For some amateur radio operators, putting up a full-length HF dipole is not always feasible. With today’s postage-stamp-sized lots, some hams can only dream of installing a dipole for 80 meters—or even 40 meters.
The most common solution has been to add loading coils to reduce length. The Alpha Delta DX-DD, for instance, offers 40/80m coverage in half the space of a typical dipole. Loading coils work, but it comes at the cost of reduced bandwidth and efficiency. There are other options.
A linear-loaded dipole might just fit your available space. Instead of using a full-length wire, a portion of the wire is folded back on itself to reduce length. Linear loading results in a significant reduction in size while maintaining good electrical performance compared to its coil-loaded equivalent.
Know When to Fold ‘Em
According to the ARRL Antenna Book, linear loading introduces minimal loss and has a low Q, which allows reasonable bandwidth compared to using loading coils. This method of shortening radiators can be applied to almost any antenna configuration to reduce its physical length while maintaining overall signal radiation. Folding the elements can significantly reduce the required length for resonant antennas. For example, you can make a resonant antenna that’s 30 to 40% shorter than an ordinary dipole for a given band.
Performance depends on what section of the antenna is folded, by how much, and the spacing of the conductors. When experimenting with linear loading, add about 10% to the calculated conductor length as a precaution. Use the cut-and-trim method to tune while taking care to adjust both the spacing and length of the folded section to optimize the match. Feeding the antenna with a ladder line connected to a tuner is another option.
Linear loading is superior to using an inductive coil because the loading is distributed along the entire length of the element rather than being lumped. The result is significantly improved radiation efficiency and a greater bandwidth than the coil-loaded equivalent. A simple dipole can be linearly loaded to shorten its length, making it possible to operate on the lower bands despite the disadvantage of being located on a small city lot.
What Makes it Work?
Linear loading is a technique to make a shorter, yet resonant and efficient antenna on the frequency for which it’s designed. To explain how the linear-loaded dipole antenna works, let’s look at the theory of resonant circuits and apply it to an antenna.
A dipole antenna at resonance is a resonant circuit. It’s the result of the combination of inductance (L) and capacitance (C). Think of a dipole antenna as a pair of coils that have been stretched out to form straight wires, one on each side of the feedpoint.
With linear-loaded dipoles, we simulate a large one-turn coil on each side of the feedpoint by folding each half of the dipole antenna back onto itself. The folded portion of wire interacts with the antenna wire above or below. Each side of the antenna becomes part of a resonant circuit, comprising inductance (due to the length of the wire) and capacitance (formed by the proximity of the folded-back wires). The resonant frequency of the dipole antenna is the result of the self-coupling of the two wires on each side of the antenna feedpoint.
Be aware that the linear-loaded dipole will be less efficient on bands other than its fundamental resonance frequency. In the example below, it is in the 40-meter band with a third harmonic in the 15-meter band. Of course, it can be coaxed into working on other bands with an antenna tuner. The tuner will make your transceiver think it’s properly matched to your antenna system.
Want to go vertical? Electrically, the antenna appears and behaves like a standard vertical 1/4 λ radiator but with a higher impedance at the feedpoint. On receive, it works almost as well as a full-size radiator over the same ground plane. Upon transmission, it appears to perform equally well, with the same signal reports being received when switching between a linearly loaded and a full 1/4 λ radiator. As with all vertical antennas, a good ground plane is required.
Can the linear-loading technique be applied to a multiband fan dipole? Yes, it can. However, other antennas near the 40-meter linear-loaded dipole will likely cause it to detune somewhat. You may need to adjust the lengths of the multiple elements, requiring some trial and error until they collectively work well with the others. Can you linear-load a G5RV? Yes—search online for details.
Construction Tips
Here are some tips to help you construct a suitable linear-loaded antenna for your individual application.
- You need to install an RF choke or clip-on ferrite chokes on the coaxial cable before it enters the shack to prevent common-mode problems. You can tell that you need additional chokes when your tuner struggles to stabilize during transmission.
- Linear loading is used to reduce the physical size of an antenna, particularly in applications where space is limited, such as in small gardens or for portable use.
- The final size is a compromise between the desired bands, available materials, and location. This is one of the best aspects of designing your antennas: You can tailor the design to fit the space and surroundings according to your specific circumstances.
- A more efficient version would be to feed a linear-loaded dipole antenna directly with 450-ohm ladder line or 300-ohm twin lead terminating in a weatherproof box. Run 50-ohm coax into the shack to your tuner—as short as possible. Ladder lines can also make excellent elements for linear-loaded antennas since they come with built-in spacers.
Linear-Loaded Dipole Approximate Dimensions
| Band | Frequency | Length* |
|---|---|---|
| 10M | 28.5 MHz | 11.5 ft. |
| 12M | 24.9 MHz | 13.2 ft. |
| 15M | 21.1 MHz | 15.5 ft. |
| 17M | 18.1 MHz | 18.1 ft. |
| 20M | 14.1 MHz | 23.2 ft. |
| 30M | 10.1 MHz | 32.44 ft. |
| 40M | 7.1 MHz | 46.14 ft. |
| 80M | 3.6 MHz | 91.0 ft. |
| * With a Bit Extra, Just in Case | ||
The lengths given in the chart are based on calculations by Claude Jollet, VE2DPE, for the M0PZT dipole pictured earlier. The above are the full overall length of the top half of each side of the linear-loaded dipole. The dimensions include the width between the balun connections.
You may have heard that short ham antennas are not as efficient or effective as a full-length half-wave dipole, regardless of the configuration. Good news! Linear-load antennas are probably the next best thing, especially for those with restricted space.
