[Top] [All Lists]

Re: [Amps] RF in the Audio

Subject: Re: [Amps] RF in the Audio
From: Jim Brown <>
Date: Sat, 24 Sep 2011 13:50:16 -0700
List-post: <">>
On 9/24/2011 12:07 PM, Rob Atkinson wrote:
> I may have missed something but I was unable to find any sort of
> common mode choke on the DX Engineering website,

MANY of the DX Engineering so-called baluns are common mode chokes -- 
indeed, what is commonly called a "current balun" IS a common mode 
choke. Many DXE baluns that transform impedance are ARRAYS of common 
mode chokes connected in series and parallel.   If you open up some of 
these you will clearly see chokes would not with coax, but with parallel 
wires.  And DXE DOES sell a common mode choke. I haven't bought one, 
because I can rolll my own that are probably better for one-sixth of the 

I HAVE inserted the bifilar chokes between the output of a Titan 425 and 
the antenna tuner and tested at 1.5kW keydown for several minutes from 
1.8 MHz to 28MHz. At that point, the choke sees ONLY the differential 
field, and there is VERY little heating because the field from one 
conductor cancels the field from the other.  Dissipation due to common 
mode current is a very different matter, and is discussed at length in 
the tutorial. In essence, if the choke as sufficiently choking high 
impedance and the antenna is not very poorly balanced, the common mode 
current, and thus the common mode dissipation, is reasonably small.  If 
conditions of the application (for example, impedance transformation) 
place very high common mode voltage across a choke, the common mode 
impedance must be much higher.  In a testing situation, I have set up 
very high common mode voltages and placed two chokes in series to 
withstand them.  DXE builds some of their impedance transforming arrays 
of chokes that way.

As to mismatch -- a study of the fundamentals of transmission lines 
would lead one to the conclusion that the loss due to mismatch in the 
short length of 100 ohm line that comprises the choke is quite small.  
After all, one of the most common uses of parallel wire line (notice 
that I do NOT repeat the fiction of calling it a balanced line) is to 
minimize the loss due to mismatch when feeding antennas that are wildly 
mismatched, like the "one-size'fits-all" dipole that is nowhere near 
resonance on most frequencies where it is used.  Think about this -- 
we're connecting an antenna that could be anything from 5 ohms to 5,000 
ohms, plus reactance, to a feedline  that is, perhaps, 400 ohms.  The 
insertion of a 24 inch piece of 100 ohm line simply modifies (and not 
very much) the impedance of the antenna as seen by the line. And, if 
wound using #12 copper, as the chokes I have described are, the loss is 
VERY VERY small, as confirmed by my tests.

Now, I'm a guy who plays by the rules, and shares my work FOR those who 
play by the rules, and my testing is done at that power level, at duty 
cycles consistent with serious contesting. Someone who wants to run more 
than 1.5kW can design and test his own solutions. :)

73, Jim Brown K9YC
Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>