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Dipoles v Doublets


Dipoles vs Doublets

So what is the difference between a dipole and a doublet? They are both antennas fed in the middle and used on the HF bands and both are made out of wire; but that is where the similarity ends.

Starting with the dipole:

The dipole is fed in the centre and is 50Ω impedance at the feedpoint. This means it can be fed directly with 50Ω coaxial cable and run straight to the radio. The length of the dipole will be a 1/2λ at the frequency required – if you are going to construct one it is best to cut it slightly longer than required and then trim the ends (same off each side) whilst checking the VSWR. This, however is a mono-band antenna so if you want to use multiple bands, you will need multiple dipoles cut to the required length.

Although the dipole is an excellent antenna, the need for a different one on each band can upset the wife/husband and even the most tolerant neighbours may have cause to complain of the spider web of wire hanging around your garden. It will also need separate coax feeds and a good switching system in the shack. A dipole can be used also on the 3rd harmonic (i.e. a dipole cut for 7MHz will also work on 21MHz as this is the 3rd harmonic).  This then brings us to the doublet:

The doublet consists of a random length of wire, the longer the better, fed in the centre using balanced twin feeder or ladder line. This is available in 450Ω or 200Ω varieties. This then needs to be fed to a balanced tuner (ATU) and then to your transceiver. This is a multiband antenna. It is not actually resonant on any band hence the need for the ATU. One well known variation on the doublet is the G5RV designed by Louis Varney (SK) which is a popular first time antenna for many newcomers to the hobby of amateur radio. This differs slightly in the fact that it has a choke balun at the end of the twin feeder, enabling it to be fed with coaxial cable. Although the performance of a doublet is inferior to that of a dipole, usually due to losses within the ATU, this is outweighed by the fact that it has multiband capabilities.