Are all spiky antennas the same?

Foundations of Amateur Radio

The world is filled with antennas. You'll find them on towers, buildings, cars and on your next door neighbour's roof. They come in an astonishing variety, to the point where you might start thinking that antennas are a fashion accessory that vary with the season and if you start digging through the history books you'll come across designs that dial that variety up to eleven.

Possibly the most visible antenna today is the television antenna and when you start noticing them, the more variation you'll discover.

Their basic shape consists of a vertical pole, the mast, with a horizontal pole, the boom. Attached to the boom are various different shapes, or elements, that often vary in length according to some pattern.

The shape is designed to collect as much electromagnetic radiation from a particular direction, or in the case of a transmitter, focus as much energy as possible into one direction. This focus is called gain. The more focus, the more gain.

One of the oldest designs for this kind of antenna, still in use today, is the Yagi-Uda or Yagi antenna. It was invented in 1923 by Shintaro Uda at the Tohoku Imperial University in Japan and popularised to the English speaking world by his boss Hidetsugu Yagi who claimed to be the sole inventor in his Japanese patent application. He went on to file similar patents in Germany and the United States.

Gain for a Yagi varies depending on design. Generally more elements means more gain. Sometimes you'll see a Yagi with weird shorter elements along the boom. This is a design to make the antenna work across multiple frequencies. Another way that this can be achieved is by adding traps along an element. They look like a thick stubby tube at some distance along an element. You can have more than one of these to allow for more frequencies. These improvements allow for several Yagi antennas to share elements and boom space, essentially combining several independent antennas into one.

It can be tricky to discover in which direction a Yagi is pointing, but essentially the boom indicates the direction and the end with the shortest element is the front.

There's another type of antenna that to the casual observer looks similar. It's called a log periodic dipole array, LPDA or log periodic antenna. It was invented in 1952 by John Dunlavy whilst he was contracted to the United States Air Force. He wasn't credited because it was classified as "Secret", later changed to "Restricted". In 1958 Dwight Isbell built a log periodic antenna as an undergraduate student at the University of Illinois at Urbana-Champaign. He was part of a larger team that included Raymond DuHamel, John Dyson and Robert Carrell. Later Paul Mayes developed a variant that improved performance.

Before I dig in, I'll also note that this antenna caused all manner of legal issues that are still in force today. The so-called Blonder-Tongue Doctrine states that a patent holder isn't permitted to re-litigate the validity of a patent that has been held invalid previously. It was a result of the University attempting and ultimately failing to protect its patent for the widely copied antenna design. Reading about this is a fascinating discovery in how a single Judge can make a massive impact on law and society.

The log periodic antenna is designed in a way that to the uninitiated looks very similar to a Yagi antenna. It's based on the idea that you can design an antenna made up from independent dipoles that are spaced in such a way that they form an antenna where each dipole radiates to take advantage of its neighbours. Generally a log periodic antenna looks like a triangle. Often the elements are on two separate booms, alternating side-to-side, or you'll see a zig-zag structure that causes the antenna signal to alternate side-to-side.

One characteristic of an antenna is called bandwidth. It's a measure of how many frequencies it can operate on within the constraints of the antenna. The wider the bandwidth, the more frequencies you can use with the same antenna.

A Yagi antenna typically operates within about four percent of the design frequency. If you combine multiple Yagis by adding traps or different length elements, you'll end up with several frequencies, each with a similar range.

A log periodic antenna on the other hand is designed to be used across a large range of frequencies. In shortwave broadcasting there are log periodic antennas that operate between 6 and 26 MHz. In more common use today you'll find log periodic antennas used for higher frequencies. It's not unusual to find log periodic antennas that operate between 400 and 4000 MHz.

For even more confusion, you can share the boom of a log periodic antenna with a Yagi antenna as is popular in fringe television reception areas.

Some other things to note are that for a Yagi most of the elements are passive and only one is generally a driven element, in a log periodic antenna all elements are driven. For a Yagi antenna, more elements means more gain, whereas for a log periodic antenna it means more frequencies.

I'll also point out that there are experiments where the frequency range for Yagi antennas is being increased to more than twenty percent of the main frequency by varying the design. Much of this is achieved by using computer simulations to test many different virtual antennas until one promising design pops out. This optimisation technique can also be applied to log periodic antennas resulting in very interesting shapes that look nothing like the antennas you see on the roof today.

I've completely skipped over how these antennas are actually fed, as-in, how is the coax connected to the antenna. That's a whole different topic of conversation worthy of many hours of research and discussion.

Next time you look at a spiky antenna you should be able to discover if it's a Yagi or log periodic, or both and why.

I'm Onno VK6FLAB