Help me out here. If you have 2 frequencies say 1 at 3khz and one at say 50khz, would the 3khz have better ground penetration? All ground conditions being equal, since the 3khz has more a wider band than the 50khz, at the same amplitude wouldn't the the lower frequency travel farther?
Not sure what you mean by a wider band. Do you mean bandwidth? Bandwidth generally describes the width of a signal when modulated. As an example (I am referring to analog signals here), your AM broadcast radio has less bandwidth than your FM higher-fidelity and stereo sound. Since the FM stereo hi-fi signal contains more information that needs to be decoded, it takes up more space in the spectrum to contain all that information. A television signal takes up even more space...it is 6 MHz wide (including the audio).
Let's backtrack a bit. What does a metal detector do, and how would we want to optimize it (in a basic technical sense...not lots of fancy indications and bar graphs and the like)..
The search coil basically radiates an electromagnetic field that rapidly switches polarity at
some frequency. The coil is being fed with an AC voltage. The magnetic field then radiates in a conical shape below the coil. When it strikes a metal object, it induces currents in the metal, which create a magnetic field of their own. This is the field that is detected and processed by the processors in the detector.
In my mind, this means two things need to be optimal.
1) The frequency of the AC feeding the coil needs to be such that the ground looks "transparent" to both the transmitted and received signal. The ground
absorbtion or
reflection is the lowest possible. After all, we aren't detecting the ground, but objects in it.
2) The frequency used may have to be different for different types of ground conditions, due to their different absorbtion (and reflection) characteristics. (Mineralization)
I think that there is probably an "average for most soils and ground conditions" that has been experimented with trial-and-error over the years.
And with the automatic ground balancing that is on the detectors now, the frequency could be changed for best results on the fly.
This is an interesting question. I, too, would have thought that a lower frequency would penetrate the ground better, they're used for submarine communications after all,
That is true, but you are talking about transmitters with a hundred kilowatts of output power feeding antennas that cover entire states. Not really comparable to metal detectors and their weak signal strengths. Plus, maybe those frequencies are best for communicating through the entire planet. (Low absorbtion & reflection)
I will close by describing an example of different frequencies behaving differently with some equipment I have worked with before. I work with industrial lasers, and in a past job the company used a type of laser called an excimer laser. It produced a beam of ultraviolet light (electromagnetic radiation) at a wavelength of 248 nanometers. The mirrors used with this laser were made of magnesium flouride. The mirror was fully reflective to the UV laser; yet you could see through it like it was a piece of glass. An example of how a material can be reflective at one wavelength (frequency), and transmissive at another.
I think in general the ground is similar in it's behaviour to the best frequency to use for metal detecting. It varies.