Thanks Jim!
I use your data and the G3TXQ data for choke design.
*Using K9YC CM choke data*
Let's determine the maximum power loss of the feedline CM choke when
running 1500W key down. Taking the sloping vertical with two elevated
radials 10' over 0.005/13S GND I establish a "low enough" impedance
counterpoise below the two elevated radials and 1' above GND. By "low
enough" impedance I mean a counterpoise presenting an impedance to GND of
less than the choke impedance by a factor of, let's say, at least 5X. To
that end I place a dozen 50' radials 1' above GND. A wire representing the
feedline shield connects that counterpoise to the two elevated radials.
Driving the antenna with 1A EZNEC shows 0.5A in the feedline shield wire
which won't do.
Let's place your *#31 Toroid,2.4-in O.D. RG400 CM choke having 18 turns*
and CM impedance of 10k in series with the feedline in the 160 meter band.
That is the first CM choke in Table 2. For worse-case analysis I will call
this 10k ohms resistive. The feedline shield current is 1.8 mA which seems
low enough. Changing the RF source from 1A to a power level of 1500W
antenna current is 7A while feedline shield current is 12mA. The CM choke
power loss is Pd = I^2 x R = (12mA)^2 x 10k ohms = *1.4 watts*. Without
bothering with thermal calculations we know that will work.
*Using G3TXQ CM choke data*
The 17T on RG58 on an FT240-31 toroid is between 2k and 4k ohms. Let's call
it 2k ohms and put that in the EZNEC model as a load in the feedline shield
wire. EZNEC reports 58mA of feedline shield current when running 1500
watts. Pd = I^2 x R = (58mA)^2 x 2000 ohms = *7 watts*. With this the choke
will get plenty hot during key down conditions but might be fine even in
contest operation. For worse-case CW contesting CQing I estimate duty cycle
to be around 40%. So, 40% x 7W gives us 3 watts and I think the CM choke
will be hot but okay with RG400 coax. I can run thermal calculations or try
a 4-core design and skip the thermal stuff. G3TXQ shows 17 turns on 4
stacked FT240-31 cores to present an impedance of 4k ohms in the 160 meter
band. Placing this in the EZNEC model at 1500W shows 28mA of shield
current. Pd = (28mA)^2 x 4000 ohms = *3 watts.* This spread over 4 cores
will surely run cool enough.
NH6V and I run a 4-core balun on an essentially identical sloping wire
ground plane.
There may be mathematical errors so check carefully.
Dave KH6AQ
*K9YC chokes* http://k9yc.com/2018Cookbook.pdf
*G3TXQ* http://www.karinya.net/g3txq/chokes/
On Thu, Dec 18, 2025 at 10:49 AM Jim Brown <jim@audiosystemsgroup.com>
wrote:
> On 12/18/2025 12:14 PM, Dave Cuthbert wrote:
> > I will post wire lengths and common-mode choke construction.
>
> I wrote the book on that. It's here.
>
> http://k9yc.com/2018Cookbook.pdf
>
> 73, Jim K9YC
>
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