[TowerTalk] BALUN revisited again

[Jim Brown]

On Fri,4/8/2016 10:13 PM, Jim Thomson wrote:
> ##  say what ?   AFIAIK, he uses  FOUR type 31 cores for the CMC-230-5K ,
> 2 x cores  for each assy.

Could be -- can't tell from the photos. That helps with power handling, 
and it also adds L, which lowers the resonance.

>    8000-11,200 ohms of RS is nothing to sneeze at. Its
> rated at 5 kw pep out.   It wont blow up with 1.5 kw cxr.  Its been tested. 2-30 mhz
>
> ##  The BL2006-3k  is a 14-54 mhz balun.  Single core.   5400- 8600 ohms of RS
> is again, nothing to sneeze at.   Rated for 3 kw pep out.   It  wont blow up on 20-6m,
> with 1.5 kw cxr.

There's a serious fallacy here about power ratings for chokes that use 
the resistance at resonance (which is the right way to do it). 
Dissipation in this kind of choke depends STRONGLY on the common mode 
voltage, which in turn depends strongly on the antenna system, INCLUDING 
the feedline and its length.

> ##  Your  single  core overheated on 80m.   His BL2006-3k  Is rated for 14-54 mhz.
> Its not intended to be used on 80m!

Operating frequency is only one factor. The choke I fried was more turns 
to move the resonance down so that it would cover 80M, and it provided 
about 5K ohms resistive. But that antenna SYSTEM needed at least twice 
that choking Z to run US legal limit.

Dissipation in a choke is the SUM of the differential mode power and the 
common mode power. N6BV wrote an excellent piece that ran in QST a 
couple years ago on the issue of differential mode dissipation in chokes 
with severely mismatched antennas (he predicted disaster), and ran it 
past me first. I told him that his analysis of differential mode was 
right on, but that he had failed to consider common mode, which makes 
matters even worse!

Another thing that bothers me about these chokes is that they are in 
what appear to be sealed enclosures, which can limit the ability of the 
assembly to conduct heat away from the choke, reducing its power handling.

The key to power handling is to understand that power is I squared R, 
and that the choking Z must be high enough to reduce the current so that 
dissipation is not excessive. Because power is increasing (or 
decreasing) as the square of the current, you CAN outrun your tail by 
making the choking Z high enough, because the power is dropping twice as 
fast as the current. :)

The differential mode dissipation must be modeled using transmission 
line techniques -- a Smith Chart or N6BV's TLW (free with the ARRL 
Antenna Book) will get you right answers. We must model the common mode 
part of the problem by adding the equivalent circuit of the choke(s) to 
an NEC model, where the choke is added to a single conductor the length 
of the feedline running between the feedpoint and ground (or wherever 
the coax is connected). Remember -- the equivalent circuit of a ferrite 
common mode choke is NOT best described as a series R + jX, but rather 
as a parallel resonant circuit, where L is the mid-to-low frequency 
inductance, R is the Z at resonance, and C is the stray C that resonates 
with L to establish the resonant peak. That simple circuit works for #43 
and for #31 above about 7 MHz. For #31, it's more complicated than that, 
because it has both a circuit resonance and a dimensional resonance that 
give it a double humped response (like a stagger tuned IF), which is 
what gives #31 its extended bandwidth.

Much of this is developed in the tutorial stuff on my website.

73, Jim K9YC