where's the DD???

[grungymike]

Just finished reading for around the 5th time bbsailors fascinating design article on building a PI mono coil for the hammerhead.

I'm wondering if/when the article on DD coil design for the hammerhead II might be released by the same author?

Can anybody reccommend any other articles on the design and construction of DD and/or concentric coils?

Was also wondering if a dual coil design is really that much more effective in mineralized ground, as opposed to a well shielded mono.

Any thoughts ?

[homefire]

Yea! I read it too!

BBS is spot on the money.

It is better on hard ground but the Concentric is better on the Deep!

I think you would be better to follow the leeds on the Concentric as it's more complete.

His paper goes into it most well if you read it.

but others have gone further.

[Reg]

Hi Mike,

If you have read the article on a mono coil, many of the same concepts hold true when trying to build a DD coil. Now, one of the differences is the fact that you simply build two coils rather than one.

Mono's only use a single conductor coax, but a DD requires a dual coax for the best results. Some of the better video cables work just fine for a DD coil wiring.

I have found it is best to keep the two coils isolated, meaning both coils are shielded, but insulated from each other. Since each coil has a ground connection, then the shield for that coil connects to its ground or coax shield.

Now, remember that on a DD coil, you have to have a separate damping resistor on the receive coil also. If the pc board provides for that, then fine, but I have found it to be best to simply add the damping resistor inside the coil across the receive coil.

A DD coil has a fundamental design of two elliptical or D shaped coils that overlap a small amount. Fortunately, on a PI, the overlap is not as critical as it is on a VLF coil. So, one can be quite sloppy in the design or the amount of overlap and the coil will work just fine.

A DD coil will have less ground response simply because of the coil itself and its natural ground canceling features. In other words, part of the coil(s) generate a response that effectively counters some of the ground signal, thus reducing the overall effect of the mineralization.

Now, as for depth capability, there is a difference between that of a DD and a mono but that difference really occurs when trying to detect objects over a certain size. The DD generally does better on the really small stuff such as gold nuggets in the few grain range. For beach hunters, this means real small gold earrings or gold chains will generally be detected better by using a DD coil.

When a detector such as the Hammerhead doesn't have any ground balance feature, then the DD coil provides some relief when trying to use the detector in really mineralized ground.

So, for someone getting started, it certainly doesn't hurt to build a DD coil and compare it to a mono of similar size just to see what really happens. What will be surprising will be how well the DD really works.

Reg