A coil of coax has capacitance between turns. This makes the coil look
like a parallel resonant circuit -- like a trap -- at some frequency.
The most effective choke is self resonant at the frequency of use. The
diameters and numbers of turns for coax choke baluns given in the _ARRL
Antenna Book_ are ones that result in self-resonance at the center of
the recommended operating range. These were determined by measurement
with a vector impedance meter. An antenna analyzer can be used just as
well for measurement of choke impedance, by tying the inner and outer
conductors together, connecting one end of the choke to the analyzer
"hot" lead and the other to the analyzer ground. The higher the
impedance, the better.

Roy Lewallen, W7EL

Roger Leone wrote:
Ken:

You haven't constructed a trap. A trap is a parallel resonant circuit,
meaning the inductance and capacitance are in parallel with each other. At
resonance it presents a high impedance. A trap can be constructed with
coaxial cable, but to get the inductance and capacitance in parallel you
have to cross connect the inner and outer conductors. See:
http://www3.sympatico.ca/wrgraham/ve...coaxtraps.html

You have wound the coax into a choke inductor but the capacitance you
calculated is between the inner and outer conductors and is not in parallel
with the inductance of the windings.

If your calculation of the inductance is correct (2.342 uh), by itself it
doesn't have a very high impedance at 10 mhz and doesn't serve very well as
a choke. It may be doing more harm than good since a resonant dipole should
have better than a 3:1 SWR. Have you tried the dipole without it?

Roger K6XQ