Design of Yagi UDA Antenna

For good efficacy and directivity for reception on radio spectrum, we need a good Yagi-UDA antenna. This antenna was invented in the early 20^th century by two Japanese engineers named Uda and Yagi. The Yagi antenna is a narrow-band antenna designed to work only on FM channel. It has the best gain for its sizes and a correspondingly narrow main lobe (beam). A well designed yagi antenna as per the design given below can cover a line of site distance of even 5 KMs with just 1 watt RF power.

For the highest gain or discriminating against an interfering signal 20deg-40 deg azimuth off the desired signal, we use a yagi antenna. Basically an yagi antenna consists of one reflector (in the rear), one driven element and one or more directors (in the direction of direction/reception).

The above fig shows a yagi antenna consisting of four elements. The middle element is the simple half-wave, folded dipole. It is termed the ‘driven element’ because it is only element that is directly connected to the FM transmitter, driving the entire antenna. The other three outer elements are called parasitic elements. One is called the director elements. The reflector reflects RF energy and the director directs the RF energy. Typically, the reflector element is 5 % longer than the driven element and the director is 5% shorter than the driven element.

The antenna parameters element lengths and spacing are given in terms of wavelength, so an antenna for a given frequency can be easily designed. The lengths of various antenna elements are related to the frequency (f=106 MHz) is as follows:

• Reflector length           =150/f(MHz)  =150/106=1.41 Meters
• Driven element length =143/f(MHz)  =143/106=1.35 Meters
• First Director length     =138/f(MHz)  =138/106=1.30 Meters
• Second Director length=134/f(MHz)  =134/106=1.26 Meters
• Length of Boom=(43/106) + (45/106) + (45 /106)= 1.25 Meters approximately

Making a simple Yagi antenna by not using aluminum pipes but cheap plastic material and some wire , However a standard antenna with aluminum pipes is the best

Following materials are required for a low cost trial Yagi antenna.

1. Get from any electrical shop 1” wide Plastic casing 12feet long – 1 Nos
2. Get from any electrical shop ½ inch wide 12 feet long casings -2 Nos
3. Any pole wooden / bamboo stick /iron pipe to hold the antenna
4. Some cello tape
5. A knife to cut the plastic case
6. Get flat 2 core ribbon cable about 10 meters or more depending (Generally used for TV antenna, in early days) to be used as feeder wire from the transmitter circuit to the antenna.
7. Use the 1” casing as main support, let us call it boom
8. Cut the ½ inch casing for reflector, driven element and the directors as per size as above
9. Run a simple insulated or un-insulated wire along the boom on the top (not inside)
10. Run similar wire across the directors and the reflector on the top (not inside)
11. Join the points to the wire on the boom for directors, reflector by soldering
12. Take another wire more than the double of the driven element
13. Join the center to the boom point
14. Fold both the ends of that wire and bring them   at bottom of the casing
15. Take out 2 ends for connection and use cello tape to keep them in firm place
16. These 2 ends are meant for feed from the transmitter through the flat ribbon cable
17. Use cello tape to keep all the directors, reflector and the driven element like a yagi antenna

Tuning Process of Yagi Antenna

1. Connect the transmitter output to the antenna by the flat ribbon cable
2. Adjust with a non ferrus screwdriver, the trimmer for oscillator frequency till you hear complete silence at around 106 MHz in a FM receiver or your cell phone if it has FM receiver facilities
3. Use a multimeter in 250 mA range in series with the battery supply
4. Adjust the current to around 75 mA or the peak by antenna  trimmer. From the peak while you turn clockwise current will fall .You turn anti clockwise it will also fall. That that is the best for full power delivery to the antenna.

To know about the project working on this Yagi UDA Antenna, click on Long Range FM Transmitter.

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