Log-Periodic Dipole Array or LPDA
- skylarkcolo
- Jun 18, 2024
- 6 min read
Updated: 2 days ago
This is a purposefully designed antenna, not a compromised one, which was part of a United States Air Force "Secret" project in the early 1950s. It provides more gain than Hex-beams and most Triband Yagi beams. Then in 1957, R.H. DuHamel and D.E. Isbell of the University of Illinois published the first work on what was to become known as the log periodic dipole array. The impedance of the Log-periodic antenna is a logarithmically periodic function of frequency. The main advantage of an LPDA is its consistent characteristics over a wide frequency range, offering very broad bandwidths without moving parts, fiberglass, traps, or wires that can break, and it is directional. It delivers more gain than a Hexbeam or a small Tri-band Yagi. This is why many services and agencies rely on LPDA antennas for maintaining reliable long-distance communication links for embassies, energy companies, high-frequency stock trading groups, the FBI, FAA, ship-to-shore communications, MARS, emergency operations centers, FEMA, and armed forces worldwide. The Log can handle very high RF power, and many commercial short-wave broadcast radio stations use Logs with up to 500,000 watts. A Log is an excellent replacement for the no longer produced SteppIR antenna.
The above photo shows one of my LPDAs being tested (no tuning is ever required).
Understanding the Log-Periodic Antenna Design
The Log-periodic antenna is a fascinating and sophisticated design that employs half-wave dipoles for its construction. In this configuration, every element of the antenna is actively driven, meaning that each dipole contributes to the overall performance of the antenna, rather than relying on a single active element as in some other antenna designs. The elements of the Log-periodic antenna are arranged in decreasing lengths along the boom, which is the horizontal support structure of the antenna. This variation in element lengths is crucial, as it allows the antenna to effectively cover a wide range of frequencies.
Broadband Capabilities and Functionality
The electrical functionality of a Log-periodic antenna is that of an active broadband array. This means that it is capable of operating efficiently over a wide spectrum of frequencies, simulating the performance characteristics of a series of full-sized two or three-element Yagi antennas. The Yagi antenna is known for its directional properties and gain, and while the Log-periodic antenna shares some visual similarities with the Yagi design, its operational principles and capabilities differ significantly.
Operational Mechanism
The Log-periodic design operates in a manner that is quite similar to that of a three-element Yagi antenna. However, what sets it apart is the way in which the dipole elements are fed. In a Log-periodic antenna, the elements are connected through a common transmission line, allowing for alternate feeding of the dipoles. This connection is vital as it enables the antenna to shift its active region in response to changes in frequency. As the frequency increases, the active region of the antenna shifts forward along the array, moving towards the shorter elements. This dynamic adjustment ensures that the antenna remains efficient across its operational bandwidth.
Frequency Response and Active Regions
One of the most intriguing aspects of the Log-periodic antenna is its ability to handle changes in operational frequency. As the frequency shifts upward, the active region of the antenna does not remain static; instead, it migrates among the various elements. This means that not all elements are active at any given moment, allowing the antenna to maintain performance across different frequencies. The design cleverly utilizes the principle of having some elements act as passive radiators, which play a crucial role in enhancing the overall signal reception and transmission capabilities of the antenna.
Structural Similarities and Differences with Yagi Antennas
Although the Log-periodic antenna might look similar to a Yagi antenna due to its linear element arrangement and boom, their electrical properties and operational mechanics are fundamentally distinct. The Log-periodic design includes both active and passive elements, enabling a more intricate signal interaction that can enhance performance across various applications. The passive elements help direct and reflect signals towards the active elements, thereby boosting the antenna system's overall gain and efficiency. In essence, the Log-periodic antenna is a versatile and efficient broadband design, using half-wave dipoles of different lengths to cover a wide frequency range. Its unique feeding mechanism and ability to dynamically shift the active region make it a powerful tool for diverse communication needs, setting it apart from traditional designs like the Yagi.
