The end-fed half-wave, a versatile multi-band antenna

The end-fed half-wave (EFHW) antenna has become a favorite among hams for its versatility and easy setup. It’s great for multi-band operation with just one radiator, which is perfect for those with limited space or who need something portable.

Radiator length and band selection

The EFHW antenna is a hit with hams because it's easy to set up and works on multiple bands with just one radiator. For multi-band operation, most operators (in the UK at least) focus on the 40m to 10m bands due to space constraints. A 40m EFHW antenna, with a radiator length of approximately 66 feet (20.1 meters), can effectively operate on 40m, 20m, 15m, and 10m bands, a good fit for small gardens, typical in the United Kingdom (although they are getting smaller with each passing year, but that's a whole other story).

While 160m and 80m operation is theoretically possible, the required lengths (270 feet and 135 feet, respectively) are often prohibitive for many installations. Thus, the 40m to 10m range represents a practical compromise between performance and size.

Matching transformers: The key to efficiency

A crucial component of the EFHW antenna is its matching transformer, often referred to as an "unun" (unbalanced-to-unbalanced). This device is essential for efficient power transfer between the high impedance at the end of the wire (typically 2000-3000 ohms) and the 50-ohm impedance of most transceivers and coaxial feedlines.

For optimal performance, especially at higher power levels, the choice of transformer core material is critical. Common options include:

1. Ferrite cores (e.g., FT240-43 or FT240-52) for moderate power levels
2. Powdered iron cores (e.g., T200-2) for higher power handling capabilities

The trick here is to pick a core that does not overheat at your operating power level. You want to radiate all of that RF energy, not heat up the core! Furthermore, core saturation is a key concern when a matching transformer overheats, leading to signal distortion and potential spurious emissions. The heat can also detune the antenna system by altering the transformer's impedance ratio, affecting SWR and overall performance. 

Physical damage is a another risk, with the possibility of the core cracking or breaking, and in extreme cases, melting wire insulation. Overheating can introduce non-linearities that may cause intermodulation distortion, especially in high-power setups. Finally, as the core temperature rises, its power handling capability decreases, creating a cycle of increasing inefficiency and further heating.

 Bottom line - don't skimp on your cores!

Unun ratios

The most commonly used unun ratios for EFHW antennas are:

1. 49:1 (7:1 turns ratio)
2. 56:1 (7.5:1 turns ratio)
3. 64:1 (8:1 turns ratio)

The 49:1 ratio is often preferred as it provides a good compromise between efficiency and bandwidth. The 64:1 ratio can offer slightly better efficiency on some bands but may have a narrower bandwidth.

Counterpoise wires: To use or not to use?

The use of a short counterpoise wire in EFHW antenna systems is a topic of ongoing debate. While some operators report improved performance with a counterpoise, others find little to no benefit. A counterpoise wire, typically 0.05 to 0.1 wavelengths long at the lowest operating frequency, may help in the following ways:

1. Reducing common-mode currents on the feedline
2. Improving the antenna's efficiency, particularly on higher bands
3. Potentially lowering the SWR on some frequencies

However, the effectiveness of a counterpoise can vary depending on the specific installation and surrounding environment. Experimentation is often necessary to determine its impact on a particular setup.

Radiation patterns

The radiation patterns of an EFHW antenna vary depending on the operating band and installation configuration. Using a 40-10m EFHW as an example:

EFHW radiation pattern for typical 40-10m example

1. On 40m (fundamental frequency): The antenna exhibits a pattern similar to a half-wave dipole, with maximum radiation broadside to the wire.
2. On 20m (2nd harmonic): The pattern resembles that of a full-wave antenna, with four main lobes at approximately 45-degree angles to the wire.
3. On 15m and 10m (higher harmonics): The pattern becomes more complex, with multiple lobes and nulls. These patterns can be advantageous for working distant stations at various angles.

It's important to note that the actual radiation patterns may be influenced by factors such as antenna height, surrounding objects, and ground conditions. 

According to your location and the direction of desired operation, an EFHW antenna can be oriented to take advantage of these radiation lobes to provide optimal radiation in the desired directions.