On Sep 28, 9:29?am, Michael Coslo wrote:
wrote:
On Sep 27, 10:53?am, Michael Coslo wrote:

Why not,

[use a 1/4 wave dipole]

other than the fact that such a short dipole will present a
feedpoint impedance that has a low resistive part and a high reactive
part?

That's a pretty good reason, though. 1/4 wavelength dipoles - make sure
to read that as total antenna length, not 1/4 wavelength per leg, are
just a troublesome combination that a lot of tuners can't tune very well.

I'm not convinced. Depending on the transmission line impedance and
length, the shack-end Z could be within the matching range
of the Transmatch.

I don't consider the G5RV and OCF to be 'trick' antennas.
They're
simply intelligent combinations of dipole and feed systems that
have been worked out to present reasonable impedances so that
line losses and Transmatch requirements are reasonable.

They are very clever. The OCF especially is a joyful
playground to work
on with antenna design programs.
It is a great way to learn both the
programs and antenna theory.

Agreed. But so are other implementations such as trap dipoles and
the classic dipole-with-ladder-line feed, where you try different
dipole and transmission-line lengths.

Often the idea of "low SWR" is put out as if it is the sole criteria.

Too often.

In
defense of SWR lovers, modern Rigs really hate High SWR,
especially
reactance of the capacitive kind.

Well, that depends on how you define 'modern'...

But a 50 ohm resistor has 1.1:1 VSWR,
and some manufacturers have taken advantage
of that sort of thing in the past.

Not just in the past. Google "Maxx-comm matcher" (try different
spellings). Those folks are still in business.

The problem as I see it is that most new folks these
days start out with an "all band" radio, and are inclined to want a
antenna that is likewise all band.

Which may or may not be a good idea. If the only bands that
are open when you have time to operate are ones where your
antenna works poorly, the result is going to be frustration.

A lot of Western and Eastern EU, Great Britain, Iceland,
Norway, most of
South/Central America, about half of Africa, Israel, Australia,
Antarctica, and a couple others.

On which bands?

No JA or far eastern countries, but I
haven't tried really hard, I just work 'em if I happen to hear them.

Of course from EPA, VK-land is about the antipodes.

I once accidentally worked a fair part of a contest once on 75
meters on
probably around 3 watts, mostly into California - I had tuned the
antenna, and forgot to turn the power back up. I had works
around 25
QSO's before catching that one.

Situational awareness, that's all. With my rig, the power level is
pretty obvious.

Of course, that isn't quantified data, it's just anecdotal. But running
at QRP levels does make for a more stern test of an antenna's
abilities,
especially if there isn't obvious signs of it, such as not getting calls
answered. Sold me on the thing.

The Ultimate Test is "what have you worked on it?" Theory is great
but the real proof is in the QSOs.

The technical details are that it is a 96 foot total length dipole, up
around 55 feet, the center support is a short length of pvc tubing.

I suspect that the 96 foot length was decided because that's what
would fit in the available space.

The ladder line is soldered to the respective dipole wire. Ladder
line makes
an almost straight drop to the Shack window. Definitely not the
best
thing going, but not too bad.

Actually, what you describe is pretty close to optimum for a simple
multiband antenna system in limited space, which I suspect is the
main issue.

Are you using true ladder line, or "window line" (Twin Lead with
holes punched in the insulation)? True ladder line (heavy wire, wide
spacing, mostly air insulation) has lower loss and less weather
effects. If the line is short there's not much difference, but as
frequency and line length increaseit can be worth changing out.

Depending on the shack-end impedances, different Transmatches
can have more or less loss. The worst-case scenario is where
a 4:1 balun is used with a shack-end impedance that has low resistive
and high reactive values. The poor Transmatch has to
try to deal with one quarter of the resistive part!

73 de Jim, N2EY

wrote:
On Sep 28, 9:29?am, Michael Coslo wrote:
wrote:
On Sep 27, 10:53?am, Michael Coslo wrote:

Why not,

[use a 1/4 wave dipole]

other than the fact that such a short dipole will present a
feedpoint impedance that has a low resistive part and a high reactive
part?
That's a pretty good reason, though. 1/4 wavelength dipoles - make sure
to read that as total antenna length, not 1/4 wavelength per leg, are
just a troublesome combination that a lot of tuners can't tune very well.

I'm not convinced. Depending on the transmission line impedance and
length, the shack-end Z could be within the matching range
of the Transmatch.

A few years back I did a modeling of an antenna that was 1/4 wavelength
long at I think 40 meters. The SWR of the antenna was approaching
infinite. If I get the chance, I'll model it again tonight - I'm doing
the mass mailing for the PAQSO party tonight, and if all goes well, I
should have a little time.


