On Sun, 08 Nov 2009 20:11:53 -0800, Richard Clark
wrote:

On Sun, 08 Nov 2009 19:46:16 -0800, Jeff Liebermann
wrote:

High VSWR also
has highly undesirable side effects such as, mangled gain pattern,
radiation from undesired conductors, loss of gain, and loss of
efficiency. Resonance is a good thing, but not absolutely necessary
for proper operation. Resonance would be where the reactive
components are zero. Since I don't see any adjustment(s) to tune out
(resonate) the inductances introduced by the relatively long exposed
coax leads, I don't think this antenna is particularly close to
resonance.

This is very problematic.

Groan. Now, where did I screw up?

High SWR may be a product of unintended radiators (like the pigtail
going from the choke bead to the feed point), but far-field radiation
lobe pattern shape is NOT affected by SWR due simply to mismatch.

Agreed. However, I was thinking that the added inductances at both
ends of the coax are going to mangle the function of the balun, which
will create pattern changes.

There's a lot going on in that statement, so I'll try it again this
way:

Added, unintended radiative elements cause mismatch AND pattern
distortion AND gain reduction (to the degree of mismatch). This is
the basis for concern about the pigtail.

Yep.

A perfectly implemented design that presents an Z other than that
expected (mismatch) causes gain reduction (to the degree of mismatch).
The pattern's shape is not altered except that its gain values at any
angle are depressed equally by the degree of mismatch.

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.

(I'm resisting the temptation to borrow or by an MFJ-1800 antenna and
bench test it.)

Resonance is desired for match AND efficiency.

Going further:

The degree of pattern distortion is a complex function of this
additional pigtail radiator. There is every chance that it won't
perturb the pattern much unless you are very concerned about nulling
out interfering sources.

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.

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.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558