Hi Lionel,
For use in a PI detector, the coax contributes resistance and capacitance to the transmitter circuit. The r.f. characteristics of coax do not come into play at the frequency components of the TX and RX signals. The forward and return path in the coax adds to the coil resistance and serves to limit the TX current. However, there is often some additional series resistance added on the pcb to limit the current and also improve the coil time constant (L/R). The resistance contribution of the coax is therefore small for the lengths involved in treasure detectors. The capacitance is about 100pf per metre, so for the 2.25m used on the Deepstar, there is an additional 225pf in parallel with the coil self capacitance, plus any additional capacitance to any grounded screen. All this capacitance lumped together tunes the coil inductance to a certain resonant frequency and in the absence of any damping resistor, would be clearly seen as a ringing waveform at the end of each TX pulse. As the pulse delay becomes shorter, for higher sensitivity to small objects, this capacitance becomes more critical and the coax has to be shortened and/or the coil inductance reduced. This is because, for best signal, the TX switch off must be better than 5 times shorter than the time constant of the smallest object you wish to detect. The switch off speed is governed by the damped self resonant frequency of the coil circuit. Critical damping gives the fastest rate, but generally the coil is slightly over damped.
The other thing to watch for with coax, is that the braided screen can give an eddy current signal which can cause false responses. This usually occurs where the coax enters the coil shell or former. Moving the coax about can give false signals. This is why I recommend spiralling the coax fairly tightly up the shaft and securing near the coil with a Velcro tie. Some coaxes are better than others in this respect. Even RG58 varies from different manufacturers. Belden RG58 seems to be one of the best. I test it by passing a straight length over the search head of a working detector. If it doesn
