Discussion:
[Elecraft] OT: Vertical doublets (was: Home made Sigma-GT5 & KRC2 or SGC?)
JAKidz
2004-03-23 15:46:01 UTC
Permalink
Greetings:

Any suggestions on the optimum dimensions of a ladderline-fed
Sigma-GT5-like hatted vertical doublet that the KAT100 will tune
efficiently between 10 and 40 m? A Sigma-GT5-like design posted on
Usenet by AB6SJ takes advantage of the availability of aluminum tubing
in 6 foot lengths so it is a bit bigger than the GT-5.

W4RNL suggests a 44 foot horizontal doublet fed by ladderline is
an optimal doublet length for 10-40m. It's tempting to turn this doublet
vertical and shorten it with bar hats like the GT-5 if it is efficient.
But at least one antenna guru (whose call I can't find at the moment)
has stated repeatedly that W4RNL's doublet design is very inefficient
because of tuner losses. Who is right? What are the tuner efficiency
tradeoffs of having the structure resonance low, high or mid-frequency?

73,
John, K7JG
Don Wilhelm" (Don Wilhelm)
2004-03-23 16:35:02 UTC
Permalink
John,
The answer to your question does not have a simple answer.
If you know the antenna feedpoint impedance, you can use a program like TLW
(I believe you can download it from the ARRL site) to try different feedline
lengths that will be practical for your installation and see if you can find
a length that will be within the range of your tuner.

L B Cebik's 44 foot length is chosen because of the single lobe radiation
pattern for all bands 40 through 10 - LB assumes (correctly) that any
impedance can be matched by some method. Longer lengths will develop lobes
on the higher bands - and for a vertical that should translate to a
(desirable) low angle of radiation, but ground reflection will determine
just how low that angle really will be. Your proposed capacity hatted
dipole would have different feedpoint impedance than LB's 44 foot wire
dipole, but the radiation pattern (meaning = no extra lobes) should be
similar.

As far as tuner loss is concerned, that depends on the tuner characteristics
and how it is used. Commonly used T match tuners can have multiple settings
that will provide a match, and some of those settings are quite lossy. If
properly used, the tuner loss can be kept to a minimum. A properly
configured L network usually has a lower loss than the T match tuners, so
perhaps a KAT2 or KAT100 would be a better choice than a T match tuner.

A 44 ft dipole is not resonant on any ham band, so your question about high,
low resonance does not compute - it is in the class of a non-resonant
antenna.

73,
Don W3FPR

----- Original Message -----
Post by JAKidz
Any suggestions on the optimum dimensions of a ladderline-fed
Sigma-GT5-like hatted vertical doublet that the KAT100 will tune
efficiently between 10 and 40 m? A Sigma-GT5-like design posted on
Usenet by AB6SJ takes advantage of the availability of aluminum tubing
in 6 foot lengths so it is a bit bigger than the GT-5.
W4RNL suggests a 44 foot horizontal doublet fed by ladderline is
an optimal doublet length for 10-40m. It's tempting to turn this doublet
vertical and shorten it with bar hats like the GT-5 if it is efficient.
But at least one antenna guru (whose call I can't find at the moment)
has stated repeatedly that W4RNL's doublet design is very inefficient
because of tuner losses. Who is right? What are the tuner efficiency
tradeoffs of having the structure resonance low, high or mid-frequency?
Stuart Rohre
2004-03-23 22:28:00 UTC
Permalink
A lot easier than worrying and computing the exact impedance at the end of
the parallel feeder at the rig is to have handy 5 and 10 foot extensions of
line to add in to correct the impedance presented to the antenna tuner, IF
it has a problem.
-Stuart
K5KVH
JAKidz
2004-03-23 17:15:01 UTC
Permalink
Post by Don Wilhelm" (Don Wilhelm)
The answer to your question does not have a simple answer.
Dang.
Post by Don Wilhelm" (Don Wilhelm)
L B Cebik's 44 foot length is chosen because of the single lobe
radiation
Post by Don Wilhelm" (Don Wilhelm)
pattern for all bands 40 through 10 - LB assumes (correctly) that any
impedance can be matched by some method.
Understood. I'm aware of other arguments that this design is very
inefficient because of tuner loss. Translating from feedpoint impedance
calculated by a program such as Multinec to a tuner and what is
matchable and what isn't with the KAT100 is an area I don't understand
(yet).
Post by Don Wilhelm" (Don Wilhelm)
A 44 ft dipole is not resonant on any ham band, so your question about
high,
low resonance does not compute - it is in the class of a non-resonant
antenna.
Sorry for not being clear. I meant that given the effect of size on
feedpoint impedance and ultimately the tuner, how should the structure
be sized, the reasonance point for a smaller structure being higher
frequency and a larger lower frequency. Playing with Multinec, it looks
like the higher angle lobe that appears on 10 m with a 44' foot vertical
doublet is reduced with a shorter hatted doublet. It looks like the
physical structure of the Sigma 5 is resonant toward the higher
frequency size with matching done at the antenna feedpoint. Is this
still the design goal if ladderline is used and matching is by the
KAT100 at the transceiver feedpoint?

