Log-Periodic Dipole Array or LPDA

Updated: Oct 19

It is a deigned antenna, not a compromised antenna, which was part of a United States Air Force "Secret" project in the early 1950s. Providing more gain than Hex-beams and most Triband Yagi beams. The Log-periodic antenna impedance is a logarithmically periodic function of frequency. The main advantage of a LPDA, is that it exhibits constant characteristics over a large frequency range of operation, providing very wide bandwidths, and is directional. That is why many Services, and Agencies rely on the LPDA antennas to maintain reliable long distance communication links for Embassies, Energy companies, FBI, FAA, Ship to Shore, MARS, Emergency Operations Centers, FEMA, and Armed Forces around the world. The Log can handle very high RF power, and many Commercial Short-wave Broadcast radio stations, are running Logs using up to 500,000 watts.

The above photo is one of my LPDA being tested (no tuning is ever required)

The Log-periodic uses half-wave dipoles where every elements is driven, and reduces in size on the boom, (different element lengths). The electrical function of a Log is an active broadband array, which simulates a series of full size two or three-element Yagis. The Log-periodic design operates similar to a three-element Yagi antenna. The dipoles elements are fed alternately through a common transmission line connected together, and shifts with frequency, every element is driven. When the frequency shifts upward, the active region shifts forward on the array to the shorter elements. So when you have a change in operational frequency, the active region shifts among the elements, so all the elements will not be active only on a single frequency. The Log antenna looks a lot like a Yagi antenna, however electrically is quite different. The antenna elements are arranged in such a way so that some of the elements are also passive radiators, which means they help bounce signals to the other elements.

A complex impedance matching arrangement is not required to drive a LPDA, most use a 300 ohm balance, or 50 ohm unbalanced fed system.

With a gain of +6 dB gain, and when installed at 1/2 wave above the ground, there is 6 dB more ground gain. And operating over a 2:1 to 4:1 bandwidths are common, the nominal half power beam width is 65 degrees. A 10:1 Bandwidth can be achievable on VHF/UHF designs and for TV reception. The wide bandwidth is thr great advantage of this antenna compare to a YAGI, and other antenna types, and no traps or moving parts.

Below: A VHF/UHF LPDA

Below:

A HF 7-30 MHz array that I had up in the 1990s, on a large tower at the K0UOs KS QTH

Below : Currently my K0UO Kansas station has two Tennadyne Logs, one at 100 feet, installed in 2023. The other at 195 feet installed in 2006, that has been through many heavy ice, and +80 mph wind storms over the years.

Mark K5YAC, at Tennadyne is manufacturing an excellent antenna.

See the Tennadyne web site https://www.tennadyne.com/

More info about Tennadyne history and owner

https://www.tennadyne.com/lpda-systems/

https://youtu.be/YJf5A4h-Dns?si=0MGX2FOfEkU6nTA-

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 up over 35 years, using a Collins Log-periodic 237B-3 RLP/ LPH-1B (LP1002-5/30 ) setup for 5-30 MHz use, antenna at 90ft. It has been through a number of hurricanes over the years. A large LPDA, however this large Log has 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/

Above: The DOD/FBI/ US Government LPDA standard setup mostly using the old Collins 237B-3 RLP design, now LPH 1C, or US Products LP-1005AA see http://w6dsr.com/LP1005AA/index.html

and http://w6dsr.com/LP1005AA%20Restoration/index.html

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:

https://www.changpuak.ch/electronics/lpda.php

https://hamwaves.com/lpda/en/index.html

https://play.google.com/store/apps/details?id=com.radioacoustick.lpdadesigner&hl=en_US&pli=1

https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1959/11/page11/index.html

https://wb0dgf.com/LPCAD.htm

Below: A wire element 2-30 MHz Log, rated for 25KW

Below: A very high gain Log on a mountain top firing down hill

Below: Four fixed wire LPDA

Below: A fixed vertical Log HF antenna which can receive ground waves, and vertically polarized Sky-waves, also it can detect very weak signals

Below: A friend of mine took this antenna down from a SW Broadcaster in Mexico a few yeas ago and is planing on installing it at his ham station in east Texas. Look at the rotator used to turn it.