Jeffrey Angus wrote in news:j60h8s$ao0$1@dont-
email.me:

On 9/28/2011 7:31 PM, Owen Duffy wrote:
The auto tuner can be a path to hiding shortcomings in an antenna
system, in search of the holy grail, low VSWR.

This is a 102' doublet with 40' of open wire balanced feed line.
Except for the input of the tuner looking like 50 ohms to make the
transmitter happy, there is no such thing as "low VSWR" on this
type of antenna.

Well to "make the transmitter happy" is jsut the new language for low
VSWR.

... There
isn't room inside the case for a 4:1 current balun.

But there is probably room for an effective 1:1 Current Balun.


If you disconnect the open wire line from the internal voltage balun
in an MFJ tuner and replace it with an external current balun you will
find that the tuning of the match on the tuner is a lot smoother as
opposed to almost erratic. (But still obtainable.)

Ok, so in your experience, you haven't yet come across a load that could
be matched with the voltage balun, but not with an external current
balun.

Due to the location of my station on the 2nd floor over looking a
covered porch, having a "good RF ground" is not going to happen.
The current balun forces the antenna and feed line into a truly
balanced condition.

Ideal conditions like "forces" and "truly balanced" don't often exist in
the real world. It would be of more interest if you had measured and
reported the differential and common mode current at various
frequencies.

The whole purpose of the tuner is to allow the transmitter to see
a 50 ohm unbalanced load that it was designed for. Accomplishing
that, allows the most power to be transferred to the antenna. The

Yes. It is the meaning of "most" that is relevant. Most doesn't need to
mean 100%, or close to it, you make compromises for frequency agility
and multiband use, but "most" is often unknown. Perhaps some DX QSL
cards can substitute.

102' doublet with open wire line seems to do a remarkable job at
radiating. (And equally well at receiving.)

Yes, they can be a good antenna, but it is not a no-brainer.

For example, a correspondent recently reported problems with just such a
thing on 40m. Turns out his feed line length was such that at 800W into
the feed line, the voltage between the wires was some 4000+V and was
causing flashovers in a 3kW rated ATU. In this case, I recommended that
since he could not lengthen or shorten the feed line enough, that he
shorten the antenna so solve the problem.

He had previously smoked up a CWS Bytemark 5kW rated current balun on
80m with another antenna, caused by unlucky feed line length.


That leads into the question of whether a 4:1 balun is 'better'.

Better than what? A 1:1 balun or none at all,

1:1.

4:1 reduces the voltage impressed on the ATU components, which is good
for high impedance loads, and poor for low impedance loads.

There is no simple thing that always works best on a random set of loads
(which is the case for many multi band antennas).

which seems to be
the selling point of the G5RV antenna and it's clones.

Perhaps to some. There is good argument for use of a balun with a G5RV
and not-balanced transmitter. Varney conceded that in one of his later
articles. Most people who are adament about what Varney did or did not
describe have not read his articles. Antenna manufacturers are not a
good source of factual information.

Owen

On 9/28/2011 8:54 PM, Owen Duffy wrote:
Jeffrey wrote in news:j60h8s$ao0$1@dont-
email.me:

On 9/28/2011 7:31 PM, Owen Duffy wrote:
The auto tuner can be a path to hiding shortcomings in an antenna
system, in search of the holy grail, low VSWR.

This is a 102' doublet with 40' of open wire balanced feed line.
Except for the input of the tuner looking like 50 ohms to make the
transmitter happy, there is no such thing as "low VSWR" on this
type of antenna.

Well to "make the transmitter happy" is jsut the new language for low
VSWR.

You're missing the point. The "Holy Grail" is low VSWR at the antenna.
Obviously having the tuner present a nice resistive 50 ohm load to a
transmitter designed to be loaded with a resistive 50 ohm load is a
"Good thing(tm)".

But with a random length, i.e. non-resonant, antenna, VSWR at the
antenna is not a meaningful term.

... There
isn't room inside the case for a 4:1 current balun.

But there is probably room for an effective 1:1 Current Balun.

But my point was MFJ chose to use a voltage balun for two reasons.
They're cheaper and they still work to a fashion.