73,
John K7JG.
Don Wilhelm" (Don Wilhelm)
2004-03-23 18:50:01 UTC
Permalink
John,

I think we are mixing too many things together here - and that results in
more variables than my brain can handle!!! It is not really possible to
answer your question directly without a LOT of work, but I can give you some
pointers.

Firstly, a length of transmission line CAN be a tuner all in itself, it will
behave that way anytime there is SWR present on the transmission line - it
will transform impedance from its input to its output. That is one big
reason that one length line will work in a particular installation while
another will not. The half wavelength (or multiple) feedline case is easy -
that is the only time a feedline with SWR will have the same impedance at
the input and output ends. If one does not understand that, he (she) will
continually scratch their head wondering why a particular antenna fed with
ladder line works and another one doesn't - it depends very greatly on the
feedline length, and the characteristic impedance of the feedline is NOT the
impedance that is to be matched - these are quite different things.

First consider the antenna for its radiation pattern, then next for its
feedppoint impedance. (Or just the feedpoint impedance alone if the pattern
is not important to you.) Then separately consider the length of feedline
sufficient to reach from the antenna to the shack (this is the minimum
length, you can always add, but not subtract from this). That (after a few
calculations) will tell you what impedance you must feed in the shack. Then
finally, ask the question 'What kind of network do I need to transform that
impedance to the 50 ohms resistive that my transmitter needs?'. If you want
to avoid the calculations, you can put it up and make measurements with an
antenna analyzer and proceed with 'cut and try' methods.

I don't know of any other method to do it right. As for tuners, some tuners
are more efficient than others. It all depends on the Q of the components
and the magnitude of the circulating current - primary losses result from
the current through the inductor(s). Folks who want to generalize this by
saying that all tuners are lossy are technically correct, there is no free
lunch, but there are things that can be done to minimize the loss. We all
have to figure out just how much is tolerable for our installation and
budget - lower losses usually translate to increased cost - larger wire in
the coils to reduce the series resistance, better quality capacitors, lower
loss transmission line, and the list can go on and on.

One just has to 'make your choices' and do the best with what we have based
on those choices. There are no 'magic' answers.

73,
Don W3FPR

----- Original Message -----
Post by Don Wilhelm" (Don Wilhelm)
Post by Don Wilhelm" (Don Wilhelm)
L B Cebik's 44 foot length is chosen because of the single lobe
radiation
Post by Don Wilhelm" (Don Wilhelm)
pattern for all bands 40 through 10 - LB assumes (correctly) that any
impedance can be matched by some method.
Understood. I'm aware of other arguments that this design is very
inefficient because of tuner loss. Translating from feedpoint impedance
calculated by a program such as Multinec to a tuner and what is
matchable and what isn't with the KAT100 is an area I don't understand
(yet).
Post by Don Wilhelm" (Don Wilhelm)
A 44 ft dipole is not resonant on any ham band, so your question about
high,
low resonance does not compute - it is in the class of a non-resonant
antenna.
Sorry for not being clear. I meant that given the effect of size on
feedpoint impedance and ultimately the tuner, how should the structure
be sized, the reasonance point for a smaller structure being higher
frequency and a larger lower frequency. Playing with Multinec, it looks
like the higher angle lobe that appears on 10 m with a 44' foot vertical
doublet is reduced with a shorter hatted doublet. It looks like the
physical structure of the Sigma 5 is resonant toward the higher
frequency size with matching done at the antenna feedpoint. Is this
still the design goal if ladderline is used and matching is by the
KAT100 at the transceiver feedpoint?
73,
John K7JG.
Stuart Rohre
2004-03-23 21:59:01 UTC
Permalink
If you use ladderline feed to any antenna vertical or horizontal, you do not
need to match at the center feed point in Theory. Your Antenna Matching
Unit will tune both the line and the antenna to a match. You operate a
balanced (ladderline) at high SWR normally, and because of the higher
impedance of such lines and limited dielectric construction of them you
still have very low losses compared to coax feeds (even with SWR line loss).
The Sigma 5 of Force 12 only needs coils at the center feedpoint to match it
to coax without use of a perfect tuner. IF it were fed with balanced line,
the coils could go away, except for their effect in moving the length toward
resonance on the lowest bands, and easing the job of matching thereby.

