Metal Detector Search Coil - Antenna Or... ?

[Steve Herschbach]

9 hours ago, MikePfeiffer said:

Basically you want the antenna tuned (impedance matching) to the TV for best reception. Same applies to the metal detector.

Detector coils are not antenna. They are part of a highly tuned inductive coupling system.

[Tye]

2 hours ago, Steve Herschbach said:

Detector coils are not antenna. They are part of a highly tuned inductive coupling system.

Whatever, I am sure we will know the details soon.

The detector coils are most certainly either a set of transmit receive anntenas or a single antenna doing double duty .... the detector signal is transmitted into the ground and the disturbance to that electrical field is received by the Antenna (coil)... with out an antenna nothing would be transmitted....or received by the detector....

Also, impedance matching is important to enable the flow of electrical energy but that is only part of the picture...the coil must also be electrical tuned to the operating frequency to reach full transmission power.. 

Transmit and receive antennas are an inductively coupled affair ....and that's why tunning them is so important...otherwise nothing gets induced...

Cheers Tye

[Geotech]

Detector coils are not antennae, they are designed as a transformer. There is no EM propagation, only a local magnetic field.

The coils don't have to be impedance-matched to anything. Most RX coils are just connected to a high-impedance preamp, with some token load resistor (typically 10k). Some designs throw in a few extra components to reduce EMI.

[Tye]

If they transmit/induce  an electrical field into the either buy design ... that is the very definition of an antenna ...not To mention the fact that they are tuned to the output of the detectors transmitter... a local field is propagation .... it's just not going very far ....

Cheers...Tye

[Geotech]

That's just it, they don't. A detector coil is no more an antenna than the transformer hanging on a power pole.

[Tye]

Sorry....but transformers on power poles do not radiate electromagnetic energy by design .... that is the big difference .... there is a little leakage however

Thanks for the info about how the coil is impedance matched to the output of the detector....that's pretty common in other sorts of electronics....

 

Cheers

[Geotech]

And metal detectors don't radiate electromagnetic energy by design, too. Only a local magnetic field, just like a transformer. We do our best to suppress the electric field with shielding.

[Steve Herschbach]

Tye, in case you do not know, Geotech is Carl Moreland, currently Engineering Manager, First Texas Products (Bounty Hunter, Fisher, Teknetics). He is the long time owner of the top notch Geotech website and co-author with George Overton of this book...

Inside the Metal Detector explains theory and offers numerous experiments and projects that demonstrate the theory. You can build an off-resonance pinpointer, a GEB-discriminator, and a microprocessor-controlled PI detector. Even if you're not inclined to build a detector, the concepts learned from ITMD will help you better understand how your own detector works and what all those controls are really doing. The book can be purchased in paperback or Kindle versions on Amazon

For what it is worth I have in the past talked about coils "being like antenna" or "basically being antenna" so am guilty of spreading that line of thought myself. The analogy works in some regards but is not technically correct so I am trying to clean up my act in that regard.

Here is a great article by Carl - Coil Basics

If you think about what "induction balance" infers it is a system in perfect balance, neither transmitting or receiving but creating a magnetic field.

[MikePfeiffer]

Steve,

Thanks for telling Tye.

Carl,

Thanks for replying. I was mentally building more into the circuit than you are describing, However, I do not fully understand. Why would you not impedance match and get all the signal strength you can. What about the TX coil. Please don't tell me it is just 50 turns of wire attached to a 3904 transistor.

(Sorry Carl, I think I got it. Your just ringing a coil with a specified frequency.)

I guess I am going to have to go back and reread your book again.

Mike

[Steve Herschbach]

See the link to the article by Carl I just added to my post above Coil Basics and the Geotech website itself is loaded with do-it-yourself metal detector projects.