Ok, so in your experience, you haven't yet come across a load that could
be matched with the voltage balun, but not with an external current
balun.

Not yet. If it matches at all with one, it will match with the other.
The difference being that the current balun results in much less
erratic/sudden/critical tuning of the antenna tuner itself.

Due to the location of my station on the 2nd floor over looking a
covered porch, having a "good RF ground" is not going to happen.
The current balun forces the antenna and feed line into a truly
balanced condition.

Ideal conditions like "forces" and "truly balanced" don't often exist in
the real world. It would be of more interest if you had measured and
reported the differential and common mode current at various
frequencies.

Um, actually, yes they do. That's the whole point of a current mode
balun.

The whole purpose of the tuner is to allow the transmitter to see
a 50 ohm unbalanced load that it was designed for. Accomplishing
that, allows the most power to be transferred to the antenna. The

Yes. It is the meaning of "most" that is relevant. Most doesn't need to
mean 100%, or close to it, you make compromises for frequency agility
and multiband use, but "most" is often unknown. Perhaps some DX QSL
cards can substitute.

As an example then. How about working RA3DA in a pileup competing with
the "big guns" in New York running at or above the legal limit? Sure
I didn't get him on the first call, but I did it running under 100 watts
and into a $25 antenna.

102' doublet with open wire line seems to do a remarkable job at
radiating. (And equally well at receiving.)

Yes, they can be a good antenna, but it is not a no-brainer.

For example, a correspondent recently reported problems with just such a
thing on 40m. Turns out his feed line length was such that at 800W into
the feed line, the voltage between the wires was some 4000+V and was
causing flashovers in a 3kW rated ATU. In this case, I recommended that
since he could not lengthen or shorten the feed line enough, that he
shorten the antenna so solve the problem.

He had previously smoked up a CWS Bytemark 5kW rated current balun on
80m with another antenna, caused by unlucky feed line length.

See above. Why is it that hams seem to think they absolutely need to run
800 (or more) watts of power?

That leads into the question of whether a 4:1 balun is 'better'.

Better than what? A 1:1 balun or none at all,

1:1.

4:1 reduces the voltage impressed on the ATU components, which is good
for high impedance loads, and poor for low impedance loads.

There is no simple thing that always works best on a random set of loads
(which is the case for many multi band antennas).

All I've said is that this appears to work perfectly for me.
In the time honored tradition of Usenet "Your mileage may vary."

which seems to be
the selling point of the G5RV antenna and it's clones.

Perhaps to some. There is good argument for use of a balun with a G5RV
and not-balanced transmitter. Varney conceded that in one of his later
articles. Most people who are adament about what Varney did or did not
describe have not read his articles. Antenna manufacturers are not a
good source of factual information.

You're hear equally unsubstantiated claims about how well an off center
fed Windom antenna works with no tuner as well.

Jeff-1.0
wa6fwi

--
"Everything from Crackers to Coffins"

"Jeff Liebermann" wrote in message
...

... am now a practicing member of the Cult of
75 ohms. The difference is subtle. While worship of the 50 ohm idol
results in the maximum power transfer, the change to 75 ohms results
in minimum coax loss and somewhat easier antenna design. However, the
major benefit is it allows one to minimize the tithe paid to vendors
as CATV coax is cheap and readily available. I suggest you consider a
religious conversion.

Yes. Somebody in this group (you, maybe) previously promoted 75 ohm cable.
This past spring, I was modeling my 20m Field Day dipole, varying size and
height for best pattern shape and minimum VSWR. At optimum, it had almost
exactly 75 ohm feedpoint impedance. Having plenty of TV coax, I didn't
hesitate to go with it. The antenna worked wonderfully well at FD.
Transmitter seemed to like it just fine.

"Sal"

"Sal" wrote in :

Yes. Somebody in this group (you, maybe) previously promoted 75 ohm
cable. This past spring, I was modeling my 20m Field Day dipole,
varying size and height for best pattern shape and minimum VSWR. At
optimum, it had almost exactly 75 ohm feedpoint impedance. Having
plenty of TV coax, I didn't hesitate to go with it. The antenna
worked wonderfully well at FD. Transmitter seemed to like it just
fine.