I don't consider the G5RV and OCF to be 'trick' antennas.
They're
simply intelligent combinations of dipole and feed systems that
have been worked out to present reasonable impedances so that
line losses and Transmatch requirements are reasonable.
They are very clever. The OCF especially is a joyful
playground to work
on with antenna design programs.
It is a great way to learn both the
programs and antenna theory.

Agreed. But so are other implementations such as trap dipoles and
the classic dipole-with-ladder-line feed, where you try different
dipole and transmission-line lengths.

Often the idea of "low SWR" is put out as if it is the sole criteria.

Too often.

In
defense of SWR lovers, modern Rigs really hate High SWR,
especially
reactance of the capacitive kind.

Well, that depends on how you define 'modern'...

Non-tube? Don't get me wrong, I LOVE tube rigs.


But a 50 ohm resistor has 1.1:1 VSWR,
and some manufacturers have taken advantage
of that sort of thing in the past.

Not just in the past. Google "Maxx-comm matcher" (try different
spellings). Those folks are still in business.

That's the one I was referring to. Hard to imagine they are still doing
business.



A lot of Western and Eastern EU, Great Britain, Iceland,
Norway, most of
South/Central America, about half of Africa, Israel, Australia,
Antarctica, and a couple others.

On which bands?

Mostly 20 and 40. Just a few of the near out of country neighbors on 80.


The technical details are that it is a 96 foot total length dipole, up
around 55 feet, the center support is a short length of pvc tubing.

I suspect that the 96 foot length was decided because that's what
would fit in the available space.

Yup, I tried a few dipoles that were longer, and made a Z shape. They
worked okay, but were a lot more maintenance. Lots of tree whipping in
storms here, and I almost made plastic pully's into the end insulators
to combat the extra movement. But I went back to the 96 foot length



The ladder line is soldered to the respective dipole wire. Ladder
line makes
an almost straight drop to the Shack window. Definitely not the
best
thing going, but not too bad.

Actually, what you describe is pretty close to optimum for a simple
multiband antenna system in limited space, which I suspect is the
main issue.

I agree. What really surprised me was that the performance on 75 meters
was pretty acceptable. I mostly get down there during contests, and have
had nice results. Worked enough people to make it worthwhile.



Are you using true ladder line, or "window line" (Twin Lead with
holes punched in the insulation)? True ladder line (heavy wire, wide
spacing, mostly air insulation) has lower loss and less weather
effects. If the line is short there's not much difference, but as
frequency and line length increase it can be worth changing out.

I use the window line. I have heard of the advantages of the true ladder
line, and certainly the higher impedance is one of them. I've heard of
some of the drawbacks of window line, such as it's performance when wet.

I did take issue with the test method cited by many, in which the window
line was dunked in water that included a wetting agent. My contention is
that the experiment showed the effects on window line with wetted line.

My experience has been that window line does not wet in this manner.
When the experimenter has to add a chemical to coat the line with water,
it is altering the conditions and producing results germane to only
those conditions.

I wonder what would happen to open wire ladder line under those conditions?

"Michael Coslo" wrote

A few years back I did a modeling of an antenna that was 1/4 wavelength
long at I think 40 meters. The SWR of the antenna was approaching
infinite. If I get the chance, I'll model it again tonight - I'm doing the
mass mailing for the PAQSO party tonight, and if all goes well, I should
have a little time.

I'd think a strict 1/4 wave, regardless how it's fed, would be pretty
horrific on that specific band. (But I couldn't explain why...it's just from
what I've read.) That's why I made mine (55 feet) so that it was under 1/4
wave for 40, and more than 1/4 for all the higher bands.

As for window line being affected by water... yes, it is, but I never found
it a big deal. As I recall, I just retuned some of the transmatch settings
to accommodate. I used both 450 and 300 ohm window line... and even tv
twinlead. What fun I had one night when I heard a cat playing on my flat
roof, and I could tell he was playing with the transmission line that was
suspended about a foot off the roof... I transmitted 100 watts and heard him
take off like a shot!

BTW, I wonder what happened to the OP? Did we drive him away? ;-)

Howard N7SO

On Oct 2, 3:20?pm, Michael Coslo wrote:
wrote:
On Sep 28, 9:29?am, Michael Coslo wrote:

Depending on the transmission line impedance and
length, the shack-end Z could be within the matching range
of the Transmatch.

A few years back I did a modeling of an antenna that was 1/4
wavelength
long at I think 40 meters. The SWR of the antenna was
approaching infinite.

???

With reference to what sort of feedline? SWR only has meaning
wrt a particular line impedance.

If I get the chance, I'll model it again tonight -

TNX

In
defense of SWR lovers, modern Rigs really hate High SWR,
especially
reactance of the capacitive kind.

Well, that depends on how you define 'modern'...

Non-tube? Don't get me wrong, I LOVE tube rigs.

