Coil size versus Performance?

I am curious as to how the size of the coil effects detector depth.  I am a technician with a strong electronics background and am curious as to exactly what kind of increase in field shapes a different size coil will have compared to my current one.

  About me...

  I use a Pioneer 505 with the 8in. and 4in. coils.  I was very very pleased with the performance of my Bounty Hunter 4in coil.  If any0one wants a review of this "prospector" coil, then I will just say that it exceeded  my expectations in every possible way.... period.  I found a silver quarter in a spot right outside my office door that I had gone over dozens of times over the past year.  It was up against a tall slab and in a whole lot of trash.  The 4 in coil snifed it out plain as day once I got right over it.  The only catch is that it takes three sweeps to eq

Linkback:

You are not allowed to view links.
Please Register or Login

http://www.thunting.com/smf/index.php/topic,21968.msg152447.html#msg152447

Tri City Ditty

This is the second time you have informed us of your "strong electronics background" and followed it with a question that sort of makes me wonder about that claim. The size of the coil changes the distribution pattern of the electromagnetic field at the rate of the distance squared, just like all radiation does. To generate a field in a larger coil requires MORE power, not less power, as AU suggests. You will also find that there is a practical limit to how large a coil you can use with any given machine. The larger coil gives you more depth as long as the power to the coil is adjusted for the larger area of radiation.

Feeding the same amount of power into a larger radiator simply diminishes the field strength but hey, it does make it larger. Not in the distance from the coil but in the overall area it covers. Most machines that can accept coils of different sizes react to the coil inductance for frequency and the impedance for power.

I've been beating a dead horse here for years trying to get people to understand that the resistance of a coil is a meaningless parameter by itself but your question illustrates the answer quite well. The inductance is the value of the coil in Henries. The impedance is a combined measurement of inductance, capacitance and resistance and also includes components you cannot see which are inside the electronics box of the detector. For this reason, design of coils for metal detectors is not a simple matter of wrapping wire round a form and measuring the resistance or even the inductance.

Linkback:

You are not allowed to view links.
Please Register or Login

http://www.thunting.com/smf/index.php/topic,21968.msg152771.html#msg152771

		Quote:			Posted by TriDitty
	Well 1950, I have no intentions of getting back into textbooks and discussing the differences between d/c resistances and a/c impedances.   The purpose for my post was to get a feel from more experienced hunters as to what to expect from changing to the 10 inch coil from the 8 inch coil that I have been using.   I have gotten a few very good insights into this so far.  I highly doubt that the manufacturers will be changing the impedance of the coils when I change to the search coil so I guess that I will be working with that slightly weaker field you mentioned.  Perhaps I can get an impedance meter and let you know if Bounty hunter does increase the load seen by the detector to compensate for the increase in field size.

Basically my response to you is over some apparent misconceptions you have over coil size. The subsequent performance issues of changing the coil are not minimized just because you don't want to break open those old textbooks.

If you just want to know about things from a detector user's view, do not ask with the preface of you being a technical whiz kid. Just ask without saying you are an experienced technician. When you do that it appears to me and a few others here that you are searching for a highly technical reply. If you want a user level reply, drop the chest thumping. Sound like a plan? Then you won't be overcome with technical advice.

Since you clearly don't want or need a technical answer, this is for the rest of the folks following this. And, contrary to what you may believe, there are many who read my technical advice regularly. My PM box overflows.

The answer about overall impedance being changed by manufacturers is that they DO change it with every coil that they fit to their machines. Some of them have an extra pin (or two) which indicate which coil is attached and adjusts the internal impedance as well. If you want to learn about the various types of machines out there and the manufacturing specifications, go to the USPTO web site and search for patents on metal detectors. You'll have several thousand to browse with all sorts of insights for you. You may even spot one or two of my own patents, mate.

Linkback:

You are not allowed to view links.
Please Register or Login

http://www.thunting.com/smf/index.php/topic,21968.msg152928.html#msg152928