Round vs Elliptical DD

Maybe someone out there can shed some light on this subject for me.
With mono coils, if you have a 18"x12" coil, the rule of thumb is that you add the 2 figures (18+12=30) and then divide by 2 (15). So a 18"x12" ellipical coil would have roughly the same depth as a 15" round coil.
I am questioning whether this rule applies with DD coils as the ellectromagnetic field is in the center of the coil and the outer windings don't appear to contribute to this field. Therefore, if the outer wiring does not contribute to the field, then the "mono" rule shouldn't apply and a 18"x12" elliptical DD coil should have the same depth as an 18" round coil and not 15" like for the mono. Can anybody verify this or has anyone done any tests to speak of?

hi mate
the mono rule does not apply
but a 18x12 dd will not have the same depth as a 18 round because

the windings in a 18x12 dd are roughly 18x6.5 each side
the windings in a 18 round dd are roughly 18x9.5 each side

so the 18 dd round recieve windings being a bit bigger should hear deeper targets. (the recieve windings are the most important)

this is just the way i understand it and would welcome any input from some one that knows better

cheers fencejumper

Thx for that answer fj. I was under the impression that it was in the centre that the send went out and the receive picked up, therefore the outer wiring dident matter, but it's all my theory, hence I post the question. Using your method though, if we use the 18x12 coil example, mono is roughly equivallent to a 15" round, what is the the 18x12 DD equivallent to in round?

not sure mate as i don't believe in that theory of adding and dividing


i.e if you had mono a coil 100 inches long and 6 inches wide..i don't believe it would go as deep as a 53 inch round

cheers

FJ, what you say makes sense. In your example, I would say that coil (100"x6") whould go slightly deeper than a round 6" mono, say equivalent to a say 8"round mono, except coverage would be more with each sweep.
Just has me wondering, that's all. Anyone else have any theories (or facts) on the subject, especially for the DD?

A round Mono coil will go deeper than an out of round coil as per recent discussions on this forum. A DD coil is basically an elliptical design from a transmit point of view, the signal is generated in the centre of the coil because that is where the transmit and recieve windings cross, they create a blade pattern receive area from tip to toe following the cross over point of the windings.

Receive is everything to a PI, a DD runs quieter because the proximity of the second winding nulls things down thereby helping to ignore mineralistion, however due to the receive windings being smaller (to make room in the housing) they will have more turns to get the inductance right, the more turns of wire in the coil the better the sensitivity especially the receive winding. As an example an 11" DD is essentially two 5" elliptical Monoloop coils crossing each other in the center, as you can imagine a 5" monoloop could be pretty sensitive going by reports of the new 8" Nugget Finder monoloop, but because of the second winding they are not quite as senstivie as a comparible Mono.

Hope this helps,

JP

Hey Fencejumper do you mind removeing or replacing your Avatar. I find myself looking at her more and not reading your posts. I can't concentrate.
Wombat

Thx for taking the time to explain JP, it does makes sense.

If we go with your explanation that an 11"DD is esentially 2x 5" monos (TX & RX), then I'm only guessing that the depth of an 11"DD will be slightly more than an 5" mono (say 25% more due to the concentration of the field at the crossover), therefore an 18"x12" is esentially two 18"x6" monos so the depth of that would be, if we used the same formula, about 25% more than an 6" mono. I understand that the depth of an 18"x12" elliptical would be more than a 12" round, but by what percentage seems to be an unknown hence the "rule of thumb" of adding the 2 diameters together and dividing by 2. I understand using FJ example though that this is not always the case.

A formula can possibly be worked out using the different radius points of a coil at all the different degrees, that would cater for the longer elliptical as well as the rounder almost oval elliptical, but I think it's something that needs alot of thinking time devoted to it, & I doubt the misses will allow me that luxury right now.

So if we take all things talked about into account and put a little guessing into the equation, the reason behind my question is I'm trying to see where I'd be better off, staying with the 11"DD round or investing in an 17"x11"DD or a 14"DD. Generalising a little here, but the 11"DD will have let's say 25% more depth than a 5"mono (from the last paragragh) and the 17x11 will have the same plus a little due to the elliptical properties.

I suppose what's next is the sentitivity issue, as a recent test that I did comparing an 18"DD to an 11"DD to a 1/2g lead buried at about 3". All other factors where constant. The 18" totally surprised me by when it virtually didn't recognise that the target was there, but the 11" screamed it in. Hence the need for an "inbetween" coil. So the next question is whether the 18"x12" example that we used above would be more sensitive than the 18"DD round, or a 14"DD round. Personally, I think it would be than the 18" but not sure on the 14".

CostasDee wrote:Thx for taking the time to explain JP, it does makes sense.

If we go with your explanation that an 11"DD is esentially 2x 5" monos (TX & RX), then I'm only guessing that the depth of an 11"DD will be slightly more than an 5" mono (say 25% more due to the concentration of the field at the crossover), therefore an 18"x12" is esentially two 18"x6" monos so the depth of that would be, if we used the same formula, about 25% more than an 6" mono. I understand that the depth of an 18"x12" elliptical would be more than a 12" round, but by what percentage seems to be an unknown hence the "rule of thumb" of adding the 2 diameters together and dividing by 2. I understand using FJ example though that this is not always the case.

A formula can possibly be worked out using the different radius points of a coil at all the different degrees, that would cater for the longer elliptical as well as the rounder almost oval elliptical, but I think it's something that needs alot of thinking time devoted to it, & I doubt the misses will allow me that luxury right now.

So if we take all things talked about into account and put a little guessing into the equation, the reason behind my question is I'm trying to see where I'd be better off, staying with the 11"DD round or investing in an 17"x11"DD or a 14"DD. Generalising a little here, but the 11"DD will have let's say 25% more depth than a 5"mono (from the last paragragh) and the 17x11 will have the same plus a little due to the elliptical properties.

I suppose what's next is the sentitivity issue, as a recent test that I did comparing an 18"DD to an 11"DD to a 1/2g lead buried at about 3". All other factors where constant. The 18" totally surprised me by when it virtually didn't recognise that the target was there, but the 11" screamed it in. Hence the need for an "inbetween" coil. So the next question is whether the 18"x12" example that we used above would be more sensitive than the 18"DD round, or a 14"DD round. Personally, I think it would be than the 18" but not sure on the 14".

An 11" DD will not provide the same outright depth as a 5" Mono thanks to the performance loss due to the proximity of the second winding (although technically the DD should have more receive surface area than the Mono so not sure on the acual mathmatical equation for this scenario (not sure if the Tx winding actually plays any part in the receiving). A straight out 5" mono in DD mode (or Mono mode for that matter) will obtain greater depth than a 11" DD with its 5" Tx and 5" Rx windings. Generally speaking there is a 20% to 30% depth reduction in outright depth across the board (depending on ground conditions of course) when going to a DD coil of similar external size to a Mono due to two factors, smaller out of round Tx/Rx windings and the reduction in depth due to the crossover of the two windings nulling the coil somewhat.

There is no concentration of field at the crossover point, a DD coil transmits just like a Mono (is essentially a small out of round Monoloop coil) and then manifests its response where the Rx coil passes over/under the Tx winding. If you want to trick the detector into receiving across the whole coil when using a DD (GP series onwards) then use the Mono mode but there is a further reduction in outright performance when using this method.

Hope this helps,

JP