A doublet antenna is a versatile, centre-fed dipole antenna designed for multi-band, high-frequency (HF) operation, typically fed with low-loss parallel-conductor (ladder) feedline. It operates efficiently across various bands (often 80m–10m) by using an antenna tuner to manage high SWR, making it ideal for amateur radio operators seeking a simple, inexpensive, and robust multiband solution. 

Doublet Antenna Design

You can also use a balun to transition from the balanced feedline to 50-ohm coax. While this isn’t an optimum approach, it works. Textbooks often recommend a 4:1 balun, and that’s a good starting point. If you go this route, I recommend keeping the coax as short as practical. If you run into matching problems on some bands, try another balun ratio (e.g., 1:1), or change the length of your feedline. 

The second drawback is that you need to be careful of how you route balanced lines. You need to avoid getting it too close to metal or laying it on the ground. Both can upset the line’s balance. This can cause it to radiate or introduce losses. If you have excess line, don’t coil it up. Operating outdoors, I found these restrictions aren’t very difficult to work around, you just need to be mindful of them. 

Key Characteristics

1. Design: A centre-fed wire antenna (usually horizontal or inverted V) using balanced feeder lines, such as 300-ohm or 450-ohm ladder line, rather than coax.
2. Multiband Flexibility: The length of the radiating element is not critical, allowing it to function as a half-wave or longer on multiple bands.
3. Low-Loss Operation: Because of the ladder line, the system has low feedline loss even when the SWR is high.
4. Tuning Requirements: A balanced antenna tuner is required to match the wide range of impedance, often located in the shack. 

Advantages

1. High Performance: Very efficient,, particularly for long-distance (DX) communication, depending on height.
2. Simple Construction: It can be made from simple wire and homemade or purchased ladder line.
3. Versatility: Works well in many configurations (horizontal dipole, inverted V, Sloper). 

Construction and Usage

1. Length: A common size is roughly 100–130 feet (approx. 30–40m) for 80m-10m coverage, though other lengths work.
2. Feed Line: Open-wire line should be used to feed the antenna, with lengths sometimes chosen to make matching easier on specific bands.
3. Deployment: The center can be supported by a tree or mast, with ends tied off using insulators. 

The doublet is often confused with the G5RV, but it is more flexible as it does not rely on a specific length of coaxial cable to transform impedance.