IB coils - if they're not quite right...

[Jeff Kinzli]

So, working on building a coil for my Whites IDX. If I don't get the coil just right, i.e. the L isn't on, or impedance is too low/high, what would I expect? Relating this to antenna theory, the VSWR would be high (mismatch) and hence radiated power would be low, and lots of reflected power back to the transmitter. Same on detectors?

[Reg]

Re: IB coils - if they're not quite right...

Hi Jeff,

Probably the most obvious thing you will notice is it will not work. The coil has to be tuned to the same frequency as the factory unit. If it isn't, the ground balance will be way off and if you have an external ground balance control you may or may not be able to adjust it in so it will work. It depends on how far you are off.

You will probably see an increase in current draw if you are off. However, trying to use current draw as a reference could be a mistake. I am not sure if Whites tunes their coils right on the operating frequency or not.

Normally, only the xmit coil is tuned, but on your detector I am not sure about the receive. I would suspect it is not tuned but don't know for sure.

You might take a freq generator and feed the xmit coil with a resistor between the coil and the generator and watch for a peaking of the signal across the coil with a scope as you sweep the generator frequency.

Another technique would be to pulse the xmit coil with a brief pulse via resistor and observing the ringing of the coil to get the proper frequency.

Matching xmit inductance isn't as critical as matching the resonance frequency of the coil. Normally increasing the inductance of the xmit coil while keeping the receive coil the same will result in a reduction in sensitivity. How much of a reduction is a function of the turns ratio, coupling, etc.

Reg

[Carl]

Re: IB coils - if they're not quite right...

As Reg said, the most important thing is resonance tuning. I believe that on White's, the TX side is a Colpitts oscillator so the TX coil inductance+capacitance is tuned for the operating frequency. On most detectors, the RX coil is also tuned to minimize noise, but I'm told that some manufacturers tune it slightly off of the TX frequency. If you don't tune it, it should still work but might have lower sensitivity.

M/D coils don't behave as classical antennas so VSWR and matching are not an issue. They are really just plain old air-core inductors and we utilize the near-field effect.

- Carl