The doublet has a long history and its longevity is perhaps down to two factors: ease of construction and its being an effective multi-band radiator. No wonder then that LB Cebik, W4RNL picked the doublet as one of his top five multi-band backyard antennas [Cebik, 2009]. The doublet has several good features:
- it will cover a wide range of frequencies
- it is an efficient radiator
- it is very simple to make
- it is not critical on dimensions or materials.
The downside is that due to the wide range of impedances encountered at different frequencies at the center of a doublet, it must be fed with open-wire line to be effective. It will also require an antenna tuner with a wide matching range for best results.
Cebik  suggests that a good length for coverage of 10 through to 40m is 44 feet (22 feet per side). This length gives a broadside pattern across all of the bands.
It might be tempting to start by cutting two 22 foot lengths of wire. Avoid this temptation! Instead work out how long the open wire line you need will be and add 22 feet to this length. Thus if you decide on 18 feet of feedline, cut two pieces of wire 40 feet long (18 + 22 feet).
Measure 22 feet from one end of each wire and tie a figure of eight knot. Thread a cable tie between the knots; this forms the centre insulator. Depending on how you are intending supporting your antenna you may need a loop of cord and a top insulator. If you are supporting the centre from a tree branch you may want to attach a throwing line.
Next make the feedline by clipping on spacers. Try to make them parallel and evenly spaced. This does not make the antenna work any better but it sure looks pretty! Aim for a spacer every 18 inches or so.
At the ends of the doublet you may wish to use insulators. Tie cord onto the ends and, if you want to make life easier, use Wire Winders at the ends of the cords to make handling in the field simpler.
That’s pretty much it. Your new doublet is a balanced antenna and will need a balanced antenna tuner. For a discussion of balance line tuners, check out Owen, VK1OD’s webpages.
My Top Five Backyard Multi-band Wire HF Antennas, LB Cebik, 2009
http://www.users.on.net/~bcr/files/backyard%20wire%20antennaes.pdf LINK DEAD AUG 2015
Balanced ATUs and common mode current, Owen VK1OD, 2012