![]() ![]() ![]() In 1990, when I was a senior in college, I was frustrated that the kind of mathematical models that are available for acoustic suspension and bass reflex boxes were unavailable for Transmission Lines. I prefer the sound of a TL to a ported box. However when well-designed have a rich, dimensional sound in the lower registers which carries a depth and ease that sealed and ported boxes lack. I don’t think TL’s are necessarily more “accurate” – they undoubtedly have flaws. Fans say TL’s sound less resonant, less boxy, more open. The Transmission Line is a seldom-used speaker design which is preferred by many audiophiles. 25Hz-20KHz range with very warm, rich bass, creamy midrange, extremely detailed high end and outstanding impulse response. It uses 6mm aluminium tube for the radials, 4mm brass tube for the driven element and is built around an N-Type chassis socket mounted to a 40x40mm aluminium square, held together with rivets.This system features a Morel MW266 woofer in lightly stuffed TL tuned to 27 Hz and a Morel CAT378 tweeter. The next two images show one I built for the 70cm band. I use one at home on 70MHz, click here to see construction details and more images. They are very forgiving due to the low impedance feed. I have made quite a few of these antennas over the years with good results. (42° is the theoretical perfect angle for 50Ω feed, but who’s measuring!) ![]() For VHF and below, as the elements get bigger, some more structured design is needed.Ī quarter wave monopole mounted against a perfect ground will have an impedance of around 36Ω but by bending the radials down at an angle of 45°, we increase this to around 50Ω whilst at the same time lowering the radiation angle more towards the horizon. These antennas can easily be built for UHF or above by using a chassis mount N-Type (or SO-239) connector, some solid wire and solder. You could cut a little on the large side and trim the antenna for best match at your desired frequency if you have the equipment. The velocity factor is set to 95% which should be fine for most people. This is a true unbalanced antenna, with a feed impedance of around 50Ω and therefore a great match to 50Ω unbalanced feedline. ![]() There are usually four radials, three being a minimum, but you could use up to six. The radiating element is a quarter wave (λ/4) and the radials are 12% longer. Ahh, the good old quarter wave ground plane! This calculator can be used to design a Quarter Wave Ground Plane antenna, with radials. ![]()
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