Attn. John Kah and Eric Foster

[Anonymous]

Regarding the cable used for PI's. I would like to share a different trick with you and get your feedback as to what you think of it. Looking at the length of coax that is needed for a detector to be used as a hip mount pushed me in the following direction. RG58U has 28.5pF per foot which is added to the coils capacitance. (RG59U is a little better at 20.5pF per foot) The rated impedance of the cable is of no concern at the frequencies a PI uses. The capacitance is what slows things down.
I settled on using RG174U which has 30.8 pF per foot. The trick is that I only use 18 inches of it. For a Dual D coil I use two 18 inch lengths, one for transmit and one for receive. The cable goes to a small waterproof, shaft mounted box, which contains both the transmit power switching and the receive coil amplifier. The other end of the box is connected via a cheap four conductor shielded cable to the main control box. This method lowers the capacitance of the coil and speeds the coils response a great deal! The cables capacitance is now fixed at a very low 46pF.
The cute part is that RG174U is only about 1/8 inch thick. This makes connecting a balanced coil many orders of magnitude easier than when using thick, nasty, RG58U or RG59U. The RG174U is also VERY strong which makes things even better. I run the two cables in a straight line to the box.
The Transmit / Receive box or T/R box for short is clip mounted to the shaft above the coil. I have found it so much easier to balance a Dual D coil using the RG174U than using thicker cables. This method is cheap, painless, and works very, very, well. With a seperate T/R box for each coil it is a simple matter to provide the exact damping resistor value/s for each and every coil you wind. Gentlemen, Ladies, Others,Your feedback please. * * *

 

[Anonymous]

Dave my good friend.
That sure is a novel approach, - but Im not sure if it is commercially do able, sure would be a bit more messy than the normal method. When the push is towards havinmg most all PIs waterproof, - splitting the electronics into two or more parts is not the way to go in my humble opinion.
Cheers John

 

[Anonymous]

John,
I must admit that it would most likely cost a bit more to produce than a regular coil. However, the box is a very small, el cheapo waterproof unit and the electronics inside it are very cheap.
The method surely does provide a fast coil and the much lighter, screened audio type cable used helps a lot when you need a long connection to the coil such as when using a hipmount.