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Originally Posted by Roberts
I will need to see your loop choise ready here, so later to design rx circuitry according it.
Deal..?
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I don't know how to design a loop. This is the reason I asked you. The loop transmitting antennas I have seen in this forum are generally similar to metal detector coils with the Faraday shield removed. I doubt any radio engineering went into these coil transmitters, but they have been experimented with for a long time to find the best operation through trial and error.
While I don't know how to properly design a transmitting loop antenna or receiver, I do know some basic parameters that the system should maintain:
Since this is a field unit, the entire transmitter and receiver must be lightweight and portable, and should run on battery power for at least 4 hours operation before the batteries are discharged. The largest coil should not be more than 30 cm diameter. The preferred frequency is between 40 KHz and 160 kHz because this is a range that has shown the best stray noise immunity for this application in actual field testing. But 455 KHz will also work if necessary.
I believe the idea is to transmit a modulated carrier with the transmitting coil, then try to detect minute fluctuations in the received demodulated signal. The fluctuations may be seen as phase shifting or amplitude fluctuations, and maybe other very small artifacts in the received demodulated audio square wave. In this kind of transmitter and receiver, the receiving coil is carefully fixed in a position where there is no magnetic coupling to the transmitting coil. The only received signal should be an RF signal that is returned from the transmitting coil by reflection or other methods.
The received signal can be compared to the transmitted signal in order to find fluctuations. The receiver could also be watching for changes over adjustable time constants to find phase shifts or other irregularities in the signal. I suppose there are many methods to find fluctuations, and I don't know which is best.
I believe the transmitter is not so critical as long as the frequency and the power transmitted remain constant. I would think less than a watt is adequate for testing purposes. The receiver is the critical part. It must discern minute fluctuations in the signal, and must also be directional enough to find the source of the fluctuations.
I don't mind building the coils and transmitter and receiver, but I really don't know how to do it with any expertise. The only thing I know about this is any apparatus should be built without any metal parts near the coils, especially iron. If you can give some instructions of how to construct the device, then I can start making the coils. What I need to start is the wire size, and lengths, how many turns, and what kind of insulation. After the coils are ready, then we can start building the circuitry.
Best wishes,
J_P