On 10/26/2011 12:37 AM, Owen Duffy wrote:
Jim wrote in news:j846r5$7h3$1
@news.jpl.nasa.gov:

There's a goodly bit of empiricism in trap design.

And a goodly bit of misinformation in some of the traditional ham sources.

I have had an interest in the so called coax traps, and just in the last
week or so, make some headway with some good measurments of the underlying
inductor formed by the coil of coax shield. There were some surprises. For
those interested, see http://www.vk1od.net/antenna/coaxtrap/index.htm .

Short of performing Medhurst style measurements on a range of inductors,
there is no way to be sure that the effects observe apply generally, or
what a more general model might be.


There is a fair amount of literature on L and C (and loss) for shielded
inductors, which is what a lot of traps look like.. the ones which use
the C to the surrounding tube, anyway, like in the 4,5,6-BTV. Some
papers have generic cookbook-ey design equations which might be useful
(although I don't have any citations off the top of my head).

For a bare coil of the appropriate L/D ratio, Medhurst will get you in
the right starting place.

The problem would be things like manufacturing variability, if you're
copying (or writing instructions) one-off design. Stuff like 'how thick
is the enamel/polyurethane/PVC insulation'.

Sort of like the measurements of Z and loss for zipcord. The dielectric
properties aren't controlled in manufacturing, so what you measure on
brand X, 16 Feb 2001 may have little or no relation to what you measure
on brand X, 20 Oct 2010 vintage.