Sal, you will have discovered that the feed point impedance of a half
wave dipole at resonance is dependent on height above ground, type of
soil etc.

This need not be a choice between 50 ohms and 75 ohms.

My 40m inverted V dipole uses 75 ohm feedline (RG6), and the impedance
looking into the feedline is close to 50+j0. Dipole height was
determined prior to design of the matching system (ie height was not
constrained by the matching solution, rather the height set a
requirement for the matching solution). The transmitter has its rated
load impedance, the feedline is efficient and inexpensive. There is a
W2DU style choke balun near the feedpoint, and measured common mode
current is quite low.

Of course, the feedline has standing waves, and the dipole is not
resonant, to be frowned upon by some (many?). The antenna system can be
readily modelled, and it has higher efficiency than a resonant half wave
with comparable coax (say RG8X), indistinguisable pattern, marginally
higher gain.

Owen




"Sal"

On Wed, 28 Sep 2011 23:26:24 -0700, "Sal" wrote:

"Jeff Liebermann" wrote in message
.. .

... am now a practicing member of the Cult of
75 ohms. The difference is subtle. While worship of the 50 ohm idol
results in the maximum power transfer, the change to 75 ohms results
in minimum coax loss and somewhat easier antenna design. However, the
major benefit is it allows one to minimize the tithe paid to vendors
as CATV coax is cheap and readily available. I suggest you consider a
religious conversion.

Yes. Somebody in this group (you, maybe) previously promoted 75 ohm cable.
This past spring, I was modeling my 20m Field Day dipole, varying size and
height for best pattern shape and minimum VSWR. At optimum, it had almost
exactly 75 ohm feedpoint impedance. Having plenty of TV coax, I didn't
hesitate to go with it. The antenna worked wonderfully well at FD.
Transmitter seemed to like it just fine.

Probably me. I tend to promote the use of 75 ohm coax and systems.
Most of the coax cables to my rooftop antenna farm are 75 ohms. The
justification was convenience and price. I obtained five 1000ft rolls
of RG6a/u mutations (some with messenger wire), and some used hard
line, and couldn't resist using it. The only problems seem to be
getting accurate readings on my test equipment and having enough
F-to-N or F-to-BNC adapters.
http://www.qsl.net/n9zia/wireless/75_ohm_hardline.html

Note that using 75 ohm coax on a VHF/UHF repeater is a really bad
idea. Duplexers, isolators, circulators, cavities, and such are very
sensitive to mismatch. For repeaters, use only 50 ohms.

--
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 Wed, 28 Sep 2011 19:26:49 -0500, Jeffrey Angus wrote:


3. Vertical antennas load nicely. So do Bird Termalines. And they
radiate about the same.
4. See #3. If I had room for a bunch of radials to make the vertical
work properly, I'd have room for a real antenna.


I have an MFJ-928 ATU at the feed point of an elevated 43' vertical, with
a half dozen radials of various random lengths. It works great for
30/20/17/15. It starts making significant multiple lobes above 21 MHz.
I'm going to try a coil for 80.

On Wed, 28 Sep 2011 21:22:55 -0500, Jeffrey Angus
wrote:

But my point was MFJ chose to use a voltage balun for two reasons.
They're cheaper

What is the price differential in winding one wire outside the core,
instead of winding it inside the core? Such "savings" accrue only in
the production run of millions of units.

and they still work to a fashion.

If both sides are unbalanced - you mean that fashion?

MFJ is not making a killing on this particular poor winding (OK, call
it historical inertia) practice, and their market for these internal
BalUns is dipoles, not monopoles.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

....
MFJ is not making a killing on this particular poor winding (OK, call
it historical inertia) practice, and their market for these internal
BalUns is dipoles, not monopoles.

Hi Richard,

I see MFJ getting mention, and whilst they may deserve a bit of a
flogging for some things, I spring to their defence on this occasion.

MFJ make the claim "More hams use MFJ-949s than any other antenna tuner
in the world!" and gauging from questions in online fora, they are
indeed popular, the claim may be correct.

The MFJ949E uses a Ruthroff voltage balun.

