[TowerTalk] Coax, conduit and toroids

Steve Hunt steve at karinya.net
Wed Jun 27 11:48:42 PDT 2012


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)

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

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.

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.

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?

Steve G3TXQ

On 27/06/2012 19:20, Jim Brown wrote:
> On 6/27/2012 9:59 AM, Steve Hunt wrote:
>> Jim's comment should not be misunderstood. A 1:1 choke balun feeding a
>> _perfectly_  balanced load does not have zero common-mode voltage across
>> it; the choke CM voltage will be exactly half the voltage across the load.
> This is incorrect.  First, it is a CHOKE, not a "balun."  Second, if the
> CIRCUIT is balanced, the common mode voltage and the common mode
> current, by definition, will be zero.  Third, both the differential
> circuit and the common mode circuit are complex -- they include the
> antenna, the feedline, and the termination of the feedline at the
> transmitter.  The contribution of the feedline is also exquisitely
> dependent on its length AS A COMMON MODE ELEMENT, especially if it is coax.
> 73, Jim K9YC
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