antenna is the most commonly used among the Beam variety. It's inventor
was mr Uda Yagi and there it has got it's name from.
Above is a simple drawing of a 3 elements yagi looking from the top at it.
The longest elements
is called a REFLECTOR. This side is often called the "Back of the
Antenna" and although it does "reflect" the signals coming
from the back of the antenna it also has a function in the Gain.
The second from
the back is the RADIATING element (the dipool) and is always shorter
than a reflector and longer than a director. This is where your coax
cable is connected. Most yagi's have an impedance of around 20/30 ohms.
A impedance transformer is put on a yagi the in this case a Gamma match, but there are other types (hair-pin match, omega, t-match) More to this in the chapter connecting the coax to the antenna.
The Black line to the left at the above diagram, is called the first DIRECTOR. You can add as many directors as you want. All designs in chapter beams have 1 reflector 1 radiating element and 0 to 8 directors.
The most important
factor of a yagi is not the number of elements, but the boom length
A "standard" rule you can handle for gain, if you want 3 dB's more you need to double the boomlenght!
As just described
it is true: that with a full size yagi. Say a 3 element with a boomlenght
of 4 meter compared to a 4 elements with 4 meters boomlenght.
You will need computer
programmers when you want to start modeling Yagi antennas. Free examples
are those from MMana or Yagimax.
Not exactly of course but you get the general idea, the FB and Gain are not at their high values where the SWR is the lowest although many people think this is the case. So please do not worries to much about that SWR.
And you will not find one person who says: I changed the SWR from 1:7;1 till 1:1:1 and o boy....what a difference.!!..there isn't....
The Gain you might
expected from a yagi is shown in a diagram on the page beams.
With that diagram
you can decide for yourself , which length of boom you need for the
you should use:
The thickness of
the radiating elements does influence the bandwidth, Gain, Front to
back of the antenna the thicker it is the wider the bandwidth
Finally you will need to know if the antenna is doing its job properly. Read chapter measurements and Gain to make your conclusion.