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Title: Rf Autotransformer Design Post by: KD6VXI on March 05, 2016, 07:57:52 PM looking at a short vertical dipole. Will be OWL fed. I can't go real high because of proximity to BFL, so the pair of 80 foot towers looks like they will be sold. My inverted v (32 foot apex, 90 degree angle) suffers from CRS for DX.
I know I might suffer from only hearing QRM, but it's worth a shot for a TX ant at least. Feed point impedance will be low on 80, and very low on 160. I was thinking of an autotransformer to step the impedance up. How do you design it? I get N turns / X turns on a typical binocular core, and understand how a toroidal rf transformer works..... How can I figure the autotransformer step up? I want it to make the feed point Z a realistic value on 160, but not create an unrealistic impedance on ten to match. Tried Google. Lots of non specifics returned. Anyone know of a paper or ? they could point me to? --Shane KD6VXI Title: Re: Rf Autotransformer Design Post by: Steve - K4HX on March 05, 2016, 09:42:43 PM Lots of 4:1 and 9:1 designs for receiving antennas. No, they aren't autotransformer, but the turns ratios will be the same. Scale up to handle the transmit power, as needed.
Title: Re: Rf Autotransformer Design Post by: w7fox on March 05, 2016, 11:33:12 PM Resistance may be low, but reactance will be high unless you tune it out with an inductor. If you are going that far, you may as well use coax. According to various sources, feeding a short antenna (less than 1/4 wave) with OWL will result in a very lossy system. I use OWL on 160 through 10 meters, but my antennas are about 1/2 wave or more on all bands, and work very well.
Title: Re: Rf Autotransformer Design Post by: DMOD on March 06, 2016, 12:53:12 AM Hi Shane,
Go down to http://amfone.net/Amforum/index.php?topic=36511.0 I have two AD5X designs down there that use autotransformers. Phil - AC0OB Title: Re: Rf Autotransformer Design Post by: Steve - K4HX on March 20, 2016, 10:48:29 AM Came across this design today in an ARRL Antenna Compendium. It's an autotransformer designed for 50 Ohms on one end and taps from 5.6 to 42 Ohms.
Core: 140-61 12 bifilar turns of #15 Polythermalized wire (do not twist wire, lay it side-by-side) Connect the end of the first wires to the opposite end of the second wire. You now have 24 turns around the core. Taps Ohms Turn/Tap 5.6 8 8.7 10 12.5 12 17.0 14 22.0 16 28.1 18 34.7 20 42.0 22 Title: Re: Rf Autotransformer Design Post by: KD6VXI on March 20, 2016, 11:06:51 AM Which one, Steve? I have (I think) all of them.
I bought a hundred feet of 1/4 inch tubing, was going to wind a coil and play with it, but I'll check the antenna compendium books too. --Shane KD6VXI Title: Re: Rf Autotransformer Design Post by: Steve - K4HX on March 20, 2016, 11:13:49 AM Volume 7, page 18.
Title: Re: Rf Autotransformer Design Post by: w1vtp on March 20, 2016, 02:10:05 PM Hi Shane, Go down to http://amfone.net/Amforum/index.php?topic=36511.0 I have two AD5X designs down there that use autotransformers. Phil - AC0OB Couldn't have said it better. Those PDFs are a good read |