2. Design a Yagi antenna
The reflector, director, active element, and total length of the "Bamu" antenna will vary depending on different needs, materials, and environments. I have not yet found any equation that describes the relationship between these parameters and other conditions. However, I did find a Yagi Antenna Design document authored by the U.S. Department of Commerce / National Bureau of Standards. This document is illustrated with diagrams and includes many valuable charts. My subsequent designs were largely based on this document.
However, the actual manufacturing situation does not match the measurement conditions specified in this document. Therefore, this document should only be used as a rough reference.
2.1 Basic Target Parameters – What kind of Yagi antenna do I want?
（1）Target frequency f – I want my Yagi antenna to help me communicate on 145.100 MHz, so I will set the target frequency to:f = 145.1 MHz。
（2）Target wavelength λ – The wavelength corresponding to the target frequency, calculated using the formula:λ = c/f, includingcIt refers to the speed of light (the speed at which electromagnetic waves, including radio waves, propagate). In this case, the wavelength corresponding to 145.100 MHz is λ = 206.8 cm.
（3）Total number of elements – I would like my Yagi antenna to include a reflector, an active element, and a director. This makes a total of three elements.
（4）The diameter of the resonator cross-section, d – the reflector, active element, and waveguide for an "八木" antenna are typically made from metal rods (here, I chose a brass rod). The diameter of these rods is estimated to be 4mm. This depends on the products you can find.
Please note: When selecting the metal rod material for the resonator, it is recommended to choose brass rods instead of stainless steel rods. Brass rods can be cut and adjusted in length using a pair of pliers, while stainless steel rods cannot. Additionally, the cross-sectional diameter of the brass rods can be less than 4mm, as experience has shown that cutting 4mm brass rods by hand is quite difficult.
（5）Main beam cross-section diameter D – Material for the main beam: PVC pipe. Diameter: 20 mm.
（6）Resonator spacing S – the distance between an active resonator and a guide, or between multiple guides (if any). I chose 0.2λ – 41.4 cm. You can choose one within the range of 0.2λ to 0.4λ.
（7）Total length of the main antenna beam – the sum of the spacing between the resonators, plus the appropriate length of the handle part. In this case, the total length of the main beam is 0.2λ + 0.2λ + 25 cm (handle). You can use a longer PVC pipe if necessary to adjust the spacing. I used a 120cm PVC pipe.
（8）Relative length – the ratio d/λ of the resonator cross-sectional diameter to the target wavelength, and the ratio D/λ of the main beam cross-sectional diameter to the target wavelength. These parameters will be used later.
2.2 Verify the length of the oscillator.
Refer to the following curve —
(▲ Oscillator length - d/λ curve)
When selecting the initial spacing, S, in previous iterations, it is recommended to choose one of the existing spacings shown in the figure, such as 0.2λ and 0.25λ located in the bottom right corner, for ease of obtaining reference data.
Verify the parameters corresponding to the "八木" antenna (Yagi-Uda antenna).E, my value is A, and then I use the 'd' obtained previously./λ corresponds to the x-axis. In the "REFLECTOR" curves (the top two), confirm the reference length of the reflector relative to the target wavelength. In the "DIRECTORS" curves (the bottom five), confirm the reference length of the directors relative to the target wavelength.
For example, I obtained d/λ = 0.0019 and S = 0.2λ. This corresponds to a length of R = 0.488λ = 100.9 cm and a length of D = 0.462λ = 95.5 cm, as shown in the diagram.
You can also adjust the oscillator length based on the D/λ data. Refer to the following curve —
(▲ Corrected length-D/λ curve)
Map the D/λ data to the x-axis, and obtain the corresponding value for the y-axis – that is, the ratio of the corrected length to the target wavelength. Add this data to the previously obtained length of the oscillator, and you can essentially complete the correction of the oscillator length. However, if you want to place the oscillator on the outer side of the main beam instead of passing through it, as I do, then I think this correction seems pointless – at least, when adjusting, I found that the actual center frequency was much smaller than the expected frequency.
Additionally, you can easily determine the length of the active transducer – one-quarter wavelength (total length is one-half wavelength, and it is cut into two pieces) – by specifying the target wavelength.
[References]Yagi Antenna Design
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