Needing a paired 49:1 transformer and a common mode choke (CMC) for a permanently installed EFHW, I whipped one of each up and mounted them in some 50 mm PVC pipe.

Parts:

• Toroid: Fair-Rite 2643251002. This is a high mass core, great for use with higher power or poorly matching antennas.
• 1300 mm Teflon coated wire. The teflon coating has a very high heat rating, just in case.
• 100 pF, 500+ V capacitor.
• 2 of 50 mm PVC pipe end caps.
• 50 mm PVC pipe, 150 mm long.
• M6 stainless eye bolt.
• M6 stainless hex bolt.
• SO-239 socket.

I had already wound my CMC, so needed to quickly whip up a 49:1 auto-transformer.

The schematic calls for 14 turns total, with a tap point after the second turn. I created the pinch point 200 mm from the end.

Drill two holes in one PVC cap and mount the hex bolt and eye bolts, with a solder tab. I drilled the holes slightly undersized so that the bolts cut the thread into the plastic, then used some Loctite to create a waterproof seal.

As can be seen, the primary of the transformer is connected directly to a short length of coax with the CMC on the other end. When mounted, the secondary gets a connection to the antenna and the counterpoise (CP).

The PVC sleave can then be slid over the assembly, and the CMC soldered to the SO-239 socket in the other cap.

A piece of cardboard was included as a stand-off, so that the two cores didn’t end up one atop the other.

I really wish my artistic skills were a little better, but hopefully you understand how it’s deployed. The eyelet is shared between the suspending rope and antenna, whilst also being the electrical connection for the antenna. The lower bolt is for the counterpoise, whilst the coax exits the rear.

That’s it, it works and replaces my older much larger separate transformer and CMC setup.

Lance

September 2025