Like any smart shopper it's important to search around for the best quality product for the best possible price. Purchasing a commodity item, such as an apple, it's easy to compare like-with-like as we understand the mechanics - we know where it comes from, how it is made, and so on. However when it comes to antennas it's easy to be mislead by false claims due to the shroud of complex mathematics, postgraduate qualifications, and social awkwardness required to fully grasp antenna design.
A very common question we get asked is why, when other retailers are offering 18dBi, 21dBi, 22dBi, and even a whopping 24dBi, don't we have cellular-band Yagi's bigger than 16dBi? The quick answer is - because they don't exist. It wouldn't be right for us to name and shame as these unsuspecting retailers have likely been mislead by their manufacturer themselves.
The driven element is the key part of the Yagi antenna, essentially just a piece of metal physically attached to the coaxial cable. The electrical current being transmitted up the cable causes the metal to resonate and give off electrical energy in the form of radio waves. If you have a basic understanding of radio or physics you'll recall that an object's resonant frequency is directly related to it's physical length. An antenna is typically tuned to resonate on one frequency by cutting it's elements to a particular length - it's resonant length. A conventional Yagi antenna cannot operate on two distinctly different frequencies - unless the second frequency is a harmonic of the first, which the radio spectrum is divided so this not the case (or else towers would accidentally be broadcasting on other bands). If that's gibberish, don't worry it's not too important to understand - the key point to take away is one Yagi can only operate on one frequency band.
The driven element on it's own is simply a dipole antenna and has a gain of 2.15dBi and is the starting point of all Yagi's. A dipole is converted into a Yagi by placing a longer reflector element behind, and a shorter director element in front, causing a substantial increase in gain due to a combination of inductive reactance and capacitive reactance. The easiest way to visualise this process is to picture the combination of the reflector and director causing a phase shift, resulting in wave cancellation (reduced amplitude) at the rear of the yagi and wave-forming (increased amplitude) in the forward direction. The addition of more director elements furthers the wave-forming, resulting in more energy being transmitted forward and less to the rear (and sides).
While there's no convenient closed-form equation to designing the perfect Yagi, the maths behind antenna design demonstrate that in order to double the gain, we have to double it's length. Because decibel being a logarithmic scale, doubling the gain of an antenna is a 3dB increase. In Yagi design, it's no surprise that the overall length roughly doubles per 3dB increase. It's this doubling nature that causes a 1m long 850MHz Yagi with a gain of about 13dBi to become 2m to produce a 16dBi gain. Consequently to reach the wild claims of over 20dBi your Yagi would have to be at least 8m in length!
Other than maximising gain, the number of elements and measurements must be carefully chosen in order to minimise a metric called Voltage Standing Wave Ratio (VSWR). VSWR is effectively a measure of how much power is leaving the cable onto the antenna, versus the power being reflected back down the cable to the device. Reflections are caused by something called impedance mismatch - a disharmony between two materials at a particular frequency. We won't delve too deep into VSWR, the key point is that a high gain Yagi will perform badly if operated at a poorly resonant frequency - as the device's power output will not be effectively transferred onto the antenna in order to be broadcast. A good antenna should have a VSWR under 1.5:1, and a multi-band antenna less than 2:1.
As a picture says a thousand words, we purchased and modelled a Yagi currently retailed that claimed to produce a 21dBi gain across 2G, 3G, and 4G networks - 824-960MHz and 1710-2190MHz. Alarm bells ringing yet? Let's have a look with some computer simulations. In the below diagrams, the balloon shapes are visualisations of the directions RF energy leaves the antenna.