Jeff Liebermann wrote:
On Sun, 08 Nov 2009 14:33:19 -0800, Jeff Liebermann
wrote:

The point I was trying to make is that the fairly long and exposed
leads at the connector, are perfectly acceptable for low frequencies
(HF) but are NOT acceptable for microwave work at 2.4GHz. The exposed
wires are inductors and/or radiators. My guess is there's a total of
about 4mm of exposed conductor. With a wavelength of 12.5mm, that's
1/3 of a wavelength. Before hitting the balun (or whatever that's
suppose to be), most of the RF will be radiated by the exposed section
of the coax, not the antenna.

Ok, let me try again, this time while not talking on the phone, eating
lunch, and watching TV.

One wavelength at 2.4Ghz is 12.5cm. Guessing from the photo, there's
a total of about 15mm of exposed conductor. That's about 1/8th
wavelenth, which will still radiate rather badly, but not as badly as
I previously erroniously assumed.

Assuming the radiator is actually resonant then the vswr doesn't really
matter but as you point out the exposed centre conductor will radiate
badly and certainly not a design to be emulated by effectively stopping
the reflected rather than matching correctly .

On Mon, 09 Nov 2009 10:11:42 +1000, atec7 7 "atec
wrote:

One wavelength at 2.4Ghz is 12.5cm. Guessing from the photo, there's
a total of about 15mm of exposed conductor. That's about 1/8th
wavelenth, which will still radiate rather badly, but not as badly as
I previously erroniously assumed.

Assuming the radiator is actually resonant then the vswr doesn't really
matter

Wrongo. VSWR does matter. VSWR is a measure of impedance matching.
Failure to match impedances means that your antenna is no longer
working at the optimum power transfer point (i.e. maximum efficiency).
It will still work with a high VSWR, but not as well. 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.

but as you point out the exposed centre conductor will radiate
badly and certainly not a design to be emulated by effectively stopping
the reflected rather than matching correctly .

Yep. It's like fixing the symptoms rather than fixing the source of
the problem.

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

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.

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.

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.

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.

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

73's
Richard Clark, KB7QHC

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

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

Jeff Liebermann wrote:
On Mon, 09 Nov 2009 10:11:42 +1000, atec7 7 "atec
wrote:

One wavelength at 2.4Ghz is 12.5cm. Guessing from the photo, there's
a total of about 15mm of exposed conductor. That's about 1/8th
wavelenth, which will still radiate rather badly, but not as badly as
I previously erroniously assumed.

Assuming the radiator is actually resonant then the vswr doesn't really
matter

Wrongo. VSWR does matter.
Depends on the feed method long as the maximum transfer of enegy takes
place
I remember as a youngster open feeder balanced into the back of the
old tube tx , still use open feeder today with good success
VSWR is a measure of impedance matching.


take a breath son getting excited can be bad for the heart on old blokes
like us

Failure to match impedances means that your antenna is no longer
working at the optimum power transfer point (i.e. maximum efficiency).
It will still work with a high VSWR, but not as well. 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.
yes BUT it may not offer a good match no ?

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.
The radiator may dip fine but the energy transffered will be radiated
badly into the ether I suspect

but as you point out the exposed centre conductor will radiate
badly and certainly not a design to be emulated by effectively stopping
the reflected rather than matching correctly .

Yep. It's like fixing the symptoms rather than fixing the source of
the problem.
Agreed , the manner of feeding also happens to radiate which of course
is bad as I did some testing a while back on some commerial yagi's and
with a fiddle the actual vswr hardly changed but energy transfer was
markidly improved