On Sun, 08 Nov 2009 21:09:27 -0800, Jeff Liebermann
wrote:

Well, I previous guestimated that the 6 mm of exposed center conductor
at the coax connector was good for about 3 nH or about 45 ohms at
2.4Ghz. If the balun represents 50 ohms from the antenna, then the RF
power is roughly split evenly between being radiated by the 6 mm
"leak" and going to the antenna or connector. Its close proximity to
the driven element and reflector suggests that there may be
considerable re-radiation.

Hi Jeff,

Actually, the inductance is shunt, not series to the drive. Look at
the drive point connection and you will see the shield/center open up
with very little dressing needed, basically that span fills the loop
creating a virtual drive point at the end of the braid. At that point
looking back towards the beads is where the shunt reactance lives.

As for its contribution to skewing the pattern, that is a function of
the match to that shunt section, and its radiation resistance.

No doubt Roy will chime in if I've jumped the tracks here.

True if the "leak" is far away from the driven element. In this case,
it's fairly close. I would expect some coupling and therefore some
pattern distortion.

Coupling is certainly a confounding factor to my explanation above.

It probably won't affect the match much
either as the driven element Z will probably swamp out the
contribution from the pigtail Z.

45 ohms reactance in series with the antenna is certainly going to do
bad things to the VSWR. For it to be at resonance, there has to be a
tuning cazapitor in there somewhere to tune out this added inductance.

Or in parallel.

73's
Richard Clark, KB7QHC