One of the challenges of operating from an 'urban' environment is the constant battle to reduce the received noise on the HF bands. Since a lot of that noise is conducted via common-mode signals, ie; on the outside of coax and control cables, part of my new antenna effort includes placement of RF chokes at critical locations to block the noise. As well, since the tower will be used as part of the 80m/30m sloper antenna system, chokes are required to keep the RF from running back into the shack on the outside of the cables.
The choke at the top of the picture is the feedpoint choke for the yagi, a 3-element Mosley S-33 17/20/40m. The other three go at the bottom of the tower. The one on the left is for the rotor control cable and the two on the right are for the antenna feedlines, one each for the yagi and the sloper. All the antenna chokes are made from 13 turns of RG-142 teflon coax wound through a double-stack of mix 43 ferrite rings and enclosed in a weatherproof PVC box. They should provide a high choking impedance right across the HF spectrum and be good for full legal power and a modest amount of SWR, although I probably wouldn't want to push them too hard at any non-resonant frequencies.
A good source of information on how to tackle RFI issues and keep the common-mode noise at bay is Chuck W1HIS. His paper on the subject is amazing and anyone building a station needs to give it a read. I had to think hard about what he wrote for a long time before I decided that he's not crazy, although his station does seem to employ a significant fraction of the world's ferrite production.. If you have read this far and it is still the weekend of the Sweepstakes contest you will have undoubtedly surmised by now that I will NOT be on the air for the contest. I had intended to get the new tower and antennas up in time for CQWW SSB at the end of October but here it is SS Phone weekend in November and I'm still QRT. Life has a way of messing up the best intentions and between a bout of the flu, a string of unexpected trips out of town for various reasons, and the ever decreasing amount of daylight available, things have just not come together as quickly as I had hoped. Nevertheless, I will continue to plug away and really do expect to be on the air again within a couple of weeks. Stay tuned!