SEARCH COIL TANK CIRCUIT E.M FIELD

[DOOLEY]

hello all,

i have a question about tank circuits / search coils.
which would "emit" the most E.M field.

so, here's the criteria :

if the sine voltage is fixed at 10v peak to peak,

and the driving transistor is current limited by a resistor,
to 10ma,

and the frequency is also fixed,

is it better to have more inductance ( more turns ) and a smaller capacitor,

or less inductance ( less turns ) and a larger capacitor ???

going to be building a DD soon and thought about playing around with the number of turns,
and then altering the capacitor value to get frequency right,

just wondered if more TX turns would be an advantage.

while on the subject,
when making a DD, is it better to have both TX and RX of a similar inductance,
or can the the RX be ALLOT higher ??

only ask as most DD'S projects ive seen are more or less the same inductance for both
TX and RX,
is this essential ??
i'm thinking it's poss something to do with the ability of a DD to cope with
mineralized ground ??

or am i barking up the wrong tree ??

thank you in advance for some concrete info.
cheers.

[Silver Dollar]

Well for TX amp turns equates to the field strength so less turns at the same current is best.
IE 10 turns at 10 ma = 1 ma/turn < Stronger field
20 turns at 10 ma = 0.5 ma/turn

RX is a different animal. Generally speaking more turns is better but, as more signal is picked up,
more noise is also, so there is a limit to how much is enough turns. The stock setup with TGSL
is 100/105 TX/RX turns but most have found the Minelab coil works much better which is
approximately 45/170 TX/RX turns. The TX gets more field strength and the RX gets more signal!

If you center tap the RX and run a differential front end you can use more windings and still have
low noise. My super TGSL does this but I haven't completed the mods yet to prove it works!

[Gwil]

For a given frequency and maximum voltage, the Tx coil field magnetomotive force (in amp turns, or just amps if you prefer) is inversely proportional to the number of turns of wire, or nearly so. Which means that fewer turns means more mmf, but lower coil Q and more battery current consumption.

[kt315]

the Minelab coil works much better

greedlab uses preamps in coils. ALWAYS. remember that. they are pressing contestants by this trick. the preamp gives MORE depth!
they do not use even shielded cable because think there is not need! SIC!
WHY WE CAN NOT USE PREAMPS IN COILS TOO? why we can not able to choice effective schematic and make
universal preamp coil board for everybody wishing?

[sinclairuser]

i agree kt, you would think a universal preamp "block"(for p.i. at least), would be a must have and an easy task for the wealth of minds on this forum, yet it seems no one cares.
all we see are pages and pages of math, simulations and relative noise scenarios, my industry calls this "vapourware", ordinary joe hobbyists just call it b.s.