Tuners have been widely mesured and have much lower losses in proper
adjustment than many hams believe as has been cited in the work of Witt and
others. What is rarely mentioned, is losses are related to having several
lossy components rather than fewer.

Typically, capacitors are lower loss than inductors. Having more inductors
gives somewhat higher loss. Thus, a tuner with only one capacitor and one
inductor is about the most practical minimum parts count, and this "L
network", has lowest tuner loss over Tees and Pis. A little studied area
is using the unbalanced L network to load a balanced line without use of a
balun. It can work. It would be interesting for someone to do a study on
it.
-Stuart
K5KVH
Stuart Rohre
2004-03-23 22:12:01 UTC
Permalink
But, with any reasonable commercial tuner out there on the market, You will
not have excess tuner loss nor inefficiency with the 44 foot doublet.

Experiments have not shown ANY heating in use of Dentron Super (high power)
or Junior (100 watt or less) tuners in several tests using a variety of
doublets or loops.

One has big coils and caps, the other minimal caps and coils to fit a QRP
sized case. Additional experiments with the 300 watt rated, but compact,
B&W tuner also did not show any heating. Transceivers were at 100 watts for
the tests. It was found that a Dentron or B&W did tune up with a more
readily found (higher Q) setting when the internal balun was bypassed and
the tuner was hooked by one foot of coax to an external 4:1 balun made by
Van Gorden. There was only one foot of coax from tuner coax output to the
balun input. At the time, the high impedance side of the balun was hooked
to ladder line feed of a 1149 foot circumference loop being tuned to 20m.
The Van Gorden device is obviously a larger inductor than either tuner's
internal balun, and most internal baluns of commercial tuners of that
vintage were voltage baluns. A current balun is preferred by the literature
on choosing baluns.

Don't let fear of tuner loss keep you from putting up the convenient 44 foot
antenna.
-Stuart
K5KVH
Ron D'Eau Claire
2004-03-23 17:50:01 UTC
Permalink
True, but keep in mind that ground losses (unless you live at the =
seashore)
pretty much wipe out any radiation below about 15 degrees in a vertical. =
The
44 foot vertical has maximum gain at about 15 degrees elevation, so it
actually represents probably the most efficient combination.=20

However, almost all verticals show maximum radiation at 0 degrees, and =
the
losses of any significant radiation below about 15 degrees are simply
accepted as part of the cost of not being in the middle of a huge, =
efficient
ground plane.

The "guru" who insists repeatedly that the losses in an ATU (or any =
matching
network) are significant is proof that saying something over and over =
again
doesn't make it so. The ATU easily can be the most efficient part of a
typical antenna system. As Don said, some ATU's are more efficient than
others. The L-match used in the Elecraft ATU's is, overall, the most
efficient system we have. But even the much-maligned "T" match networks =
can
be very efficient, if used correctly.=20

Ron AC7AC

-----Original Message-----
...Playing with Multinec, it looks like the higher angle lobe that =
appears
on 10 m with a 44' foot vertical doublet is reduced with a shorter =
hatted
doublet. It looks like the physical structure of the Sigma 5 is resonant
toward the higher frequency size with matching done at the antenna
feedpoint. Is this still the design goal if ladderline is used and =
matching
is by the KAT100 at the transceiver feedpoint?

73,
John K7JG.

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George, W5YR
2004-03-23 19:56:06 UTC
Permalink
Just a comment re tuner loss:

Frank Witt, AI1H, has written in QEX an excellent article on measuring tuner
loss using an impedance meter such as the MFJ 259B. I and several others
have performed numerous tests on various types of tuners and have yet to
find one with an inherent loss exceeding one dB. Most come in the range of
0.5 to 0.75 dB on 20 meters. One KW tuner was measured at 0.11 dB!

As a rule, and according to theory, L networks tend to show less loss than T
or PI networks.

Any tuner will exhibit larger losses when "mistuned" or when one attempts to
tune a load that is outside the envelope for the tuner.

There are excellent computer programs available for tuner analysis that
include calculation of losses in real components with finite Q values. They
confirm the minimal power loss in a well designed and adjusted tuner.

