A crossover common-mode choke is perhaps the simplest choke possible consisting in 10 to 12 windings around a ferrite toroid with great results for HAM radio bands.

• FT240-43 ferrite toroid,
• RG-316 antenna cable,
• SO-239 sockets

Note that typically this type of unun is made the very same way by winding the common RG58 antenna cable, which seems to work up to about 8 turns and still provides a good dampening, however ideally the antenna cable should be as close as possible to the ferrite core and the RG58 cable is not as malleable as the RG-316 such that for this choke an RG-316 cable will be used instead of an RG58.

There is some difficulty here, in particular, in dealing with the RG-316 ground braid that seems very brittle to work with and the braid must be cut and then wound into a wire to connect the ground, which has to be done for both ends of the RG-316.

Another particularity is that the SO-239 sockets are mounted on top of washers made out of wood (observable in the image) and each side of the box is fit using nylon screws in order to ensure that the ground of the antenna is isolated from the case of the unun itself.

In principle, one single toroid wound up this way should be enough to quench any EMI from teh antenna, but in this case, two toroids were used, stacked upon each other, effectively doubling the power of the choke. It might be overkill for the lower bands (higher frequency) but doubling the dampening works a lot better for higher bands like 20m.