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About

A common mode choke can be used to reduce the unwanted EMI radiating back from an antenna to the radio. The principle is that antennas tend to produce current that travels back over the outer sheath of the antenna cable to the radio. A common-mode choke, or a 1:1 unun is a particular case of a balun, that acts as a coil that generates a magnetic field using the backflow current and thus expends the current before it reaches the radio equipment. Typically a common mode choke is placed close to the antenna connection, but several chokes can be used if necessary, closer to the radio.

Placement

The following ASCII representation is a cross-section of a typical ham radio antenna with the central pin marked as $c$, and with both sides of the surrounding grounding braid marked as $s_{i}$ (or interior shield), respectively $s_{o}$ (outer shield). 

             so
            .---------.
           / .-------. \
          / / si      \ '
          | |    +    | |
          \ \     c   / /
           \ `-------' '
             `--------`

Depending on the SWR, the problem is that EMI tends to radiate back from the antenna, via the outer face of the inner grounding braid, marked as $s_{o}$ in the ASCII representation. The EMI then travels all the way back from the antenna, through the cable and into the radio where it produces the unwanted interference.

Given the former, and with a common-mode choke prepared that will cut off the EMI, the question is where the unun should be placed. The following is a representation of a typical HAM shack with a radio, with an antenna on the side and with a connecting cable between the antenna and the radio inside the shack. 

       ^
      / \
      \ /
       v
       |
       |
       |                ...............
       |               /               \
       |              /                 \
       |             / SHACK             \
       |            +---------------------+
       |            |                     |
       |  1         |                     |
       +mmm         |                     |
       |   \        | 2         3 +-------|
       |    +------mmm---------mmm| RADIO |
       |            +-------------+-------+

Assuming that the antenna is good and manages to put out a big majority of the power that the radio manages to emit, then a good choke point is 1, right next to the antenna feed and before the shack and the radio itself. Clearly though, the antenna will never be perfect, such that the cable itself will end up loaded with EMI even outside the shack, such that placing a choke at 2 is also a good option and, perhaps, the best out of all the options because it does not allow the EMI residual to enter the shack at all and perturb any of the equipment inside the shack.

Another choke point is 3, right next to the radio, which should be decent enough for the radio itself and it also depends at what distance from the radio the choke is placed, but the EMI will still be carried inside the shack and will end up interfering with the rest of the equipment. Of course, it is also possible to choke all three points, 1, 2, 3 given enough ununs but it should not be necessary and with that much of a required dampening, something else might more likely be the culprit.

When in doubt, see EMI as some sort of unwanted radiation that should rather stay outside the house and place any common mode chokes to ensure that when the line is crossed inside the house, the EMI will stay out. In other words, in the ASCII illustration, the best choke point is perhaps number 2.

Results and Expectations

One of the issues that the common-mode choke should solve is some humming that can be perceived when transmitting with the radio that should not be there. The humming is usually due to an improper antenna ground plane, which is characteristically solved with a counterpoise and/or the 1:1 unun common mode choke can also make it much better.

Otherwise, a nice side-effect, should be that the attenuation provided by the 1:1 unun will make it such that background noise will be greatly diminished. This should be observable simply by looking at a waterfall spectrum and noticing that the noise pollution should be greatly reduced.

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