> It seems that many Hams considered the 4:1 ratio some sort of magic > number, > when all it was intended to do was provide a good transfer between 300 > ohms > and 50 ohms.
I just modeled a typical 20m folded twin-lead dipole using 4Nec2 and TLD software. Had never tried that one before. The antenna feed-point Z at a half-wave height (33 FT) above average ground computes to 291-j7.7. That's surprisingly close to 300 ohms resistive. With TLD software, I then coupled the feed-point with 300-ohm twin-lead line. Finally, I varied line length between 0 and 1/2 electrical wavelength and watched the resulting Z at the line input. Z always stays between 290 and 310 ohms with very little reactance. Total system loss never exceeds 0.25 dB. The 300-ohm VSWR stays near 1.05:1 and not surprisingly, the 50-ohm VSWR stays near 6:1. While a 4:1 balun can probably result in efficient transfer of power into the line from a modern 50-ohm output Z transceiver, I think a 6:1 ratio current balun would be a better choice under these circumstances, assuming one wanted to avoid an ATU at (or within) the rig altogether. No wonder this antenna was so popular. The antenna and line all use the same inexpensive 300-ohm twin lead material. The match at the antenna is so good that line VSWR (300) and loss is negligible. Paul, W9AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
