Paul Burridge wrote:

Thanks, Roy. This is *totally* uncharted territory for me so any
advice is welcome.
I guess even if I can't get any cross sectional drawings of the
N-types off the net I could perhaps measure the required cut from
probing the connectors themselves with a dial caliper depth gauge -
which I happen to have. That ought to be the most accurate way of
going about it, I guess.
Is the amount of braid one clamps up into the plug important? Does it
matter if it gets twisted slightly (where it's doubled back down over
the outside of the outer insulation)?
Thanks,
p.

Details of how the braid is clamped will affect the physical ruggedness
of the assembly, but it shouldn't affect the electrical characteristics.
I don't understand about doubling it back over the insulation, though.

Remember that virtually all the current is flowing on the outside of the
inner conductor and the inside of the outer conductor. What you're
trying to do is maintain a constant impedance as the signal goes through
the connector. This means that at places where the dielectric is absent,
as in the mating area, the inner conductor has to become larger and
closer to the outer conductor. If the dielectric is absent at any point
where it should be present, you'll have a high impedance in that region.
Any place where the center conductor changes diameter, there has to be a
corresponding change in the spacing to the outer conductor and/or a
change in the dielectric. The connector is carefully designed to do all
this, but as you're well aware, it'll only work out as designed if it's
assembled correctly.

Roy Lewallen, W7EL