On Tue, 2 Sep 2008 19:31:18 -0700 (PDT), wrote:

In a perfect situation, with a balanced feedline, the only kind of
current and voltage you have IS common mode!

Many US amateurs do not understand how a balun works.

How true - not even fundamental terminology.

73's
Richard Clark, KB7QHC

In message , John Smith
writes
JB wrote:

...
There ya go.
One of the problems of breaking up the path to ground on the shield is that
now static can build up. If you can provide a way to bleed off the charges
that build up on both halves, while breaking up the shield currents, now you
have made it worthwhile. But when you guys start discussing off center fed
dipoles, I step aside. I have computers in the shack. I have better luck
with fan dipoles. Also running a Butternut vertical. Works a lot better
than a 4btv, but a pain to get it right on all bands.
Thanks
Also liked the other pdfs presented. I never have time to read them
all
because of constant interruption around here. Back to work.


OCF antennas were not fully appreciated by me, a few of Cecils' helpful
insights and encouragement and I built one and was rather surprised ...
I do not have one right now but that is only due to neighbors/property
constraints.

Here is a URL for design/implementation of "non-standard"
baluns/transformers, but of a highly useable and desirable nature--or,
Dr. Sevick strikes again!:

http://www.highfrequencyelectronics....104_Sevick.pdf

Fig. 6(A) is very interesting. A 5-winding, 1:1.56 bootstrap
transformer which provides 50/75 ohm connections/substitutions.
Perfect for allowing one to use 75 ohm "junk" (or found in dumpsters)
tv coax in place of more expensive 50 ohm coax.

I have made good use of this since I have thousands of feet of NEW 75
ohm coax I purchased from a scrap dealer for next-to-nothing! A lot of
large dia coax and hard-line mixed in!

Regards,
JS

What about the loss in the balun? You need to decide if the loss in the
balun is less than that you would get if you simply used the 75 ohm coax
direct. And ask yourself 'In my application, why would 75 ohm be more
lossy?' Also, with similar physical dimensions, construction etc, 75 ohm
coax is slightly less lossy than 50 ohm (current is less, so 'I squared
R' loss is less).
--
Ian

Ian Jackson wrote:

...
What about the loss in the balun? You need to decide if the loss in the
balun is less than that you would get if you simply used the 75 ohm coax
direct. And ask yourself 'In my application, why would 75 ohm be more
lossy?' Also, with similar physical dimensions, construction etc, 75 ohm
coax is slightly less lossy than 50 ohm (current is less, so 'I squared
R' loss is less).

Simple solution, I trust my S-meter!

Regards,
JS

--
It is like a nightmare where the public servants are the people which
the police are supposed to protect us from!

Richard Clark wrote:

...
How true - not even fundamental terminology.

73's
Richard Clark, KB7QHC

When you are right, you are right; and, YOU GOT THAT ONE RIGHT! :-)

Regards,
JS

--
It is like a nightmare where the public servants are the people which
the police are supposed to protect us from!

wrote:
For a balanced transmission line, the characteristic impedance is not
expressed in differential mode terms, it IS common mode so I do
not know why you ask about differential mode characteristic impedance.

Well, there's your problem. In a properly functioning
transmission line, whether balanced or unbalanced, the
currents in the two conductors are differential, not
common-mode. The two currents are equal in magnitude
and 180 degrees out of phase and their fields tend
to cancel which minimizes radiation.

Common-mode currents only occur when the two currents
are not perfectly differential, i.e. not equal amplitudes
and/or not 180 degrees out of phase.
--
73, Cecil http://www.w5dxp.com

wrote:
I disagree. There is no choking, there is merely a polarity and phase
shift between the "primary" and "secondary" of the 1:1 balun
"transformer".

In a w2du balun, zero flux in the beads is introduced
by the transmission line currents. All of the flux in
the beads is introduced by common-mode currents. Thus
the impedance of the beads *IS* a choking impedance
aimed only at the common-mode currents.

My choke-balun is ten turns of RG-400 on an FT-240-77
toroid. It's only purpose is to provide a choking
impedance to common-mode current. That choke-balun
doesn't know if it is being used as a UNUN or as a
BALUN.

Most baluns are designed such that near-zero flux
is introduced in the ferrite toroid by the differential
mode currents. Essentially, the only flux introduced
in the toroid is due to common-mode. The resulting
impedance *IS* a choking impedance.
--
73, Cecil http://www.w5dxp.com

wrote:
By the way, I have seen so many articles about baluns written by other
hams and they tend to repeat the same mistakes and assumptions. Most
hams do not understand how a balun works.

Your following statement puts you in that category.

Inserting a balun does not "choke" the current in the shield, it
merely shifts the output phases so that the current (voltage) is
directed towards the dipole at all times (see my other post in this
thread).

This applies to a W2DU balun. Why would you want
differential current fields to be introduced into
the #77 ferrite beads? Their entire purpose is to
provide a large choking impedance and dissipate
common-mode power.
--
73, Cecil http://www.w5dxp.com

wrote:
Many US amateurs do not understand how a balun works.

Quoting "The ARRL Antenna Book" re a w2du balun:
"Maxwell made a test balun by slipping 300 #73 beads
over a piece of RG-303 coaxial cable. The impedance
of the outer conductor of the cable measured
4500+j3800 at 4.0 MHz."

The differential current emerging from the inside
of the coax braid encounters, e.g. 30 ohms, looking
toward the antenna. It encounters 4500+j3800 = 5890
ohms looking back down the outside of the coax.
Ohm's law does the rest.

Without the beads, that differential current might
see an impedance lower than 30 ohms looking back
down the outside of the coax for certain unfortunate
lengths of coax. The common-mode choking impedance
forces ~equal currents in each dipole leg at the
BALanced antenna to UNbalanced coax junction. That
common-mode choking impedance causes the balun
function to occur.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Most baluns are designed such that near-zero flux
is introduced in the ferrite toroid by the differential
mode currents.

That should have been, "Most 1:1 current baluns ..."
--
73, Cecil http://www.w5dxp.com

Richard Clark wrote:
wrote:
Many US amateurs do not understand how a balun works.

How true - not even fundamental terminology.

http://en.wikipedia.org/wiki/Balun (see 1st photo)
"This is a simple RF *choke* which works as a *balun* by
preventing signals passing along the outside of the braid."

From "The IEEE Dictionary":
"balun (1) A network for the transformation from an
unbalanced line or system to a balanced line or system,
or vice versa."

From an Unabridged Webster's:
"balun - a device for converting a balanced line into
an unbalanced line and vice versa."

10 PRINT "A W2DU balun is both a choke and a balun!"
20 Goto 10
--
73, Cecil http://www.w5dxp.com