YAGI YADA YADA YADA
It has become common usage in broadcast engineering circles to refer to any antenna that resembles a TV-type receive antenna as a "yagi." As in "did you aim the yagi back to the studio?" It's easy to see why this has happened: "yagi" is a nice short word, and it doesn't sound like anything else I can think of right now. But often enough, the antenna we're referring to isn't even a real yagi at all! Read on…
Strictly speaking, a yagi is an antenna described by a pair of Japanese university professors in the late 1920's. S. Uda invented the antenna, but it was first described in English by his colleague, H. Yagi. It was the wish of both Messrs. Yagi and Uda that their inventions be called "Yagi-Uda" antennas, but the "yagi" moniker is now so entrenched that it's here to stay. The antenna consists of a driven element, which is typically a half-wave or a folded dipole, and a reflector and one or more directors. The reflector is slightly longer than 1/2 lambda (one wavelength is one lambda), and the directors are slightly less. These parasitic elements are critically spaced a small distance apart--usually between 1/10 and 1/3 lambda--from the driven element. The spacing and the number of directors help determine the antenna's characteristic gain, input impedance, front to back ratio, the magnitude of minor lobes, and the antenna's bandwidth. Typically the more elements, the more gain. A six-element yagi may have a gain of 10 dBd or so. These antennas offer a compact solution for VHF and UHF transmitters and receivers--they are compact and have significant gain. They have a high front to back ratio, so can be used to reject undesired reflected signals. They can be easily aimed. But they are inherently narrow in bandwidth, three percent or so of the operating frequency, and this can sometimes become a problem. It means an antenna can be effective for, say, the 450-455 MHz link band, or a single TV channel, but not for multiple TV channels. The narrow bandwidth also makes the yagi antenna's performance suffer under even mild icing conditions.
Well, there are always different antennas for different needs. The yagi is very attractive on many fronts, but the bandwidth problem in particular was vexing. Many variations of the basic yagi have appeared as a result. Sometimes the parasitic elements are detuned slightly to "broadband" the antenna. Sometimes a second driven element, tuned for a second frequency, is added. Invariably, the gain of the antenna suffers horribly, but that may be acceptable in order to get the improved bandwidth that is wanted. The trick is to bend and twist the antenna slightly so that the characteristics are fairly even across the band of frequencies of interest.
As a result, we see "modified" yagis on the market that can for instance cover more than one TV channel effectively. Still, there's sometimes a need for even wider bandwidth in a directional antenna. For instance, a cable television company will often want to receive all the VHF channels using a single antenna or array at its headend. The solution here is a whole different class of antennas called "log periodics." The log periodic antenna can be designed to cover several decades of bandwidth. The gain suffers considerably compared to a yagi, but the front-to-back ratio can still be quite good, so that non-incident signals can be rejected by this antenna, too. A log periodic antenna can be recognized by the fact that the parasitic elements vary in length more than with a yagi. This antenna looks like the profile of a Christmas tree, with a triangular silhouette. The formerly common VHF TV masthead receive antenna (common before the age of cable television, that is) is usually a form of log periodic antenna. Whether a yagi or a log periodic antenna, the pointy end of the assembly indicates the direction of maximum gain.
Another variant that has become quite popular for rural TV reception is a log periodic antenna with some yagi elements added for UHF reception. The log periodic elements can produce a nice smooth pattern over the span of VHF and FM frequencies, and the additional yagi elements increase the gain at UHF frequencies, where that extra gain is often needed.
More on yagis, log periodics, and their relatives in our next installment!