Coil Radius

[RogerInHawaii]

What is it that determines the radius of the transmitter and receiver coils? Is it determined by the "radio" frequency that's used? Is it arbitrary, and any radius will do? Is the (smaller) radius of the receiver coil related or otherwise determined by the radius of the transmitter coil? Does the radius of the coils determine how sensitive the the unit is? Could I take an existing detector and build significantly larger or smaller coils for it?

[Carl-NC]

The overall radius is arbitrary. VLF coils from 3/4" to 24" are commercially available. Smaller is more sensitive to smaller targets but with less depth. Larger goes deeper but with less sensitivity to small targets.

- Carl

[Digger]

Size and shape helps to determine the detection field pattern. I believe the transmit and receive coils are bound by the same limits as a radio antenna. The are tuned to specific needs of a given detector.

[Frankn]

All the different size coils that work with the same detector have the same length antenna wire in them. The smaller coil has more wraps around it inside, but the same total length of wire. This creates a more concertrated signal in a more confined area resulting in a stronger, but narrower pattern. The larger coil has fewer wraps of the same length wire resulting in a larger coverage area at one time. The power output to either coil is the same with the exception of detectors that can be adjusted for coil size like the Whites with computer adjustments. In other words, the length of wire in the coil should be the wavelength of the transmitter frequency. It is possible to use a 1/2 wave length, but you will loose selectivity. Forget resistance, an antenna is only connected at one end. Basically what this means is Carl hit the nail on the head and I got a little wordy. Frank

[JohninNC]

The size is arbitrary, but you will find that as you wind a smaller coil you do not use exactly the same amount of wire as a larger coil. The important factor is the electronic characteristics like resistance and inductance. The resistance is not as important but should be close. I have found that a smaller coil takes a bit more wire in the send coil to get the same inductance as the large one. By the same token since you have a bit more wire, the resistance will be a bit higher. The receive coil typically uses a much smaller gauge wire so it becomes really important to measure your inductance to get a match, and then there is the resistance, and then you get to tune it for resonance. You get the idea.