Hello forum,Like I say i did some test using two coils .one to transmit and the another to receive Eric give the name of CONCENTRIC coil.
The big one is 11" for transmit ,inductance is 330 uH (the same for the mono coil) and the smaller one is 8 inch inductance is 1000 uH o 1 mH.
First I adjust for both coil the R damping for no ringding it give me 380 R for the trans. and 1 K 1 for the receive,then i installe and balance physically the two coil one in the center (R) of the another (T) with the best geometric i can,for that i use now a nice test coil it is a form board for designer , you can make your circle o D ,cut with Xknife and with a solder pen you burn a little to make a small channel where yours coil wires goes,with this technique you have a exact coil diameter with it center.
For my test in the workbench i simulate a hot mineralised ground with a red brique ( like you use in construction house) and I have also a collection of hot rock .....
With the detector and a 11" mono coil i have a treshold sound for a nickel at 30 -32 centimeter and a repeatable good signal at 27cm with indication in the signal meter...AIR TEST !!!
with the concentric coil (T/R) ...25 cm ...air test .
and for detecting the red clay the concentric give me 3 cm down in comparaison with mono.Not great for me , but to give me a better result i make the concentric coil (with my PVC techniK) and go to check in the field because you can see the real thing far from the electricals interferences....It was the same result , in a hightly mineralised ground ( i live in a silver mineralised mining zone) the CC give me a LITTLE more signal just perceptible in the Smeter, but NOT the claiming of many publicity coils advertiser....just a little better two ; three cm for a nickel and if you see the weight and the two embarassing cables ,two connectors coil and the switch in the box i prefer the mono coil, and also years ago i make the 8 configuration and many forms with T center etc... for the same not many improvment ....
In the hightly mineralised ground your signal for a nickel shut DOWN to 15 ,18 cm for a nickel ,yes at 250 mA of power transmit coil.......
I live also in the pacific coast ,here it is different some sand beach are complete neutral , your AIR test is the SAME as the GROUND test, but also you have mineralysed black copper sand with mica , and adios your search you shut down to 10 cm for the nickel.........and IT IS the TRUE reality , i check with more and more power to go to 500Ma transmit coil and nothing because in this time you receive also the same magnetised ground ............
For my detector i worked with the Basic Foster design front end (709 or 5534) with 2K2 input R -TX width 100uS,pulse delay variable from 10uS to 80uS Fcy is 660 and 1200PPs ,Sample is 20uS -MosF 740-two channels with two samples like the Goldscan design with manual pot for gnd cancel (it is the only effective design for working in a hightly mineralised ground like here in Mexico) treshold audio stage and Smeter signal energetised power with the very effective and old design of the pump charge 555 two transistors boost of the GS ( +6V - 6V , -12V, ground is the positive path) or direct for 740 coil ) and for close this door ; a pack of Nimh of 14V4 ( 12 of AA ,1600 Ma ) .Alex
Ps: when you adjust the coil receive in the coil transmit you must see with oscillo the signal , if it is inverted you must invert the receive coil turning in the other side and the signal reappears inverted allright...