[TowerTalk] Building K9YC Double Choke

[Jim Brown]

On 12/15/2020 8:00 PM, Kirk Kleinschmidt via TowerTalk wrote:

> SPACING: I see in the choke book that W6GJB has a pair of RG-400 chokes in series, spaced about an inch apart on a length of PVC pipe. Is that sufficient for nomex chokes, or should the spacing be greater? If so, what's a good guess? 2 inches. 3 inches. Etc?

The significance of spacing is to minimize capacitive coupling between 
the two chokes. An inch is sufficient.

> ENCLOSURE: I understand that TX chokes just wanna be out in the elements, but this choke needs to live in a crowded, insulated box at the base of the vertical, so I'd like some way to keep it somewhat self-contained. I was thinking about using a length of PVC pipe (3" ID) to house the chokes. End caps would provide mounts for the SO-239 connectors, and I'd cut the pipe in half length-wise so I can position the chokes and solder the wires to the chassis-mount connectors. When everything is good I can place the top "half" of the pipe in place and tape it together.
> This choke will see max 100W (typically 5-10 W). Will the PVC negatively affect the romex choke's performance? 

No -- magnetic field that makes the choke work is all in the toroid 
core. What IS critical is that the enclosure be non-conductive, so that 
connectors whose shields are bonded to it don't short out the choke.

If necessary, I could mount the chokes onto a smaller PVC pipe and 
attach metal brackets at the ends to mount the SO-239s...but that would 
make it more difficult to keep it separated from relays, tuners, 
connectors, temperature sensors, heating elements, etc.
> 
> (Yes, LDG's RT-100 remote tuner stops working below 10F, so my base box is insulated and heated by a pair of power resistors mounted onto an aluminum heat sink which, in this case, is a heat source. MN winters always go below 10F. The RT-100 has performed well, but I consider the temperature limit a bit of a design fail, but it is what is is.)
> If necessary I could mill long slits into the PVC pipe to ventilate it.

It isn't clear to me if the the pipe running through the center of the 
core, as shown in W6GJB's design, or is surrounding the core? I see no 
problems with Glen's design; I see potential dissipation issues if the 
pipe surrounds it. Also, it will be important that you do some sort of 
moderation of the heater when it's not needed.

Also, the choke MUST be on the matched side of the tuner (that is, 
between the tuner and the coax home run to the shack). And the antenna 
side of the tuner must be bonded to the radial/counterpoise system.

73, Jim K9YC