THE TWIN FEEDER TUNED DIPOLE.

This antenna, also know as the Tuned Doublet or Random-Length Dipole is very simple, yet is a most simple, effective and efficient antenna for multiband use. The antenna does not require a counterpoise or RF ground system.

This antenna is fed with open wire tuned feeders. This type of feeder or transmission line has the advantage of low loss even with long runs. You can use open wire feeder or any any commercial  300 or 450ohm ladder line.

An ATU with a balanced feed is used to take care of the wide variations of feed impedance on the different bands. If you haven't already got an ATU I strongly advise you get one. Unlike your latest radio an ATU is a low tech bit of kit and will last a lifetime. The traditional T match with a balun at the output for balanced feeders is fine. They come in different prices depending on power handling requirements and whether it has a built in SWR meter (a useful feature).

The Tuned Doublet should be at least 3/8 wavelength on the lowest frequency. A 3/8 wavelength dipole has an effectiveness greater than 98% relative to a half wave dipole and the SWR values are reasonably easy to match.

A 3/8 wavelength dipole at 3.5MHz is approximately 30m (100ft) long, which means that any length from 27m (90ft) to 30m will make an excellent radiator on all HF amateur bands, 80 through 10 metres, including the WARC bands.

If you don't have room for a 27m length of straight wire for operation on 80 metres, a 3 to 5m (10 to 16ft) portion of each end may be dropped vertically from each end support. There will be no significant change in radiation pattern on 80 and 40 metres. However, there will be a minor change in polarisation in the radiation at higher frequencies, but the effect on propagation will be negligible. This antenna can also be supported using a single pole to form an inverted V

If you are willing to forgo the 3.5MHz (80m) band then a centre fed wire half the length quoted above will work on all bands from 7 to 29MHz including the WARC bands. And remember, the dimensions of this antenna is not critical, unlike the resonant dipole or the G5RV.
While the open-wire tuned dipole is a very good antenna there are problems in many locations of bringing open wire feeder into the shack. This is because twin feeder must not be allowed to come close to metal objects such as metal window frames, guttering or flashing and particularly electrical wiring. Metal objects close to the line can cause the currents in the line to be unbalanced. On the other hand coax cable is fully screened and can be allowed close to metal objects but has greater losses. The solution, if you do not have a clear run from the antenna to the ATU is to use part coax and part balanced feeder, a solution for apartment dwellers, first described by Dick Rollema, PA0SE.

All this arrangement does is to move the balun, normally located inside the ATU, to a place outside the ATU and the shack.

 This arrangement often makes for a more convenient connecting point for open line feeder.

You don't cut the balun out of your ATU! You make or buy a new one.

 It is connected to the coax output socket of the ATU. Although the position of the balun is not critical the coax section should be kept a short as possible to keep the losses down.

Updated 15 Jan 2009