Good Coax?

[Bill]

I have been looking for some low capacitance coax and found the Beldon 1505F, seems to be a good choice at 17pf/ft.

http://bwccat.belden.com/ecat/pdf/1505F.pdf

The 1800F looks even better with 13pf/ft

anyone ever use these? Which is better, two conductor or single?

Bill

[bbsailor]

A quick test for PI coax

Bill

Here is a quick test for PI coax cable.

Assuming you have a coax cable with low enough capacitance per foot, the real test is to cut a small 6" piece and wave it under the PI coil at the lowest delay setting. A good PI coax will not produce a signal.

Typically, coax cables that have higher rated impedances, tend to have lower capacitance. When you get much below 17 pf per foot, the cable diameter starts to get a little larger.

One more consideration is the outside diameter of the coax cable that must fit inside the liquid tight pigtail http://www.hayselectronics.com/parts.htm. I believe .25" is the largest diameter that will fit.

The stranded center consuctor is desireable when you make a hip mount control box configuration as the cable will be flexing. However if you mount the control box on the coil shart/arm rest, then you have less chance of cable flexing and a stranded center consuctor is no as necessary. The difference between 32" of coax on a shaft mounted configuration is 45 pf compared to 7 ft of coax at 120 pf for a typical length used in hip mount configurations (using 17 pf per foot).

The value of the dampling resistor will be a little less with the longer coax due to the added capacitance that needs to be damped.

When making a fast PI coil for early sampling (10 us and under), try to make the coil have the lowest total capacitance which will be reflected in the measured self-resonant frequency, measured with the coax attached to the coil. A higher self-resonant frequency is better as it reflects a lower total coil/cable capacitance.

bbsailor