VK7JB wrote: > >I've been reading the discussions about RF and common mode current and >using a clamp-on RF current meter to sniff out common mode current in >the shack. I've checked the calibration of my MFJ 854 and am surprised >that it's calibrated quite well: I measure 1A of current measured >running 50W into a 50ohm dummy load. >
The MFJ-854 is quite a competent piece of equipment - see my review for Radcom, at: <http://www.ifwtech.co.uk/g3sek/clamp-on/mfj-reviews.pdf> (In contrast the same review warns against the MFJ-805, a definite "Don't Buy".) There is more info about HB clamp-on meters at: <http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm> >So, here's my question: how much common mode current is considered >"acceptable"? This meter measures down to a few milliamps. Is it an >impossible/futile goal to try to eliminate any stray CMC completely? > There is no general answer, because RFI depends not only on the level of CM current but also - very much - on the type of "victim" equipment. As we well know, there is a huge spread in sensitivity to RFI between different types of equipment, and even between different models of the same general type. Also, RFI very often shows a 'threshold level' above which you have a problem, but below which the problem is either disappears completely or is of no concern to the user. All of these things combine to prevent any general answers about "acceptable" levels of CM current. >When should you call it quits and accept what you find? If you don't have an RFI problem, you can usually call it quits and stop worrying :-) However, some of us like to understand what's really going on, so I'll at least try to answer your next question... >Just curious, because the only CMC I can measure in my shack is about >10mA measured at the coax connector on the back of one of my rigs while >tuning up with 10W output. > At 10W you'd be driving about 450mA of normal differential-mode RF current into a 50-ohm antenna feedpoint. Meanwhile, down in the shack, you're measuring 10mA of CM current on the outside of the feedline. Most victim equipment can tolerate 10mA, and you aren't reporting any RFI problems... but I'm not at all sure that everything is OK. It only means that 10W isn't enough power to cause a problem! With 1kW output the CM current would be 100mA, and at that level many kinds of victim equipment might start to fall over. So yes, I think this is worth investigating further. The first test should be to swap to a dummy load, and also to swap transmitters, to see if there's a shielding problem on that particular rig. If all the CM current disappears (as it may appear to do, with a simple diode detector) then the problem probably lies closer towards the antenna and its feedline. The next place to measure the CM current would probably be where the coax feedline enters the shack. Measure what happens if you insert a good ferrite common-mode choke at this point (eg several turns on a giant #31 snap-on bead; see K9YC's papers and web presentations). Also check for CM current on all the *other* feedlines - not forgetting the rotator cable - because the problem isn't always where you first imagined. >Interesting discussion, because I'd never thought of quantifying CMC >before. > The clamp-on CM current meter is a really valuable tool for finding and fixing RFI problems - sometimes before they even happen. And please don't imagine that QRP operators are automatically immune! Even if you don't cause any RFI problems, CM currents can also affect the noise level in your receiver so a 'common-mode cleanup' is still worthwhile. The clamp-on current meter doesn't have to be complicated - just a split ferrite bead with a 10-turn secondary windings, a simple Schottky diode detector and a 100uA meter. My website shows an example in "ugly construction" that took about 30 minutes to build. Who knows, one day some enterprising company may offer it as a 'bare-board' kit? (We'll be away for the weekend, so I look forward to reading the list mail on Monday.) -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek ______________________________________________________________ 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
