[TowerTalk] Coax, conduit and toroids

Jim Brown jim at audiosystemsgroup.com
Wed Jun 27 12:48:33 PDT 2012

On 6/27/2012 11:48 AM, Steve Hunt wrote:
> Jim,
> You'll not be surprised that I disagree strongly:
> 1) It is a BalUn - it's a device that interfaces a BALanced load (the
> antenna) to an Unbalanced source (radio + coax)

A "balun" is a very fuzzy word given to a generic black box (often a 
"mystery black box)  that can consist of at least a dozen VERY different 
things.  You may USE a common mode choke to connect an unbalanced 
circuit to a balanced one, but if you want to understand how it works, 
you must know that it a common mode choke, NOT a balun. You can use a 
transformer to connect a balanced circuit to an unbalanced one, and if 
you want to understand how it works, you must know that it is a 
transformer, NOT a balun.  You can use a section of transmission line to 
feed a balanced antenna from coax, but to understand how it works, you 
must know that it is a transmission line, not a balun. You could use an 
array of common mode chokes connected in a series/parallel configuration 
to connect a balanced circuit to an unbalanced one, but to understand 
how it works you must know that they are common mode chokes, not a 
balun. You could use an active device called a video balun to drive 
baseband video down CAT5 cable and another active device to receive it, 
but to understand how it works you must know what is in the box, not 
that it is a balun.  I could go on.

> 2) If the load is completely balanced with respect to ground, the two
> load terminals will be at Vl/2 and -Vl/2 wrt ground, where Vl is the
> differential load voltage. A choke having its output connected to that
> load, and its input connected to an unbalanced source, will have a
> Common-Mode voltage across it of Vl/2

Well yes, but I specifically said that if the antenna, the feedline, and 
the termination of the feedline was balanced, the common mode voltage 
and current are, BY DEFINITION, zero. YOUR circuit is not balanced, it 
is UNbalanced by the use of coax as a feedline.

Further, a choke that is 80 ft in the air is NOT connected to GROUND, it 
is connected to an antenna and to a feedline, and both are subject to 
the transmission line equations. AND your example was a WORST CASE one 
-- the feedline was close to a half wave at the operating frequency.

> 3) Yes, antennas and feedlines are complex structures. So I invite any
> readers to model using NEC a perfectly symmetrical dipole plus a braid
> path from one side of the feedpoint to ground. Then insert a high
> impedance load close to the feedpoint in the braid path and check what
> voltage it has across it.

If you study the literature you will find that an NEC model, including 
W7EL's documentation or his version, has serious limitations in the 
analysis of the common mode circuit.

> I just tried it with a perfectly symmetrical 20m dipole at a height of
> 35ft. I placed a 5000 Ohm resistive choke in the braid path at the
> feedpoint. Applying 1000W, the differential voltage across the feedpoint
> was 265v rms and the voltage across the choke was 135v rms - almost
> exactly half, as you would predict for a perfectly balanced antenna.

Yes, but what was the voltage for a 50 ft feedline?  A 70 ft feedline?  
More important, did you note the dissipation in the choke in your 
example was only 4 watts under keydown conditions?

And did you model a choke connected to an ideally balanced antenna with 
a parallel wire feedline with a balanced termination at the 
transmitter?  THAT is the condition of circuit balance! What impedance 
did you use for the common mode circuit between feedline and "ground?"

> 4) If a choke has no CM voltage across it, the core flows no flux; if
> there is no flux you can safely remove the choke and nothing will
> change. Does anyone really think that you don't need a choke on the coax
> just because the antenna is completely balanced?

You continue to be unable to see the forest for the trees. The PRIMARY 
reason for using a high impedance common mode choke on a transmission 
line is to prevent the feedline from being part of the antenna, both to 
minimize noise on receive and to minimize RF current in the shack 
causing RFI.  We also use a common mode choke on a feedline just as we 
would use an "egg insulator" in a guy wire -- to prevent the feedline 
from being a parasitic element of another antenna.

Real world antennas are NOT ideally balanced, so there is nearly always 
SOME common mode voltage and current.  When common mode chokes are used 
in antennas that are reasonably close to balance, as in most dipoles and 
most antennas made of aluminum, the common mode voltage across the choke 
is relatively small, even in your worst case example. BUT -- a common 
mode choke in a circuit where the antenna is seriously out of balance, 
for example, one that is fed well off center, can see a high common mode 
voltage, again depending on feedline length. This worst case scenario 
can easily be modeled by NEC, but it will still be worst case, because 
we don't know the actual common mode impedances involved.

AND if two or more common mode chokes are used in the series common mode 
circuit (whether at the same or at two different points along the line), 
the common mode voltage will divide between them, the total dissipation 
will divide equally between them, and, because the common mode current 
will be divided by two, the total dissipation will be half that for one 
choke, and each choke will dissipate one quarter of what it would if 
only one choke were in place.

73, Jim K9YC

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