In a message dated 2/19/07 7:00:22 PM Eastern Standard Time, [EMAIL PROTECTED] writes:
> A "fan dipole" is a single-band affair. It's is a way of broadening the > bandwidth of a single-band dipole by simulating a "fat" wire using multiple > wires. Typically the wires join at an apex at the center and spread or "fan" > out at the far end with cross-wires connecting the ends. That forms larger > effective radiator diameter that reduces the "Q" of the antenna; hence the > bandwidth is increased. That's what I think of when someone uses the term "fan dipole". > > It's not a commonly-used antenna because a simple wire dipole usually has > adequate bandwidth on 40 meters and up, and in practice even a large "fan" > doesn't broaden the bandwidth all that much; not enough to cover the 3.5 - 4 > MHz range with a low SWR, for example. > I disagree! Way back in the 1980s, WA3UZI and I did some tests on an 80/75 meter "fan dipole". We used four equal-length wires with spreaders at the ends, making a sort of giant bow tie. The far ends were not connected, they just terminated at the spreaders. The spreaders were horizontal and the whole thing was up about 50 feet, fed with 50 ohm coax. IIRC, if the ends were spread about six feet, the antenna had an SWR well under 2:1 from 3.5 to 4.0 MHz. The wires were a lot shorter than one would expect from the formula - about 57 feet or so. It was built so that one antenna could cover 80 CW and 75 phone on Field Day without need of a tuner, since the rigs we used could handle 2:1 SWR with no problem at all. It took a bit of pruning and testing to get the wire and spreader lengths right. It worked as well as a regular halfwave dipole. > However, like any "fat radiator" dipole, the resonant length is shortened a > bit, which can be a help in limited space. > Yup. Another approach is a cage dipole - multiple wires of the same length all in parallel, spaced around a circle throughout their lengths. I've not tried one because of the mechanical complexity compared to simple fan. Calculations (which I have not tried) predict that a cage of three feet in diameter is needed to get 2:1 SWR from 3.5 to 4.0 MHz. The big problem with such antennas is that they are heavier, more mechanically complex, and have much more wind/ice loading than a regular single wire. We had good supports at the old FD site, and the antenna only had to last a weekend. Even so, we only used it one year. > Cebik has a good write-up on fan dipoles and their close cousin, the bow > tie > antenna, at www.cebik.com as part of his discussion of small beams. > I suppose the best description of our antenna is a bow tie. 73 de Jim, N2EY _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
