Can a resister be added to correct frequency on a coil?

[kac]

Trying to design a couple of concentric coils and I can't seem to get the wire size to match up for the correct frequency. I first pick a course 25 awg for tx and set number of winds to bring the inductance in range but can never seem to find the correct guage for the Rx.
Curious if I could simply use a small pot or resister on the Rx to bring it in spec.

These are for Tesoro Delta machines.

Thanks,
Ken

[Carl-NC]

"I can't seem to get the wire size to match up for the correct frequency.." What do you mean by this? In most cases, the RX wire gauge makes little difference.

[kac]

The Tesoro Delta coils have existing ranges:
Tx 1.117-2.71 mH
4.1-4.5 ohms
Rx 18.98-21.75 mH
61.7-74 ohms

I have been using those specs as a starting point for my own coils. If I pick a wire size for Tx and get the mH in those ranges my Rx resistance is incredibly high to if I get the Rx in it's range.
I have a precision awg guide that has resistance for copper which gives me a bit of a ball park so I'm not wasting copper.

[kt315]

did you get a data from DD coil and want to use them to do a concentric coil? did i understand you right?

[kac]

The widescan and concentric coils have similar numbers.

[Carl-NC]

You give a very wide range for the TX inductance: 1.117mH - 2.71mH. It should be much tighter than this.

Here are the numbers I would use for designing a compatible coil:

TX: 1mH @ 4Ω
RX: 15mH @ anything less than 100Ω

Let's say you want a 12" (300mm) concentric. A 12"/300mm TX coil will need about 33 turns to reach 1mH, and the wire length is about 104 feet (31.6m). 25AWG gives you 3.35Ω and 26AWG gives 4.23Ω. Since this does not include cable resistance I would use 25AWG. Also, for a concentric the bucking coil needs to be accounted for so more turns are needed and likely 24AWG.

For the RX coil, let's say we're going to copy Tesoro and make it smaller than 1/2 the TX diameter, make it a 4" (100mm) coil. You'll need ~250 turns which has a wire length of 262 feet (79.8m). Let's say we want 50Ω, the wire needs to be 32AWG (43Ω). 36AWG will give you 108Ω which should still work, and any wire larger than 32AWG will only decrease the RX resistance which will still work, but just make the coil heavier.

[kac]

Thanks Carl.

Numbers I found were based off of these coils:
10x12 ws, Tx 1.21mH @ 4.4 ohms Rx [email protected]
8x9 c, Tx 1.235mH @ 4.5 ohms Rx [email protected]
5.75 ws, Tx 1.117mH @ 4.2 ohms Rx 18.98mH@74ohms
5.75 c, Tx 1.198mH @ 4.1 ohms Rx [email protected]

If I have trouble bringing the resistance down can I toss a trim pot in there or resistor?

[pito]

Just wind 70 turns. 0.5 mm wire for Tx and 200 turns 0.2mm wire for Rx. Whatever resistance you get leave it as is.
Then add capacitors to bring coils to resonance on frequency you are using.
Adding resistor will reduce Q of the coils.

[Carl-NC]

My particular 10x12 measures

TX: 0.93mH @ 3.9Ω
RX: 15mH @ 57Ω

Might be one of our meters is off a bit. In any case, no, don't add a trim pot. TX coil resistance is the important one, try to achieve 4Ω. The RX resistance is far less important, but try to keep it below 100Ω.

Just wind 70 turns. 0.5 mm wire for Tx and 200 turns 0.2mm wire for Rx. Whatever resistance you get leave it as is.
Then add capacitors to bring coils to resonance on frequency you are using.
Adding resistor will reduce Q of the coils.

Number of turns depends on what size coil he wants to make. It far easier (and preferable) to get the TX coil right so the TX frequency is correct, without having to pad with caps. Besides, if you over-wind the TX coil (Ltx is too high) then adding caps is going the wrong way.

[pito]

you over-wind the TX coil (Ltx is too high) then adding caps is = then you replace capacitor with lower capacity capacitor.
Tx, 20 cm coil and 30 cm coil, both 70 turns.
Bigger coil will produce stronger EMF, draw less current, and the sine wave will have less distortion.
This is the result of building and testing many many coils, some of them on picture.

[Carl-NC]

If kac is wanting to build a coil for a Tesoro detector and still use the other coils, then replacing the cap is not an option. You build the coil to fit the detector.

[pito]

correct

[Emersonpaz]

If you want a ready-made coil, you can use a doTianxun TX 850 sold on Aliexpress. I bought it once and even did a test using it on SMW (Serbian Most Wanted) the specifications are similar.

[pito]

Low turn numbers coils have low Q, the purpose is to increase the temperature stability of the LC resonance, so +/- 10% inductance/resistance doesn't make much difference.
Doubling the power of Tx is increases sensitivity by just a few cm.

[kac]

I have a lot of small processed iron in the grounds here. My goal is to make fairly large concentric coil that is not overly sensitive. This will allow me to use the threshold discrim and trim out the iron and reveal other targets above and below. Much how their stock 8x9 works but on a larger scale and with less separation. Large dd coils such as the 10x12 widescan tend to mask out.
I am not looking to pull out trimes at extreme depths but rather this would be for larger coins in the bronze and silver range as well as old relics that are bronze and brass. I figured something between 12-14" in diameter will get me in the ball park.
Tejon is a relatively deep machine but I don't expect it to go much past the 14" range in my area as no ib machine can.

I did use the awg precision wire specs that have wire diameter and resistance over 1000ft and put that into a spread sheet so when I enter wire size in the coil calculator and number of winds I can also get a good idea on the resistance. Would be nice if the coil calculator had that built in but no big deal.

I will try to get the Tx spot on and now that I know the Rx isn't as finicky will work on that.
I would a heavier guage wire on Rx give me less sensitivity? How low can I go on resistance on the Rx?