For those building an isolation transformer for a 160M 5/16 FCP -- PLEASE READ! IMPORTANT!
Martin has the right instincts, says it feels fishy for this. But it's worse than he thinks. And don't even consider using ferrites. The T300A-2 core and 20 bifilar turns were SPECIFICALLY chosen to produce a residual inductance value in the right order of magnitude to cancel a typical residual capacitive reactance from a 160 meter 5/16 wave single wire folded counterpoise, AND provide enough coupling to make the transformer work, AND maintain a low enough loss to operate QRO without heating, or especially to operate QRP without further handicapping the operator with needless loss. This allows the builder of the "simple solution" to prune the wire to get resonance and remain somewhere around an ELECTRICAL quarter wave radiator. So this works for T's, L's, U's, straight verticals, 1/8 wave-ish raised radials and HOA miscellaneous stealth wires. A T300 form factor is the minimum to wind 20 bifilar turns on the INSIDE diameter. Twenty turns (forty wires) fills up the inside diameter. Less than the T300A-2's 22.8 A sub L number (Amidon does not use the decimal point) and residual inductance is reduced and doesn't match the FCP. Use of a mu of 20 (#1 material) to pull this off on a smaller form factor with fewer turns adds loss, and gets into heating. FERRITES?? DO NOT use ferrites. This is a transformer, NOT a balun. There is a voltage differential across the winding. The winding will go lossy. We wind up overheating ferrites and cracking. This is not a balun with guaranteed 100% cancelling counter-currents. Powdered iron is required. The Amidon T300A-2 can be replaced with a stacked PAIR of Amidon T300-2, with a SINGLE Micrometals T300-2D, or a stacked PAIR of Micrometals T300-2. You can find the Micrometals cores on eBay. You CAN use the Amidon T400A-2, but that core is forty bux compared to the T300A-2's sixteen, and I haven't figured the correct smaller number of turns to balance the FCP, and since the T300A-2 does the job, why bother to blow 24 bux? Maybe for 5kW RF someone needs to do the work. My specification for results and success of design is predicated on the particular design of isolation transformer. Move away from that and YOU ARE ON YOUR OWN. Further, do not be deceived, to use the 5/16 wave FCP, an isolation transformer is REQUIRED. We ALREADY TRIED using regular balun designs to keep the counterpoise current off the feedline to an FCP. They do not work in this app. Been there, done that. With the wrong kind of dirt under the antenna, using a regular balun goes dummy load on you, merely lossy otherwise. Don't try to feed this with a regular balun and then come back and complain that it didn't work. OF COURSE it didn't work. WE DISCOVERED it wouldn't work. WE TOLD YOU it wouldn't work. Those of you carefully thinking this through could say that you could use a series reactor AND a regular balun. Yeah, yeah, BUT this is now a straight inductor on a core WITHOUT any counter-current to cut down on the losses. So you lose big time on two counts: First, you only saved the cost of 7.5 feet of teflon on wire vs the isolation transformer, because you STILL had to do core+single winding PLUS it also cost you the balun. Second, you lost the fairly high percentage of counter-current cancellation using the bifilar winding in the isolation transformer. So your coil plus regular balun costs you more loss for QRP and heat for QRO. If your beef is that it's too hard to make and get the right materials, Balun Designs is making a model 1142s, which you can buy ready-made, now, and does my full specification without any corner cutting. (I have no financial interest in Balun Designs) http://www.balundesigns.com/servlet/the-108/1-cln-1-High-Isolation-Balun/Detail This is 160 meters, guys. You can't use the miniature stuff down here without going lossy. You're talking about less money than taking the family to a restaurant. Why bother with cheep cheep. 73, Guy K2AV (the inventor) On Thu, Dec 15, 2011 at 8:10 AM, Martin <hamra...@vr-web.de> wrote: > For those who intend to build the FCP INV. L. with a T-200-2 (or an > antenna tuner). > Me and a few guys from our club wound different types of baluns and > chokes on different material and measured them with a > analyser. We found the T-200-2 next to useless below 3 Mhz due to a low > AL value (uH/100 turns). The T-200A-2 is a lot better, but still not > really good in balancing. You better use ferrites (FT-something) as long > as you use moderate power. > > BUT if you go QRO, you should stick with the iron powder cores. > > I once replaced the T-200-2 in my homebrew s-match with a ferrite. I > changed back the same day, cuz after a few seconds with 750W the ferrite > got so hot you would not want to touch it. The T-200-2 runs cool over an > entire contest. Next step is to replace it with a T-300A-2. > > Below you find a list of cores with their corresponding Al Value. > You can clearly see that the T-300-2 is even lower than the T-200-2. > For the FCP INV L. i think a T-300A-2 or even better a T-400A-2 is best > choice, may it be expensive. > > Core Al Value > T-200-2 120 > T-200A-2 218 > T-225-2 120 > T-225A-2 215 > T-300-2 114 > T-300A-2 228 > T-400-2 180 > T-400A-2 360 > > I'm not a technician or rf-specialist and english is not my native > language, so please forgive me if my explanations are a bit ragged. > Yes, i use a T-200-2 in a tuner, which of course is not the same as an > antenna, but my feeling tells me i'm on the right track when i say that > a T-200-2 is no good choice for frequencies below 3Mhz, > > > http://toroids.info > http://www.qrz.lt/ly1gp/amidon.html > > -- > > 73, Martin DM4iM > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
