (changes in the flow of current). The greater the non-ferrous attributes (inductance and/or conductivity) of a target, the greater the phase shift. The largest phase shift occurs with highly conductive metals like silver and copper. Smaller phase shifts occur with targets of less conductive/inductive materials. Ferrous targets demonstrate their ferrous nature best at low frequency. As the frequency increases, ferrous targets become increasingly difficult to identify. That is the reason some higher frequency detectors are referred to as "iron lovers".
The transmit signal of a detector generates eddy currents whose direction is reversed (clockwise to counter-clockwise and back to clockwise) several thousand times every second. In the case of the 3 kHz coil, the direction is reversed 3000 times per second. For the 18.75 kHz coils, the direction is reversed 18750 times per second. Both are well within the frequency range of modern VLF detectors.
George Paine's engineering designs were the basis for today's ground balancing, discriminating, target ID'ing detectors. Through rigorous testing procedures and scientific analysis, Mr. Paine determined that the ideal frequency for detecting a silver dime was 2.7 kHz. Gold coins and most US coins were best detected using a frequency lower than 10kHz. The US nickel and gold jewelry were best detected using a frequency of 16.5 kHz. And gold nuggets were best detected with a frequency greater than 20 kHz.
Knowing that three different frequencies would provide three different sets of TID numbers (for the same targets) if the same information was used, (thanks to the digital signalling of VFLEX) Minelab engineers were able to program a different set of parameters to be used with each frequency. The result being.....the TID numbers remain virtually the same, regardless of the frequency of coil you are using.
With all that in mind, here is a page from my ebook, Understanding your X-TERRA that depicts the differences in "bin assignment" for the three coil frequencies.
[attachment 230938 phaseangleillustrationandexplanation.jpg]
I hope I addressed your question. HH Randy
