What is the polarization of a standard log periodic dipole array?

The polarization of a standard log periodic dipole array (LPDA) is predominantly linear, and in the vast majority of designs, it is horizontally polarized when the antenna is mounted in its conventional orientation. This is a direct consequence of the antenna’s physical structure: the dipoles are arranged parallel to each other along the boom, and the electromagnetic wave radiates with its electric field vector parallel to the plane of these dipoles. If you rotate the entire antenna assembly by 90 degrees so that the boom is vertical and the dipoles are stacked vertically, the polarization becomes linear vertical. The polarization is inherently tied to the physical alignment of the radiating elements. This characteristic is crucial for applications like terrestrial television reception, where broadcast signals are typically horizontally polarized to minimize interference from vertical noise sources like automobile ignitions.

To truly grasp why polarization matters, we need to understand what it is. In simple terms, the polarization of a radio wave describes the orientation of its oscillating electric field (E-field) as it travels through space. Imagine a wave moving toward you; if the E-field is moving up and down, it’s vertically polarized. If it’s moving side to side, it’s horizontally polarized. For maximum power transfer, the receiving antenna’s polarization must match the polarization of the incoming wave. A polarization mismatch can lead to a significant signal loss, known as polarization loss. The theoretical maximum loss when cross-polarized (e.g., a horizontal antenna receiving a vertical wave, or vice versa) is 20 dB, which translates to losing 99% of the signal power. In practice, due to reflections and other phenomena, the loss is often around 10-15 dB, but that’s still enough to render a signal unusable.

The genius of the log periodic design lies in its frequency-independent operation. The antenna is composed of a series of dipole elements of increasing length, fed from a common source via a complex feeder line that alternates the phase. The active region—the set of dipoles that are effectively radiating and receiving—shifts along the structure as the frequency changes. A key feature is that the electrical characteristics, including impedance and polarization, remain consistent across the entire operating bandwidth. This means that whether the antenna is operating at its lowest frequency (using the longest dipoles) or its highest frequency (using the shortest dipoles), the polarization remains steadfastly linear. This is a major advantage over other wideband antennas like the discone, which can have ambiguous polarization at higher frequencies.

The specific polarization is determined entirely by the installation. Let’s look at the data for a typical commercial LPDA designed for VHF/UHF reception (e.g., 100 MHz to 1 GHz).

While linear polarization is the standard, the concept of circular polarization (CP) is important, especially in satellite communications. CP waves spiral as they propagate, making them less susceptible to signal degradation caused by Faraday rotation in the ionosphere or orientation changes of the transmitting or receiving platform (like a tumbling satellite). A standard LPDA cannot produce circular polarization. However, specialized versions can be created. One method is to use two identical LPDAs mounted orthogonally (at 90 degrees to each other) on the same boom and fed with a 90-degree phase difference. This creates a crossed log periodic dipole array. This dual-polarized antenna can be fed to produce either left-hand or right-hand circular polarization, or it can be used as a diversity antenna to receive signals of any linear polarization. This design is more complex and costly, but essential for space-to-ground links.

The mechanical construction of the LPDA directly enforces its linear polarization. The elements are rigid, straight conductors. The radiation pattern is strongest in the direction of the boom’s apex (toward the shortest elements), with the E-field aligned with the elements themselves. This is a fundamental property of dipole radiation. Any deviation from perfect linearity is considered a defect. Factors like element sag, asymmetry in the feed system, or proximity to large metallic objects can introduce cross-polarization, a small, unwanted component of the signal with the opposite polarization. For a high-quality LPDA, the cross-polarization discrimination—the ratio of wanted to unwanted polarization—should be better than 20 dB. This means the antenna is at least 99% “pure” in its intended polarization. Manufacturers achieve this through precise engineering and robust materials. For instance, a well-designed Log periodic antenna from a reputable supplier will have minimal cross-polarization components, ensuring high performance.

When selecting and installing an LPDA, polarization is a primary design consideration. You must first determine the polarization of the signals you intend to receive. For example, in the United States, digital TV broadcasts (ATSC) are primarily horizontally polarized, so an LPDA should be mounted with its boom horizontal. In contrast, many two-way radio services use vertical polarization, requiring a vertical boom mount. The mounting structure itself must be sturdy enough to maintain this orientation against wind and weather; a mast that twists over time will slowly change the antenna’s polarization, degrading performance. Furthermore, the physical length of an LPDA is proportional to its bandwidth and gain. A typical LPDA for TV reception might have a boom length of 1 to 2 meters with 10-20 elements, providing a gain of 6 to 10 dBi. The following table contrasts key parameters of a standard linearly polarized LPDA with a theoretical crossed LPDA for circular polarization.

>High (Dual feed lines, phasing harness)

In real-world environments, pure polarization is often muddled. Signals bounce off buildings, hills, and other objects, which can scatter the radio wave and change its polarization state. This phenomenon, called depolarization, means that a signal arriving at your antenna may have components of both horizontal and vertical polarization, even if it was transmitted with only one. This is one reason why a perfectly cross-polarized signal isn’t always completely nulled. An LPDA’s ability to maintain a consistent polarization pattern across its bandwidth helps in rejecting unwanted multipath signals that have undergone significant polarization change, thereby improving signal clarity. This is particularly valuable in urban canyons or hilly terrain where signal reflection is prevalent.

Ultimately, the standard log periodic dipole array is a masterpiece of predictable, linear performance. Its polarization is not an afterthought but a fundamental feature dictated by its mechanical design. This predictability is what makes it a workhorse for so many communication systems. Whether it’s perched on a rooftop pulling in crisp HDTV signals or mounted on a tower for spectrum monitoring, its linear polarization is a key factor in its reliable operation. Understanding this characteristic is the first step in deploying the antenna effectively to ensure strong, clear signal reception.