Yagi–Uda antenna is a directional and pretty effective antenna consisting of multiple parallel elements in a line, usually half-wave dipoles made of metal rods.
Best-known use of this type of antenna is as rooftop terrestrial television antennas, but it is also used for point-to-point fixed communication links and for long distance shortwave communication by shortwave broadcasting stations and radio amateurs.
In this article i will show how to build a simple 5 element antenna for 144 MHz band from widely available parts, and how to get some fun with it.
This is my antenna.
But before actual building we should discuss some theory and calculation of this type of antenna.
Every Yagi antenna contains reflector, active (or driven) element and at least one director. Number of directors may be even 15. Increasing number of this elements increases total antenna gain and directivity but this requires a much precise tuning (element sizes and distances between them).
5 element antenna contains 3 directors and this is a good balance between complexity and performance.
Calculations is very simple.
Lengths of the elements:
Reflector = 0.495 * wavelength
D1 = 0.440 * wavelength
D2 = 0.435 * wavelength
D3 = 0.430 * wavelength
Where wavelength = c/f, f – antenna central frequency in Hz and c – speed of light.
Our selected band is 144 MHz or 144000000 Hz, speed of light – 300000000 meters per second.
Wavelength = 300000000/144000000 = 2.083 or simple – 2 meters.
So our lengths is:
Reflector = 0.495 * 2 = 0.99 meters
D1 = 0.440 * 2 = 0.88 meters
D2 = 0.435 * 2 = 0.87 meters
D3 = 0.430 * 2 = 0.86 meters
And distances between elements:
Reflector – Driven element = 0.125 * wavelength = 0.125 * 2 = 0.25 meters
Driven element – D1 = 0.125 * wavelength = 0.25 meters
D1 – D2 = 0.250 * wavelength = 0.250 * 2 = 0.5 meters
D2 – D3 = 0.250 * wavelength = 0.5 meters
But what about a Driving element? How to calculate it?
There are two big types of this elements:
- 1/2 wave open dipole
- folded dipole
Folded dipole length: 2.083 * 0.5 = 1.0415 meteres
Height: 2.083 * 0.05 = 0.10415 meters
Typical distance between feed connection points is 1-3 cm, depends on your boom diameter.
Both dipoles are symmetrical types of the antennas.
Main difference between this two types is a wave impedance.
Open-type dipole has impedance near 70 ohm
Folded impedance is near 300 ohm
Typical impedance of TV coaxial cables and home receivers is 75 ohm
Impedance of the cable and antenna should be carefully matched or you will get a big losing of power.
Also is very important to transform symmetrical antenna line to the non-symmetrical coaxial cable.
For the folded dipole a simple way to do this is a U-knee which can be made from the same 75-ohm cable. U-knee acts both like a symmetry and impedance matching device.
Length of this additional cable is ~0.4 of wavelength.
On the picture below you can see my another antenna for 433 MHz band with such U-knee.
In the current 144 MHz project open-type dipole is used due to it simplicity and material saving.
For such type dipoles all you need is just transform line from symmetrical to non-symmetrical without impedance matching. Good solution may be a special transformer.
In my antenna I’m using transformer from the cheap TV antenna amplifier, like on the photo below.
Bottom part with amplifier itself was cutted.
To fasten the rod you can use plastic ties and wall fasteners for your boom diameter.
Using formulas in this article you can great a good antenna for educational uses and non-critical applications.
Of course in a real world everything little bit complex.
You need to consider with boom and rods diameter, so more precise calculations is required. I’m strongly recommended to use this calculator for such types of antennas.
So, why 144 MHz ?
This antenna is quite broadband, so you can really receive signals from the FM band and to the 160 MHz, sure.
Most interesting in this band is:
- FM & TV stations
- Aircraft communications and airports (118 – 135 MHz)
- NOAA weather satellites (137 MHz)
- HAM radio (145 MHz)
- ISS (145.8 MHz)
In the next article i will show how to receive this signals using popular rtl sdr receiver.