Log periodic antennas are exceptionally versatile for amateur radio operators, capable of covering a vast range of frequency bands from the lower HF regions, like 80 meters (3.5 MHz), up into the UHF spectrum (around 1300 MHz), all within a single, continuous feedline connection. The specific bands covered are not fixed but are directly determined by the antenna’s physical design, primarily the lengths of its longest and shortest elements. This makes them a popular choice for multi-band operation, contesting, and situations where space for multiple antennas is limited.
The magic of a log periodic antenna lies in its clever geometric design. Unlike a simple dipole cut for one specific frequency, a log periodic array consists of multiple dipole-like elements of varying lengths, all mounted on a central boom. The key parameters that define its coverage are the scaling factor (τ) and the spacing factor (σ). The scaling factor dictates how much shorter each subsequent element is compared to the one before it. For instance, a τ of 0.95 means each element is 95% the length of the previous one. The spacing factor controls the distance between these elements. These two factors work together to create an antenna where only a small, active region of elements is effectively resonant at any given frequency. As the frequency changes, this active region smoothly moves along the boom, providing consistent performance across a wide bandwidth. This is why a well-designed log periodic can offer a nearly constant standing wave ratio (SWR) across its entire design range, often better than 2:1, without needing an antenna tuner for within-band adjustments.
To understand the band coverage in practical terms, it’s best to look at the relationship between the element lengths and the target frequencies. The longest element in the array determines the lowest frequency the antenna can effectively operate on, while the shortest element sets the upper frequency limit.
| Target Amateur Band | Approximate Frequency Range | Required Longest Element Length (½-wave dipole approx.) | Practical Design Consideration |
|---|---|---|---|
| 80 Meters | 3.5 – 4.0 MHz | ~130 feet / ~40 meters | Extremely large and heavy structure; requires massive support towers. Rare for practical amateur use. |
| 40 Meters | 7.0 – 7.3 MHz | ~66 feet / ~20 meters | A large but manageable antenna for stations with sufficient space. |
| 20 Meters | 14.0 – 14.35 MHz | ~33 feet / ~10 meters | Very common starting point for HF log periodic designs; a popular size for contest stations. |
| 10 Meters | 28.0 – 29.7 MHz | ~16.5 feet / ~5 meters | Often included as the top band in multi-band HF LPAs. |
| 6 Meters | 50 – 54 MHz | ~9.8 feet / ~3 meters | A common VHF band covered by specialized VHF/UHF log periodic antennas. |
| 2 Meters | 144 – 148 MHz | ~3.3 feet / ~1 meter | Easily incorporated into a single antenna covering 6m through UHF. |
| 70 cm (UHF) | 420 – 450 MHz | ~1.1 feet / ~0.33 meters | The shortest elements define the upper limit of the antenna’s range. |
As the table shows, covering the lower HF bands requires a massive structure. This is why the most common commercially available Log periodic antenna designs for amateur radio focus on the higher-frequency HF bands and above. A typical “workhorse” HF log periodic might cover 14 through 30 MHz, encompassing the 20, 17, 15, 12, and 10-meter bands in one antenna. This is a manageable size, with a boom length often between 20 and 40 feet. For VHF/UHF enthusiasts, a single log periodic can seamlessly cover the 6-meter, 2-meter, 70-centimeter, and even 23-centimeter (1240-1300 MHz) bands, making it ideal for satellite communication, weak-signal VHF work, and public service events.
One of the most significant advantages of a log periodic dipole array (LPDA) over a trapped triband Yagi or similar antenna is its consistent directional performance across all bands. The gain and front-to-back ratio remain relatively stable throughout its frequency range. This is a huge benefit during contests or DX-peditions when you need reliable directional gain and rejection of interference from the sides and rear, regardless of which band you switch to. The feed point impedance is also designed to be constant, typically 50 or 75 ohms, which simplifies the connection to your coaxial feedline and transmitter.
However, this performance comes with trade-offs. The primary drawback is size. To achieve a low-frequency cutoff of 14 MHz (20 meters), the antenna will still be significantly larger than a mono-band Yagi for the same frequency. A 20-meter Yagi might have two or three elements on a 20-foot boom, whereas a log periodic covering 14-30 MHz will have numerous elements on a boom of similar or greater length. This increased wind load and weight require a more robust mast and rotator system. Additionally, because the antenna is essentially a large array of elements, it can be more susceptible to wind noise and ice loading compared to a simpler wire antenna.
For the amateur radio operator considering a log periodic, the decision boils down to operational priorities. If your primary interest is operating multiple HF bands efficiently with strong directional characteristics and minimal hassle (no antenna tuner needed within the design range), and you have the physical infrastructure to support it, a log periodic is an outstanding choice. It eliminates the need for a separate antenna and rotator for each band. For VHF/UHF operators, especially those active in satellite or EME (Earth-Moon-Earth) communication, a precision-built log periodic is often the antenna of choice for its wide bandwidth and consistent gain. Before purchasing or building, carefully assess the manufacturer’s specifications for the exact frequency range, gain figures, and physical dimensions to ensure it matches your station’s capabilities and your target bands.