Right. So I figure if I don't need to ever exceed 10-15 inches of line
to build a capacitative stub on HF, it's better than if I need to get
out to many feet of line to get the same Xc.

So, here's a question. I've built a piece of such line since I posted.


The outer conductor is 15 inches long, 0.995 inch inner diameter. The
inner conductor sticks out on both ends, 1.5 inches on one end, 8
inches on the other (I've not cut it yet). The outer diameter of the
inner conductor is 0.875 inches. The dielectric is air, except for 1/2
inch at either end of the outer conductor where there are some little
nylon rings to keep the inner conductor spaced.

Incidentally, Zo of this line should be 7.7 ohms or thereabouts. I
expect the velocity factor is not so far from 1.

I attached it to an MFJ-259 via a small length (about 8 inches) of
RG-58 coax cable with a connector on one end and stripped ends (only
about an inch) on the other.

At low frequency (~2MHz) the analyzer shows a purely capacitative load,
about 200pF. (some of which is from the RG-58, some from the pipe)

However, this object (pipes+coax) has a series resonance around 30MHz!
(R=0,X=0).

This was not expected as the line is only 0.04 wavelengths long at this
frequency.

If I calculate Xc from Xc=Zo/tan(length in degrees) at 2MHz
(Zo=7.7ohm), I get Xc=482 ohms. C=165pF, not too far from measured,
considering that the 8 inch RG-58 probably adds a few tens of pF.

If I calculate at 30MHz where I'm finding a resonance, I expect Xc=31
ohms and Xl (Zo*tan(length in degrees)) = 1.88 ohms, so it should still
be strongly capacitative.

Where's my resonance coming from?

73,
Dan
N3OX