[RFI] shopping for toroids
pf at tippete.net
Sun Dec 1 16:02:05 EST 2013
>>>>> "Jim" == Jim Brown <jim at audiosystemsgroup.com> writes:
Jim> Hello Pf,
Jim> As it turns out, I am currently organizing a group purchase of #31
Jim> cores for several local (Northern California) ham clubs. The parts we
Jim> are buying this time around are 0431177081, which is a 0.75-in
cool, I have this in my list too. EUR 13.03 each, in small quantities.
Jim> i.d. "clamp-on" intended for multi-turn chokes on small cables (home
Jim> entertainment systems, computer cables, battery chargers) and
Jim> 2631181381, which is a 1.4-in i.d. clamp-on that is 2.5-in long. It
mouser seems not to have it in stock.
Jim> Several years ago, GM3SEK made me aware of the very high cost of
Jim> ferrite cores in EU, so I started working on chokes that could be
Jim> effective with fewer cores. This resulted in the "bifilar" designs
Jim> that you'll find in the latest version of my RFI tutorial (about
Jim> 2010). The principle is simple -- take two insulated AWG #12 copper
Jim> conductors about 1.3 m long, tape them together, wind 12-16 turns
Jim> around a single #31 toroid, and connect them as a short section of
Jim> parallel wire transmission line. Insulated "house wire" will give Zo
Jim> of about 85-95 ohms, enameled wire gives Zo about 50 ohms. Both work
Jim> very well with antennas in the range of 50-100 ohms. 16 turns is good
Jim> for 160M to about 17M, 12 turns is better for 80M to 10M. AWG #12
Jim> copper is 0.08-in diameter.
Jim> These bifilar chokes can handle transmit power of 1.5 kW if the
Jim> antenna is reasonably well balanced and operated near resonance. In
Jim> other words, a center-fed half-wave dipole, a typical beam, and a
Jim> vertical with a good radial system. They will NOT handle high power
Jim> of the antenna is badly unbalanced (off-center fed), or if the VSWR
Jim> is VERY high (like an 80M dipole fed on 40M).
Nice, I didn't realize I had an older version of your tutorial. You're
talking about fig. 37, right? My 80m dipole, while a full-size center
fed one, is mounted in an odd way so it's going to be quite unbalanced,
although hopefully not as a OCF one.
Jim> As to the #43 parts -- the discounts for buying full boxes of #31
Jim> parts make them cheaper than #43 parts, and the #31 are more
Jim> universally useful. That is, #31 is greatly superior below 5 MHz,
Jim> roughly equivalent from 5-15 MHz, and still quite effective to 100
Jim> MHz. #43 is a dB or so better between 20 MHz and 50 MHz.
Ok, I read those graphs correctly. all #31 then. Need to do some more
reading before finalizing my shopping list.
Pierfrancesco Caci, ik5pvx
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