Last Revision: 08 April 2008
Since a while I have a monoband dipole for 40m (7 MHz) running with fairly good results. With all my antennas I have built throughout 2006 and 2007 for 40m, I found a lot of noise finding its way to the input of receiver. There were stations calling me which were just at the noise level and giving me good reports. This noise level always has been close to S7 on my TS690s. I do not know the absolute value, but it is quite high. With my full-size vertical antenna, this level has been close to the S9 level. All this happens while living in a rural area, where man-made noise levels are significantly lower than in well populated cities.
A common way for man made noise is - it is obvious, that we are talking about this - radiation from power lines, data lines and so on into the antenna. But another way is the noise which finds its way along the feed line up into the antenna. In most cases you forget about about this fact. I recommend reading the article by Chuck Counselman, W1HIS. (see below)
In short words: I tried to cut this way and for this I inserted two common-mode chokes into the feed-line. One is placed in the shack, the other I mounted close to the antenna, which already has a 1:1 balun transformer installed. But this 1:1 balun is obviousy not sufficient. So I built a common-mode choke consisting of two ferrite cores available at DARC (www.darc.de) Type RK1.
I put 4+4 turns of RG58 coaxial cable on two stacked ferrite cores. One of these chokes I placed close to the rig, the other is now close to the antenna. Furthermore it may be worth placing a third choke somewhere in the middle of the feedline because rf may be picked up here too.
The common-mode choke mounted to the 1:1 balun of the 40m monoband dipole. This balun is fixed to a 1,5m pole of plastic tube. There is no metal mast close to the antenna which would be able to carry unwanted rf to and from the antenna itself.
Oh yes, I am pleased with the result ! Noise level is now down to the S2..S3 level (from S7 before). I am hearing many more stations, which were covered by noise up to until I installed these common-mode chokes. In the evening times I now hear many more U.S. stations, which were frequently buried by noise before. So as this is my only wire in the air, I use this same antenna for receiving DRM radio stations. Reception always suffered from the noise level around. Improvement here is fun now again. Overall DRM quality has become much better. I found the most improved band however is the 30m band. Even though not designed for 10 MHz, I frequently listen there. Noise level has been around the S9 level, but now it seems completely gone. Matching of the antenna has not changed by the chokes, so I can transmit as before. The price for this improvement has been around 16 Euros for all.
So installing common-mode chokes is not only helpful with keeping RF away from the cable and of course from the shack. It can - like in my case - improve reception dramatically.