There are a couple of issues to consider: we want to reduce the CM current on the coax so that we maintain reasonable balance in the ladderline; and we want to ensure that the residual CM current through the choke doesn't cause a heating problem. Both issues depend on the choke's load impedance, and the CM impedance looking back along the coax braid.
If we assume, worst-case, that the braid impedance is zero, and assume that the VSWR(50) looking into the ladderline never gets above 4:1, a choke with an impedance >1000 Ohms should ensure reasonable current balance.
If we now assume the choke is wound on a single FT240 size core, we should try to limit its average power dissipation to 10W. To do that, if the average power into the ladderline is 100W, the choke impedance needs to be >500 Ohms.
So, to meet both criteria a choke >1000 Ohms will do the job.
Please note that only conducted TX CM current has been considered, and those calculations assume: the choke is resistive; the average power level is not greater than 100W; and the choke load impedance never exceeds 200 Ohms balanced. Change any of those factors and the answer changes
1. The impedance looking toward the antenna is 50 ohms and the impedance looking back toward the transmitter is 2000 ohms. One might not even need a choke under those conditions.
2. The impedance looking toward the antenna is 5000 ohms and the impedance looking back toward the transmitter is 200 ohms. One definitely needs a choke and a choke with that much choking impedance is difficult to come by.
By choosing an appropriate length of ladder-line that ensures a low impedance at the choke, one reduces the choking requirements and stress on the balun. Seems to me, random choices of ladder-line length tends to obey Murphy's law.
"Vary the length of the ladder line."
That is a new idea to me. Smart, but new. How do you do that? Can you make that adjustment without physically manipulating something at the radio end of the ladder line? Would it still be practical if your ladder line did not drop right into your shack? Thanks for the idea...
Am le haghaidh Dia. Téigh le Dia.
I'm experimenting with switching lengths using one DPST switch rather than 2 x DPDT switches per loop. Here's another method that I once used for varying the length of the ladder-line. It worked very well and could be automated using a screwdriver motor.
A question not answered is the type of ferrite? A FT-240-43 will work fine 80-10M and wound conventionally with RG-8X or RG-142, depending upon power and VSWR, they are in stock at Mouser under the real Fair-Rite part 2643803802 at far less than Amidon.
If you intend to build a different antenna to include 160 then the 31 mix is a better choice.
The JR method adds nothing to the performance.
Thanks for checking in Carl. I already have the the FT240-43 and the 142 coax. I also have some 31 mix cores. I guess the 43 mix it is. All I need is the appropriate weather resistant container and I'm set. Thanks very much to all who responded and shared their knowledge and experience. VY 73 and Shalom from Jerusalem. Kal, 4X1SO
The "W1JR" winding with the center crossover has very little affect on the magnitude of the broad resonance peak, but it does move that peak to a higher frequency. With the conventional winding method, the start and finish of the winding can be very close together, so the self-capacitance of the winding is quite large and also quite difficult to control. The crossover winding places the start and finish on opposite sides of the core, so the self-capacitance is not only smaller (which increases the resonant frequency) but also much more predictable.The JR method adds nothing to the performance.
Tthe crossover configuration often gives a more convenient layout as well. For many applications I simply secure the opposite ends of the winding with cable ties and let the whole thing hang in the coax. The weight only becomes significant when someone insists on using a box.
73 from Ian GM3SEK
Very true Ian but the 10M end is rarely a problem using 43 material. At 80M it becomes a problem to get sufficient choking with a limited number of turns and a single core.The "W1JR" winding with the center crossover has very little affect on the magnitude of the broad resonance peak, but it does move that peak to a higher frequency.
Putting it in a metal box as JR did is not a good idea either, at least when I tried it at the 1200W level on a G5RV up 70' between 2 pine trees. The RG-213 feed was hot in the shack. I finally ended up with a plastic box plus 12 large 43 mix beads over the RG-213 at the box end. Yes it was heavy but the #12 Copperweld for the antenna and the #12 copper for the balanced line held up well until I built a real antenna system.