Simple indoor / attic coax rf-choke

[hertzian]

Here's a shot of one of my favorite new accessories - a common mode cable choke.

The coax is just hand formed to about 3-1/2 inches and wound as a solenoid, that is each loop is next to each other. Around this are Radio Shack #273-104 snap-on ferrite chokes. I really like these ferrites as they come with some specs and documentation. Apparently the material is effective up to about 500 mhz. Make sure you snap these back together tightly.

I could wind about 2 turns of RG-6, or 3 turns of RG-59. I'm only using a short run to the attic, predominantly at 118-135 mHz so I don't feel the need to run heliax, but do check out other threads on cable attenuation.

In the pic, I use something like a "binocular" version. Other options are just stacking them on one side, stacking more on each side, or even using none at all and just winding about 5 to 6 turns together. I like to get fancy here as the ferrites provide about 12db of gain. Just kidding.

I actually use 4 ferrites - two on each side, but use just two here for illustration.

I use it very close to the feedpoint of dipoles, just below the tips of the radials of ground planes, or actually choke the feedline at 1/4 wave away from the feedpoint with other antennas to purposely use some of the common-mode current -- to trick an antenna into using a different pattern. (My deepest thanks to W8JI and W7EL)

Offset-fed antennas and those that use tv-type "baluns" need these, as the balun in an offset fed antenna is totally swamped by the severe non-symetrical setup, along with the fact that the voltage-baluns that are inside most tv-type baluns make the situation even worse. So while the tv-type balun might help with impedance transformation, you need to make up for the missing balun-action with an rf-choke.

In some cases, these chokes may stop severe common-mode current coming from your shack, and traveling up to the antenna. You may find some locked-out channels usable again. Maybe.

For a couple of turns of coax and some cheap ferrites, these go on every antenna I use these days....

[hertzian]

K6STI shows a great way to wind one for the FM broadcast band and you can see it in action on a spectrum analyzer.

Simple 75Ω Coax Balun

[hertzian]

Now that I think about it, this might be overkill for scanning multiple bands.

In this case, I just stacked about 4 of these together, but instead of tightly tie-wrapping the coax turns, I left them separated all fanned out around the chokes.