[TowerTalk] Common Mode Choke

[Jim Brown]

On Sun,5/22/2016 5:42 PM, Don W7WLL wrote:
> Has anyone some experience with the use of a CMC and typical single band mobile whip antennas?

Hi Don,

Yes. We use ferrite chokes at the feedpoint of the verticals we use for 
mobile operation, including the recent 7QP trip through 7 counties in 
NV. Simply follow the winding guidelines in my cookbook. 
k9yc.com/RFI-Ham.pdf

The designs using 5 cores are for use with high power (up to legal 
limit). For 100W, a single 2.4-in o.d. core is big enough, but you'll 
need more turns -- use the graphs in Appendix One of measured data for 
1-14 turns small diameter wire on a single core as a guide.

Or simply wind five turns through a 1-in i.d. clamp-on. Use #31 for 80M, 
#31 or #43 for 40M and above.

Another VERY important point. Modern cars go nuts with excessive paint, 
so much so that most parts of the body of many vehicles are insulated 
from each other by that paint. This can make it VERY difficult to find 
and connect to enough of the body to form a return for antenna current.

Also -- with respect to that mag mount -- the ones I've seen often fail 
to even make an electrical connection between the cable shield and the 
metal mount that couples to the vehicle roof. When this happens, there 
is, for all practical purposes, NO return for antenna current. For that 
antenna to work, it MUST have a return. If it doesn't return to the 
body, it will return via the coax. But we're choking the coax, so it 
doesn't have that either.

And, BTW, the likely reason you had RFI issues with your mag mount is 
that the mag mount failed to provide good coupling to the body. :)

Does this sound like there's no free lunch?  :)

73, Jim K9YC