DIY making Yagi antenna

Yagi antenna is an antenna lead, by a number of active and passive dipole oscillator placed in the same plane and perpendicular to the rod connecting their centers. Oscillator is usually a passive reflector, the rest of the passive oscillator as director. Because the metal bar through the oscillator voltage wave on the node, and perpendicular to the antenna, so the rod of the antenna near field has little effect. And
Active oscillator must be insulated with metal rods.
Through the data table you can see, Yagi antenna gain higher than the vertical antenna and dipole antenna. (From "antenna wave propagation", Northern Jiaotong University Xukun Sheng, Jiang Zhong Yong ed.)

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Form of the number of antenna reflector director a few number of active dipole orientation factor

Dipole 0 0 1 0 dB
Second Unit Yagi 1 0 1 3 ~ 4.5dB
Second Unit Yagi 0 0 1 3 ~ 4.5dB
Three elements Yagi 1 1 1 6 ~ 8dB
Four-cell Yagi 1 2 1 7 ~ 10dB
5 unit Yagi 1 3 1 9 ~ 11dB

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     From the table we can see that the unit Yagi antenna more directional stronger. But the unit does not increase proportional with the direction. Unit over time, result in narrow frequency band, the antenna size will be too large.

     In the short wave band, the wavelength is longer, more difficult to self-Yagi antenna, the FM band (V / U), because the wave length, can be more easily made low-cost Yagi antenna.

     Yagi antenna complex mathematical calculations (mathematical derivation I met felt dizzy), but many engineering or theoretical books are given its size, as long as in accordance with these data, we can made out of a good YAGI!
     Five Yagi antenna the size of the unit into the Figure 1

If the self-made four-cell Yagi antenna, they do not install the device D can lead, the antenna will look a little small. If you want to make seven units in the map based on the two lead unit, the length of half-wavelength, respectively 84%, 82%. Newly added unit wavelength interval is still 0.2 times.
I do 70CM band Yagi antenna, the first of four units, each oscillator and its connection rod, copper rod provided with BG4RUV (diameter 2.5mm) are made. About a month later, bought a 2 m long, 4mm diameter copper bar, and a removable system of four-cell Yagi antenna (as the connection to find a rectangular brass pole of each oscillator, the
An oscillator are fixed with screws and strut, easy to carry). First antenna resonance center frequency than expected (435 MHz) low-about 2 MHz, 430-440 MHz in the SWR is not high, the lowest SWR <1.1, the highest SWR is not greater than 1.4. The second branch antenna SWR across the band of undulating 70CM, up to about 1.2. Later, I to quell the detachable antenna to make some changes, use the rest of the material and device made of three arguments, so I Zhezhi antenna either assembled into four-cell Yagi antenna can also be assembled into seven units Yagi.
If you want a Yagi antenna, but does not require directive, you can try to do a three-unit hands-Yagi antenna. To place in their size (Figure 2).