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Yagi antennas are formerly called before as Yagi-Uda antennas. They were named as such to honor the two Japanese who invented them. Such antennas are commonly seen and used in TV receptions, it can also function as a bridge antenna linking together a site to a particular WiFi access point.
They usually come with a dipole which serves as its main “driving” or radiating element. As for its “reflector”, normally this can be found behind the driven element. It is not unusual for a reflector to add 4dB to 5dB forward gain. But this should not mean to say that all these yagi antennas come with their reflector. By adding up a few more directors to the antenna, it will likely get an average of 1dB of gain.
Performance can be further improved by adding up parasitic elements to the main element since they are known not to radiate a signal. The signal amplitude and phase of the signal would depend on the spacing and the size in which these parasitic elements come in.
So what are the likely Yagi antennas pros and cons? Why is that their use is advantageous or disadvantageous? When is it advisable to make use of it?
We consider Yagis as directional antennas. The vast majority of them come with a beam width size of a 50° to 70°. With regard to the fact that they center their input to only one direction, their natural tendency is to put out high gain with respect to omnidirectional antennas. By this measure, they become good receptors of lower strength signals.
This high gain is putting yagi antennas to much greater advantage in such a way that it is given a good range. The Yagi’s tend to get the most gain with respect to their physical size.
The Yagi designed antenna will filter out signal noise that normally comes from the opposite direction. This explains the reason why the Yagi system of an antenna is the best choice there is when it comes to high demand applications which includes telecommunications.
Contrary to how some arrays are, aiming a yagi system antenna is pretty much easier. How they are designed and built makes mounting them on vertical towers a breeze.
And since Yagi system antennas are way simpler in comparison to log-periodic antennas, they tend to cost less as opposed to a comparable LP antenna. While we have good availability of printed circuit boards, it is possible to build other with the help of simple rods provided they are properly positioned.
The frequency or bandwidth range is somewhat confining.
You need to have an extra-long antenna if you are going to aim for a high gain level. And even then, the gain would be naturally limited to 6-9dB, not unless there is more than just one Yagi system antenna underway.
The natural reaction of a Yagi antenna when you attempt to move it away or get to some distance from the frequency range you originally designed it for is to experience degradation of its electrical characteristics (feed point, gain, front to back ratios, gain). The reason behind this is that Yagis are structured or designed based on resonant elements. As for its off-resonant operations, they introduce reactance that allows for significant SWR increase.
By design, the Yagi system antennas are supposed to be balanced. This sense of balance though can get disrupted if you place a balun at the feed joint line, which links it to the drive element. With respect to the driven element design, they can either be balanced or unbalanced.
Take note that Yagi’s are balanced. Hence, it eliminates the need for a balun.
When you add some elements to a Yagi system antenna, you are further enhancing its directionality. Essentially, you are just narrowing its focus, but you are rendering it to receive better signals from that direction since it tends to improve the ratio signal.
In other words, it significantly pulls down the interference level, particularly those that are coming from the sides.