On 1/6/2019 20:12, Jim Brown wrote:
We add one or more common mode chokes to minimize the effect if the common mode circuit, which is radiation and reception, and to do that, a very good choke must be AT THE FEEDPOINT. I don't know of a way to EFFECTIVELY choke a feedline that is not matched to the antenna. Yes, you can put something there, but if it did anything useful it would fry (unless yo only ran QRP to the antena), and a choke that wouldn't fry with TX power won't do anything useful!
I have to respectfully disagree a little here. A choke specifically designed for the antenna impedance at the feed point could effectively eliminate the common mode current. The problem is, we often use balanced line with antennas which have a large range of impedances over frequency range at which they are used. (Think G5RV.) It may be impossible to design a single choke which can work over the whole range. However, feeding balanced line with a balanced tuner can work pretty well, even though the feedline may radiate a bit due to common mode current. I've only attempted this with link-coupled tuners. I'm not sure it works as well with the double T-network tuners with a choke at the input, like the MFJ-976, but it may.
For years, I've used a warped, lop-sided, short, 80 meter "dipole" fed with window line and the balanced tuner. I never bothered to try to figure out the impedance at either the feedpoint or the tuner, but the tuner matches it on several bands. I don't use it much on 80, though, as it's so close to the roof of my house that the RF gets into everything. I'm sure the feedline radiates some, but that's not where the RFI is coming from.
73, Scott K9MA -- Scott K9MA k...@sdellington.us ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