I have measured the balun losses in my '949E, and they are as I
discussed in the general case earlier, quite high on high impedance
loads. Anyone with a rudimentary knowledge of balun design would
understand that, but there are those who apparently live in an ideal who
would refute it.

The simple explanation is that most loss in practical ferrite cored
baluns usually results from losses in the core material. The losses in
the core material are related to flux density, which in a Ruthroff
voltage balun is roughly proportion to the differential voltage, and in
a Guanella current balun is roughly proportional to common mode current.
There is no reason to think that these two different types of baluns
would have identical losses.

The MFJ949E could easily be reconfigured as a Guanella 1:1 balun by
changing its end connections, but that does not make it an optimally
designed current balun. I haven't done it, and so cannot comment
further. I cannot see how the cost of manufacturing it wired as a
current balun would be any different.

I think that it is the buyers who determine the market, and savvy
sellers cater to the buyer's wants.

While anecdotal evidence abounds that 4:1 voltage baluns match up
extreme loads better, and users mostly arent't interested in finding the
root cause of the problem and fixing it, voltage baluns will be seen by
most buyers and savvy sellers as the solution.

A good demonstration of the credibility of anecodotal evidence is the
massive online support for the Array Solutions 4:1 Ruthroff voltage
balun / ZeroFive unloaded vertical combination. The configuration drives
high common mode on the coax feed line. (I should note that the sellers
recommendation has recently changed to an unun.)

MFJ is no doubt one of the savvy sellers. They do BTW have some higher
end ATUs with 1:1 current balun, as does the Ameritron label for those
who want a current balun.

My own view is that achievement of highest choking impedance in a
current balun is assisted by minimising stray capacitance to 'ground',
so I would prefer to put a current balun in a non-conductive box,
outside the ATU, on a foot of coax to the ATU. (This is one reason why I
haven't tried converting my '949E to current balun connection, the
windings are quite close to grounded metal and I expect stray
capacitance to ground is higher than desirable.)

Owen

On Thu, 29 Sep 2011 21:06:40 +0000 (UTC), Owen Duffy
wrote:

My own view is that achievement of highest choking impedance in a
current balun is assisted by minimising stray capacitance to 'ground',
so I would prefer to put a current balun in a non-conductive box,
outside the ATU, on a foot of coax to the ATU. (This is one reason why I
haven't tried converting my '949E to current balun connection, the
windings are quite close to grounded metal and I expect stray
capacitance to ground is higher than desirable.)

Hi Owen,

The internal modification would also demand a balanced tuner topology,
and not the garden variety pi configuration. Your stating stray
capacitance, coupling to ground, etc. is a preface to this.

Put simply, and as described, the 949E and its ilk are lumped
transmission line UnUn transformers. You might salvage the box in a
re-design, but it would require gutting the entire interior circuitry.

The 974 attempts to do this:
http://www.mfjenterprises.com/pdffiles/MFJ-974HB.pdf
and by their stated intentions, they are aware of the design issues.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

....
The 974 attempts to do this:
http://www.mfjenterprises.com/pdffiles/MFJ-974HB.pdf
and by their stated intentions, they are aware of the design issues.

I am sceptical of the "true balanced tuner" approach.

For one thing, I do not recall having ever seen measurement of the
common mode impedance reported.

If the objective is current balance, high common mode impedance is
essential.

Some folk seem to think that symmetric design is the sure path to
success, but it isn't. If you take a 1:1 Guanella with extremely high
choking impedance, the currents in its output wires will be almost
perfectly balanced, irrespective of the voltage from each terminal to
ground. If you placed 10pF of capacitance from each terminal to ground,
you appear to have preserved symmetry, but the currents in those
capacitors will not be equal unless the load is symmetric. In cases
where the currents in the balun wires are almost equal and the currents
in the capacitances I mentioned are not equal, then those capacitances
have probably compromised common mode impedance.

Most implementations of a "true balanced tuner" have large stray
capacitance from each side to chassis.

But, savvy sellers will offer them to the people who are attracted by
the concept of a "true balanced tuner".

The joke of balun offerings is those designs purported to work well on
isolated loads. If the concept of the load is the overly simplistic two
terminal network, and it is isolated from ground, clearly current into
one terminal MUST equal current out of the other terminal, no balun is
required.

Owen