There have been non-tube ham rigs for a couple decades now....

In a way, we have come full circle.

Look back 50-odd years, and most ham rigs could match
an SWR of 3 or 4 to 1 without a tuner, because they had fairly
wide-range pi-networks.

That was sacrificed to the desire for miniaturization, but even as
late as the early 1980s, there were rigs with tube finals that could
match 2:1 SWR or so.

Meanwhile the SS rigs wanted 50 + j0 loads for their no-tune finals,
so Transmatches became very common - so common, that rigs began to
offer them built-in, so the rig could match SWR of 3 or 4 to 1. Only
difference was automation.

The more things change....

Google "Maxx-comm matcher" (try different
spellings). Those folks are still in business.

That's the one I was referring to. Hard to imagine they are still
doing business.

The ARRL Product Review where they tested one, then X-rayed
it and opened it up to show the resistors was well over 20 years
ago.

The fact of the matter is that they *do* work - just not very well.

Mostly 20 and 40. Just a few of the near out of country
neighbors on 80.

WInter nights are coming. That's when 80 and 40 really come into
their own.

I suspect that the 96 foot length was decided because
that's what
would fit in the available space.

Yup, I tried a few dipoles that were longer, and made a
Z shape. They
worked okay, but were a lot more maintenance.
Lots of tree whipping in
storms here, and I almost made plastic pully's into the end
insulators
to combat the extra movement. But I went back to the
96 foot length

IMHO, amateur radio antenna design is no more than
10% electrical engineering and no less than 90%
mechanical engineering. In many cases it's 5%/95%.

Actually, what you describe is pretty close to
optimum for a simple
multiband antenna system in limited space, which
I suspect is the
main issue.

I agree. What really surprised me was that the performance
on 75 meters
was pretty acceptable.

Not really a surprise to me. While short, the 96 footer and lowloss
feed system will put significant RF in usable directions.

I mostly get down there during contests, and have
had nice results. Worked enough people to make it worthwhile.

CW SS is a month away....

Are you using true ladder line, or "window line" (Twin Lead with
holes punched in the insulation)? True ladder line
(heavy wire, wide
spacing, mostly air insulation) has lower loss and less weather
effects. If the line is short there's not much difference, but as
frequency and line length increase it can be worth changing out.

I use the window line. I have heard of the advantages of the true
ladder
line, and certainly the higher impedance is one of them.

Higher impedance in and of itself doesn't make the difference.
What matters is the lower loss due to more copper and less
dielectric.

The ocarc transmission line loss calculator does balanced lines as
well as coax.

I've heard of
some of the drawbacks of window line, such as it's performance when wet.

I did take issue with the test method cited by many, in which
the window
line was dunked in water that included a wetting agent.
My contention is
that the experiment showed the effects on window line with
wetted line.

My experience has been that window line does not wet
in this manner.
When the experimenter has to add a chemical to coat
the line with water,
it is altering the conditions and producing results germane to only
those conditions.

I agree. A spray with the garden hose will adequately simulate a
rainy day, I think.

I think your biggest possible improvement would be to
see how lossy your tuner/transmission line combo really is, and
improve it if possible.

73 de Jim, N2EY

wrote:
On Oct 2, 3:20?pm, Michael Coslo wrote:
wrote:
On Sep 28, 9:29?am, Michael Coslo wrote:

Depending on the transmission line impedance and
length, the shack-end Z could be within the matching range
of the Transmatch.
A few years back I did a modeling of an antenna that was 1/4
wavelength
long at I think 40 meters. The SWR of the antenna was
approaching infinite.

???

With reference to what sort of feedline? SWR only has meaning
wrt a particular line impedance.

When I came up with infinite SWR, I didn't spend a whole lot more time
on the antenna. I could probably match it with a super long piece of
coax, and attendant loss, but it is really so much easier to change the
length so it isn't 1/4 wave on any desired frequency I might want, have
the SWR at a manageable level, and use the tuner.

I look at it as one of those things that are easy to avoid.

snippage


I've heard of
some of the drawbacks of window line, such as it's performance when wet.

I did take issue with the test method cited by many, in which
the window
line was dunked in water that included a wetting agent.
My contention is
that the experiment showed the effects on window line with
wetted line.

My experience has been that window line does not wet
in this manner.
When the experimenter has to add a chemical to coat
the line with water,
it is altering the conditions and producing results germane to only
those conditions.

I agree. A spray with the garden hose will adequately simulate a
rainy day, I think.

That is the case. I have heard that some ladder line will get "frosted"
by sandstorms or even have moss grow on them, which could lead to
trouble re wetting, or being somewhat submerged in a wet substance, but
that is kind of extreme. I've got 5 year old window line that still has
the water bead up on the poly.

- 73 de Mike KB3EIA -