A quick and easy 20m Folded Dipole
Since the 20m band is still the main "long haul" band and most popular for DX, it makes sense to have an antenna handy for use on this band at all times. Especially when conditions suddenly improve or if a rare DX station is suddenly heard, there is little time to tune up a multiband antenna. The 20m folded dipole antenna presented here performs well, needs no tuning, and can be assembled and installed in no time at all. The advantage of a folded dipole over an open dipole is that a folded dipole is less susceptible to local QRM (it's far less sensitive to the near field) and therefore sounds much "quieter" than a comparable open dipole, which is obviously a valuable property when hunting for weak DX signals!
While the length of the antenna specified here is for the 20m band, its dimensions may of course be multiplied or divided as desired to make the antenna resonant on other bands. Because the feed point impedance of a folded dipole is higher than that of an open dipole, a 4:1 balun is required between the open feed line to the antenna and the radio.
Construction is fairly straightforward. Instant Ladder Line clip-on wire spreaders are used to construct both the dipole itself and the ladder line that feeds it. An insulator (e.g. a plastic "dog bone") between the two points where the ladder line is connected to the dipole takes the strain and keeps the antenna in shape. The coax between the 4:1 balun and the radio should be kept as short as possible, but if absolutely necessary the balun can be mounted outside the shack, with coax going through walls, windows etc.
While an open dipole, when fed with a ladder line, can be used as a multi-band antenna on practically any frequency, the same is not true for the folded dipole. The latter can only be used on odd harmonics of its lowest resonant frequency. E.g. a folded dipole for the 80m band can be used on 30m but not on 40m and 20!
If properly constructed, this antenna will give a 1:1 SWR at the desired center frequency, with no more
than 1.4:1 at the band edges. It can be suspended from poles, trees or any other available support. Keep in mind that higher is always better. Even more important is to keep the end points of the antenna well clear of metal parts such as masts and roof gutters, since close proximity between the antenna end points and metal will seriously de-tune the antenna.