The following is from an article on my website by George Payne about preset versus adjustable ground balance. It's a little technical but explains why manual ground balanced detectors are the better choice. George developed most of the features found on modern detectors, including ground balance, so you can pretty well take what he says about anything related to metal detectors to the bank
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<b><i>A pure ground is a soil condition that reacts like it was pure ferrite. In other words a perfect magnetic condition where no electrical conduction (eddy currents) takes place. We can think of this as a soil that produces a signal in the detector with zero phase shift relative to the transmitted signal. This is considered our reference signal of zero phase to which all other signals can be referenced to. Of course the only real life object that produces this type of signal is pure ferrite. So ferrite becomes our reference target and produces what we call a pure "X" reactive signal.
Of course real ground conditions do not behave like pure ferrite. When subjected to a detectors magnetic field small currents begin to flow in the soil. This will cause the soil signal to be displaced slightly from that of pure ferrite. We call this difference a phase shift and define it to have an angle in degrees negative relative to pure ferrite. In addition, this phase shift produces a new signal in the detector which we call the "R" component signal. We can carry this analysis one more step. Using Trigonometry the ratio of the X signal to the R signal can be shown to be the actual measured phase of the ground.
All grounds have varying amounts of magnetic and conductive properties. Therefore, the ratio of the X or magnetic signal and R, the conductive signal, will vary from one location to another. However, the phase produced by this characteristic will always be negative relative to the zero phase of pure ferrite.
From my experience most grounds produce a phase that falls somewhere between zero (ferrite) and a -5 degrees. Some highly magnetic soils can have a phase that is quite low, but it can never be zero. Once the phase exceeds several degrees the ground characteristics begin to fall into an area where it becomes more saline. This doesn't mean that its not magnetic. Its just that the R or conductive component of the ground becomes stronger in relation to the magnetic portion. Thus the phase becomes greater.
The manual ground adjustment works in this manner: When you position the “Ground Adjust” control to the phase of the target, in this case the ground, any up or down motion of the coil does not produce a corresponding change in the audio volume. For example, when you position the control to zero phase, and then move a piece of ferrite around near the coil, the audio volume will not change. In other words you have balanced out to the ferrite. However, if you now lower the coil to real ground the audio will increase in volume. Of course this indicates that you are not balanced to the ground. As you begin to turn the control counter clock wise the ground adjust control phase changes from zero to a more negative amount. Once you have reached the point of “ground balance” the control and ground phases match. Of course as the coil is moved to various locations the ground phase changes slightly and you must readjust the control for a neutral reaction. As you can see there is no one control phase position that matches every condition since the ground phase varies from one location to another.
The introduction of the Motion detector solved this problem.....sort of. In a Motion detector design you can calibrate the “fixed” ground adjust control phase to approximately +0.5 degrees and set the audio threshold for silent operation. If that is done the detector will appear not to respond to the ground. In reality it is responding. Its just that you don’t hear it since all ground reactions cause the audio to decrease in volume.
And since the audio is already silent you don’t hear anything. Remember I said that all real targets, which includes the ground, have a phase between zero and some negative value. The preset ground control phase of +0.5 degrees is in a location where no real targets ever exist. Therefore, you never have a condition where you are balanced to anything, least of all the ground. As you move the coil over the ground, the internal detector signals are continually being driven negative. Any weak positive target signal is easily over-ridden by the huge negative ground signal. Of course, if the target is close enough to the coil its positive signal can override the negative ground signal and you will hear the reaction in the audio. The greater the phase and strength of the negative ground signal the more it will mask the positive target signals. A manual ground balance design would avoid this since the operator can adjust the control for a (near) neutral reaction on the ground.
For fixed machines the phase error between the internal “ground preset balance” and the actual ground condition can be much more than “slight”. The internal preset is calibrated for +0.5 degrees. This is in an area where a real ground phase never occurs. The actual ground phase may be -2 or -3 degrees “negative“. That’s a huge difference, maybe 2.5 to 3.5 degrees. This much phase error will in effect cutoff several inches of detection depth.
When fixed ground balance (motion) machines first came out I was opposed to using this technique. I knew it was in some ways a trick into fooling the customer that there was no ground balance. The control was simply a fixed internal adjustment. However, the pressure to compete in the market place was enormous. So, I eventually gave up the argument and designed a detector using a fixed ground balance, the “Big Bud”.
The standard loop size is the best size to use on a fixed GB detector since it was the coil most likely to which the detector was designed. I took extra care to try to insure that the other loop sizes meet the same characteristics as that of the standard coil. It is also true that larger coils pickup the ground more than smaller coils. So any phase errors due to a detector-coil mismatch will make this problem worse. The only sure way to get around this is by using a detector with a GB mode and a manual ground balance.</i></b>
