Detector Coil Signals.

[jeff of pa]

Because of a Question by rcasi44 Elswhere Here it has Made me Think.
(I know Dangerous precedence here)

We all know a Signal from a detector coil Goes UP as far as it Goes Down.

IF there were a way to Block the signal from Going UP,

Why wouldn't the Signal going Down Increase ?

 

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[Captain Trips]

No, the signal going down wouldn't be changed by blocking the one going up. What you need to do is reflect the upward signal back down, THEN there would be twice the signal going down. Of course, depending on the location of the reflector, you would have a phase differential to deal with.

Think of a light bulb -- it radiates in all directions. If you place a black sheet of paper on one side, thus blocking the light from going that direction, there would still be the same amound of light going the other direction. What you need to do is place a mirror on one side to double the light going the other way.

 

[Monty]

I don't think we are dealing with light waves here, rather ultrasonic waves or radio waves if I understand the basic mechanics. But maybe I don't, I just never saw any light coming out of my coil. I don't think a mirror would do anything except maybe shield the up signal a bit? An interesting question, but I think it would take much more engineering than just a mirror? The question marks are because I am not stating this as fact, just my deduction which we all know is a little less than 100% accurate! Monty

 

[jeff of pa]

No actually you can Detect through a mirror.

I think what C T is suggesting is the Equivelent of a Mirror in Theory,
whatever that could possable Be ?

 

[Captain Trips]

I don't think we are dealing with light waves here, rather ultrasonic waves or radio waves if I understand the basic mechanics. But maybe I don't, I just never saw any light coming out of my coil. I don't think a mirror would do anything except maybe shield the up signal a bit? An interesting question, but I think it would take much more engineering than just a mirror? The question marks are because I am not stating this as fact, just my deduction which we all know is a little less than 100% accurate! Monty

Well, Monty, actually we ARE dealing with light waves here. Metal detectors use radio waves (not ultrasonics) which are basically the same thing as light waves. (Light is simply radio waves of an EXTREMELY high frequency.) I was using the light and mirror situation as an analogy, though. A mirror wouldn't work at radio frequencies. Although light and radio waves (and microwaves, infrared, ultraviolet, and even x-rays) are composed of the same thing (photons), the difference in frequency does make a difference in how they act. For example, radio waves will go through a wall where light waves won't.

Think now of an old-fashioned T.V. antenna. (Yes, pre-cable.) The kind that was on rooftops. They were made up of several pairs of metal rods. Only one pair actually was the antenna. The others were called "reflectors" and "directors." Or, let's think of a satellite dish. The actual antenna is that little piece of metal held up over the dish -- the dish is the reflector in this case. For a metal detector, we would need something of the nature of the reflector rods on a tv antenna, or a dish on a satellite receiver. The exact shape and nature, though, would need to be designed by an electronics engineer, and would depend on the type of md you are using. A PI detector would need a different type of reflector than a VLF, or a BFO.

There I go again, into the world of electromagnetic physics. But that is the science we depend on when we use metal detectors. To design them requires some hairy mathematics. Fortunately, it's much easier to use one than design one.

 

[snorton750]

Just my 2 cents added. if our machines were working in the range of 144mhz the reflector could be around 39" and the director slightly less at around 37", however at 6khz the reflector would be much much larger. In fact, I dont know the exact nrs for 6khz, but 7mhz would be in the range of 20meters long. all these nrs are using 1/2 wavelength dipole as a reference. As I have never seen the insides of a detector coil, I dont know if they are actually a 1/2 wavelength at the frequency used. If it is thats gonna be on big reflector. if you used a round metal plate above the coil to reflect the rf , it also needs to be a specific distance from the loaded coil and this also would be very long in distance. A metal plate that was grounded would just absorb the energy that it captured. so making a directed coil would be quite an engineering acomplishment. Sorry for the long winded run-on but as a ham radio operator I have already tried to figure this one out. NO JOY. If someone else has a different idea on the subject I would love to hear it. thanks and good luck HH Stan...

 

[Captain Trips]

Well, I did say it was possible. I never said it would be easy. Nor feasible. In fact, it seems to me that a better way to detect deeper would be to use a higher power coil than using a reflector.

BTW, you say your numbers were for 1/2 wave dipoles. What about other multiples? 1/4? 1/8? (Again, probably not practical, nor very efficient.) I admit, antenna mathematics were never my specialty. I understand the theories, but never learned the actual formulae.