A doublet antenna is a wire antenna used on HF that consists of a dipole-style wire antenna fed with ladder line (also known as twin feed, window line, or open line) and used in combination with a tuner. Feeding a dipole with ladder line allows it to be used as a multiband antenna, though routing and accommodating ladder line can be inconvenient.

In the provided source material, a doublet is characterised by the following features:

• A dipole or dipole-like wire antenna
• Fed at the centre with ladder line
• Used with a tuner to match the antenna system to the radio
• Tuned primarily by adjusting ladder line length rather than antenna wire length
• Using ladder line with a tuner turns a dipole into a multiband antenna.

Ladder line is repeatedly recommended in the source material for doublet antennas due to the following properties:

• It has extremely low loss
• Losses that would be severe on coax are much lower with ladder line
• Because of this low loss, SWR is not a primary concern
• A tuner can be used to match the antenna system effectively across multiple bands
• These characteristics make ladder line well suited to multiband wire antennas such as doublets.

According to the source material:

• The antenna wire itself does not need to be cut for each operating band
• The tuner, ladder line, and antenna function together as a single system
• Fine tuning is achieved by adjusting the ladder line length
• High SWR on the feed line is acceptable due to ladder line’s low loss

The construction approach described in the source material is deliberately simple:

• Install as much wire as space allows
• Feed the centre of the wire with ladder line
• Run the ladder line toward the shack
• Leave approximately one metre of slack in the ladder line
• Trim the ladder line until the tuner matches the antenna satisfactorily
• The ladder line length is treated as an adjustable parameter during setup.

The preferred configuration described is:

• Ladder line connected directly to a balanced (symmetrical) tuner
• This arrangement is described as a “true ladder line tuner”

Where a direct balanced connection is not practical:

• Ladder line is run to just outside or just inside the shack
• A balun is placed at that point (1:1 or 4:1, depending on how impedances work out)
• A short run of coax connects the balun to the tuner or radio

• Ladder line is balanced
• Coax is unbalanced
• A balun converts between the two
• Coax runs should be kept as short as possible to minimise losses

From the source material:

• Ladder line is balanced (symmetrical)
• A balun is required when transitioning to coax
• A 1:1 balun can be made by wrapping coax around a toroid
• Ladder line itself may also be wrapped around a toroid and connected directly to wire terminals
• In ladder-line systems, baluns are commonly placed at the tuner or at the ladder line–to–coax transition point

The disadvantages of ladder line are explicitly noted:

• Ladder line “hates being near things”
• It is difficult to route into a home shack
• Wider-spaced ladder line is more complex to accommodate
• Despite these drawbacks, ladder line is repeatedly recommended due to its low loss and flexibility.

The source material contrasts doublets with other approaches:

• Feeding a dipole with ladder line makes it multiband
• End-fed antennas are often used to avoid tuners
• Avoiding a tuner does not imply better performance
• Adding coax, ununs, and transformers introduces additional losses
• A tuner plus wire antenna is described as more efficient than end-fed transformer-based systems

The following issues are implied or explicitly stated in the source material:

Using long runs of coax in a ladder-line-fed system introduces unnecessary losses. The source material repeatedly advises using as little coax as possible.

Running ladder line close to objects or through cluttered environments can cause problems. Ladder line is described as disliking proximity to other objects and being difficult to route indoors.

Placing baluns without regard to feed line type can reduce effectiveness. With ladder line systems, baluns are typically placed at the tuner or at the transition to coax, not necessarily at the antenna.

With doublets, fine tuning is achieved by adjusting ladder line length, not by repeatedly trimming the antenna wire itself.