A complex impedance matching setup is unnecessary for driving an LPDA, with most using a 300-ohm balanced or 50-ohm unbalanced feed system.
With a gain of +6 dB and when installed at half a wavelength above the ground, it achieves 6 dB more ground gain, about 2 dB more than a Hex-Beam, with significantly more front-to-back ratio. Operating over bandwidths of 2:1 to 4:1 is common, with a nominal half-power beam width of 65 degrees. In ice or hail storms, the Hex-beam's wire may break. A 10:1 bandwidth is possible in VHF/UHF designs and for TV reception. The wide bandwidth is a significant advantage over a Yagi and other antenna types, without traps, wires, or moving parts that could fail.
Below the Tennadyne in use at K0UO
Unlike many LPDA antennas, the Tennadyne has minimal wind resistance. I've had one installed on a 195 ft tower for 20 years without any maintenance, enduring ice, hail, and numerous 80 mph wind storms.
Mark K5YAC at Tennadyne produces an outstanding antenna.
Visit the Tennadyne website https://www.tennadyne.com/
More info about Tennadyne history and owner
BELOW: I am using two Tennadyne Logs, one at 100 feet, installed in 2023. The other at 195 feet installed in 2006
The Tennadyne Log-Periodic Dipole Arrays (LPDA) are robust and comparable in price and size to a Hex. However, they offer more gain and improved front-to-back ratio, with minimal risk of damage. They feature no traps, lossy coils, wires that could break, or moving parts. Once assembled, the SWR remains low without needing any tuning, and you can extend the mast through the array to mount another antenna, such as a VHF/UHF antenna, on top.
Above: Is at K8CU, an LPDA stack, at one time I also had a stack, using two T12 Tennadynes.
Below: My antenna in Belize at V31KW, which has been installed for over 35 years, utilizing a Collins Log-periodic 237B-3 RLP/ LPH-1B (LP1002-5/30) setup for 5-30 MHz use, positioned at 90ft. It has withstood several hurricanes over the years. Although a large LPDA, this Log offers no more gain than a Tennadynes on 20 - 10 meters, http://www.collinsradio.org/wp-content/uploads/2022/12/Q3-2018.pdf
Now sold by https://www.usantennaproducts.com/antennas/models-lp-1005-lp-1001-lp-1002/ or https://www.antennas.com/product/lph-1b/
Logarithmic Periodic Dipole Antenna Calculator
The ratio of the successive element lengths (L_(n+1)/L_n) be equal to some constant k, and that the distance between elements (d_(n+1)/d_n) also equal k.
See:
Below: A YouTube of the repair & refurbishing of the LP-1002 Log Periodic Beam Antenna @ the Northern Utah WebSDR 4
Northern UT Web SDR Number 5, with an old 10-30 MHz KLM log
A large logarithmic antenna protel radio link ARL531 5-30 MHz being installed
Below: A large Log using the old Collins 237B-3 RLP design, now LPH 1C or US Products LP-1005AA
ABOVE: K0UO's 10 foot wide tower with LPDA 1990s
Calculator to build a LPDA
LPDA Links
COMPARISON OF COLLINS LOG -PERIODIC ANTENNA AND
MILITARY RHOMBIC ANTENNAS, SEE PAGE 79 UP https://w1op.com/237/Collins%20Antennas/Collins%20Antennas%201960.pdf
Install of a large Log
The K0UO antenna test range site utilizes the 4KS Walz airport, and its surrounding area as a practical learning environment for Scientific, Technical, Engineering, & Mathematics (STEM) antenna projects in an outdoor real-world setting. If your group has a University aerospace or antenna research STEM program, let me know.
TO SEE the complete K0UO antenna Blog list check @ https://www.k0uo.com/k0uo
I wished I had known about a Log before, I rebuilt the last three hex beams; that have been destroyed by ice and wind.
I got a T6, and it has more gain and F/B and same size as the Hex.
A Hex work but even a small T6 LPDA has a noticeable more Gain and F to B at low cost.