INCREASE RANGE OF SIMPLE LOW POWER FM TRANSMITTER BY YAGI ANTENNA

Introduction

For good efficacy and directivity for reception on radio spectrum, we need a good Yagi-Uda antenna. This antenna was invented in the early 20th 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). 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).

Step 1: DESIGNING a YAGI UDA ANTENNA: (YAGI ANTENNA)

The above fig shows a basic 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 as in the first diagram and the 2nd one is with 107.9 MHz: as per the following calculations

Reflector length=150/107.9=1.39 or rounded to 1.4 meters

Driven=143/1079=1.32

First director=138/107.9=1.28

Second director=134/107.9=1.24

Now the gap between each

Gap calculation

gap reflector to driven=(43/107.9)=0.398-rounded to 0.4 ,

gap between driven to first director=(45/107.9)=0.41,

gap between director to director = same 0.41.

Add all the gaps and that will be the boom length

Thus the boom length= 0.4+0.41+0.41=1.22 meters. Thus make the boom about 1.5 meters. But maintain the gap as calculated.

Step 2: Length Detail

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

Thus

Length between reflector to dipole=43/106= 0.4 meters approx.

Length between dipole to director=45/106 = 0.425 meters approx.

Length between director to director=45/106 =0.425 meters approx.

Using a balloon transformer would bring about a better matching impedance. Given below an example for 107.9 MHz using a small portion of the same co-axial cable as per connections seen below

Step 3: Materials Required for a Low Cost Trial Yagi Antenna Made at Home

1 Get from electrical shop 1 inch wide rectangular Plastic casing ( used in house wiring), 12 feet long - 1 no

2 Also 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 straight

4 Some cello tape

5 A knife to cut the plastic case

6 Get co-axial cable , as used by cable TV connection as per your need to get a good height to mount the antenna at rooftop. This is to be used as feeder wire from the transmitter circuit to the antenna.

Step 4: Assembling the Home Made Antenna

1 Use the 1 inch plastic rectangular casing as main support ,let us call it boom (see the diagram above)

2 Cut the ½ inch casing for reflector, driven element (also called folded dipole) and the directors as per size as above.

3 Run an un-insulated or enameled copper wire (shown as “straight wire” in diagram below) of say 10 to 16 SWG (used in motor winding) along the length of the boom on the top (not inside).If enameled copper wire is used then remove the enamel at the soldering joints and also for the at the 2 end points of the driven element ie the dipole

4 Run similar wire as per the size for the directors and the reflector on the boom wire at right angles to the boom wire. Do not place the plastic case now.

5 Join the center points to the wire on the boom for directors ,reflector by soldering, be careful as the plastic is near by.

6 Take another wire more than the double of the driven element ie the folded dipole

7 Join its center to the boom point wire by soldering

8 Place the plastic case meant for the dipole over the boom top now and tape to hold the wire firmly.

9 Fold both the ends of that dipole wire at the ends of the plastic casing and bring them at the top of the casing and tape it firmly

10 Take out 2 ends for connection and use cello tape to keep them in firm place

11 These 2 ends are meant for feed from the transmitter through the co-axial cable

12 Now place the plastic casings of reflector and the 2 directors on the top of the boom over their respective wire and tape them for the wire to be sticking to the plastic case

13 Use cello tape in cress cross way to bind the reflector ,dipole and the 2 directors on the boom firmly, to look like an olden day television antenna

14 Use any pipe to get Yagi antenna stand straight on some height while aiming the director side to the desired direction that you would like to get the range.

Step 5: Additional Notes

A) Making a simple and cheap Yagi antenna at home as explained above ,by not using aluminum pipes but plastic material and some wire , may give a range of about 1 KM for a simple transmitter that covers hardly 100 meters.The out put from the transmitter is fed to a co-axial cable (generally used for cable TV ) which nearly matches to the Yagi antenna impedance of 75 ohms for best results.

B) However a standard antenna with aluminum pipes as were, used in our TV in earlier days is the best if you can get it made to order. Or buy a TV antenna whose dipole ( driven element) is bigger than 1.35 meters. Say if its 1.5 meters then cut it to 2 pcs exactly from the center. Then make each pc 1.35/2 by again cutting them. Now fix them at the same place where the driven element is supposed to be there. Connect the co-axial cable to the driven element. No need to size the directors even if they are smaller than the design parameters