73, George W5YR
Fairview, TX
***@att.net
http://www.w5yr.com


----- Original Message -----
From: "Ron D'Eau Claire" <***@easystreet.com>
To: <***@mailman.qth.net>
Sent: Tuesday, March 23, 2004 10:48 AM
Subject: RE: [Elecraft] OT: Vertical doublets (was: Home made Sigma-GT5 &
KRC2 or SGC?)


The "guru" who insists repeatedly that the losses in an ATU (or any matching
network) are significant is proof that saying something over and over again
doesn't make it so. The ATU easily can be the most efficient part of a
typical antenna system. As Don said, some ATU's are more efficient than
others. The L-match used in the Elecraft ATU's is, overall, the most
efficient system we have. But even the much-maligned "T" match networks can
be very efficient, if used correctly.
Bob Tellefsen
2004-03-23 22:05:00 UTC
Permalink
Hi John
I'll swing at your questions.
First, you have to look at a nonresonant dipole along with its feedline
as a system. You can't take each part separately.

Let's talk about 40m for an example. With different combinations of
antenna length and feedline length, you can have a current maximum
(lowest impedance), a voltage maximum (maximum impedance), or
something in between at the shack end of the feeder.
I try to adjust such an antenna system for the current maximum at the
feedpoint. That makes a resistive load for the ATU, and allows use
of a simple 1:1 balun.

The rub is when you go multiband. Let's talk 20m now. Suddenly that
nice, tame low Z feedpoint is now a high Z feedpoint with lots of rf
voltage.
Very tough for an ATU to match. Also, not a good place for a balun.

On 15m, the third harmonic of 7 MHz, the feedpoint will again be low,
but it may not be exactly at a resonant point, meaning lowest reactance.
Ten meters again will be a high Z feedpoint. And so on. The WARC bands
fall in between these points.

Here I use a 60 ft fat non-resonant dipole on all bands, 10m down to 80m.
Since I'm out in the garage, it is easy for me to loop some additional
feedline
through the rafters to bring a middle-of-the-road impedance on all bands
to the feedpoint at the rig. This is not a perfect solution, as almost
every band
has some reactance along with the resistance. However, if it is not
extreme,
a good antenna tuner can cancel it during the matching process.

For contests, where I'm jumping from band to band, I find my 50 ohm:12.5 ohm
balun gives the best overall results. However, if I expect to spend a lot
of
time on one band, say 40m for ragchewing, I will pick my 1:1 balun, as it
works
best with this antenna/line combination in my situation. If 20m, I might
wind up
with my 50 ohm:200 ohm balun.

Since these nonresonant systems are all different, you have to figure out
what
antenna length you will have, how long a feeder you can run, and then in the
shack adjust for the best match under these conditions.

One way to do this, rather than run extra line through the rafters, is to
use extension sectons connected with banana plugs and jacks. This way, you
can have the shortest possible feedline coming to the shack, and then insert
correcting lengths as needed to
match any given band. This is an easy fix, especially if you have a band
decoder to control relays which can do the switching for you.

I hope this gives you some ideas, John.
Good luck and 73
Bob N6WG
Ron D'Eau Claire
2004-03-23 22:24:01 UTC
Permalink
I found that article useful too, George.=20

Having a several tuners handy, I took a slightly more "nuts and bolts"
approach to evaluating losses and hooked up two identical T-Match tuners
back-to-back. Measure the power in at the 50 ohm input and measure the =
power
out at the 50 ohm "output" with the second tuner running "backwards". I =
knew
what settings my antenna used on each band, so I set the ATU on the xmtr
accordingly and adjusted the second ATU for a 1:1 match at the rig with =
a 50
ohm dummy load and wattmeter attached.=20

I found attenuations of less than 2 dB across the band PROVIDED the =
T-match
ATU's were adjusted for the highest inductance at which a match could be
found. It is very easy to make it 'match' at much smaller inductance
settings in many cases, but sometimes the losses of the two tuners was =
in
excess of 8 dB in that state!=20

I suspect that's why some ops find that the inductor melts in their =
T-match
circuits.=20

One of the very nice T-network simulators that I like is W9CF's version =
at:
http://fermi.la.asu.edu/w9cf/tuner/tuner.html

I had another relevant experience using a commercial ATU that had nylon
"banana plug" connectors for wire antennas. I was loading a voltage-fed =
wire
and it worked just fine. But shortly after commencing a transmission I
smelled something and the SWR jumped off the scale. I was running 15 =
watts
output at the time.

Following the hot smell I discovered that the banana connector had =
melted
until the hot lead for the