Increase FM transmitter range by antenna

For very  Long Range use a yagi antenna fabricated at your end:-

Yagi antenna

The out put is fed to a co-axial cable (generally used for  cable TV ) which is nearly matched to the Yagi antenna  impedance of 75 ohms by trimmer TR 2  of the tuned circuit for maximum power delivery to the load ie the Yagi / GP antenna. Transmitter should never be powered without the antenna   (ie the load) in which case the total power forms a SWR standing wave ratio on the power transistor Q3 heating it up badly to result in failure.

Antenna assembled

DESIGN OF YAGI UDA ANTENNA: (YAGI 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 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).

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
  • 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.

For 107.9 MHz

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.

Yagi at 107.9

Making a simple and cheap Yagi antenna at home by not using aluminum pipes but  plastic material and some wire ,that may give about 1 KM 

 (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.)

Electrical conduit

 

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

  • Get from any electrical shop inch wide rectangular /tubular Plastic casing ( used in house wiring), 12 feet long – 1 no
  • Also get from any electrical shop ½ inch wide 12 feet long casings -2 nos
  • Any pole wooden / bamboo stick /iron pipe to hold the antenna straight
  • Some cello tape
  • A knife to cut the plastic case
  • 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.
  • Use the 1 inch plastic rectangular / tubular casing as main support ,let us call it boom (see the diagram above)
  • Cut the ½ inch casing for reflector, driven element (also called folded dipole) and the directors as per size as above.
  • Run an un-insulated or enameled copper wire  (shown as “straight wire” in diagram above) 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 only and also  at the 2 end points of the driven element ie the dipole
  • 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.
  • Join all the center points to the wire on the boom for directors ,reflector by soldering, be careful as the plastic is near by.
  • Take another wire more than the double of the driven element ie the folded dipole
  • Join its center to the boom point wire by soldering
  • Place the plastic case meant for the dipole over the boom top now and tape to hold the wire firmly.
  • 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
  • Take out 2 ends for connection and use cello tape to keep them in firm place
  • These 2 ends are meant for feed from the transmitter through the co-axial cable
  • 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
  • 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
  • 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.

Tuning Process again

  1. Connect the transmitter output to the antenna by the coaxial cable
  2. Adjust with a non iron screwdriver, the trimmer 1 till you hear complete silence at around 88 to108 MHz in a FM receiver or your cell phone if it has FM receiver facilities
  3. Use a multimeter in 250 mA DC range in series with the power supply 12 volt or a good 12 volt adaptor
  4. Adjust the current to around 75 mA (at 12 Volt )or the peak by trimmer 2. 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.

In Case the range is not adequate because of mismatched impedance of the antenna add a balloon as shown below for 107.9 MHz

baloon for Yagi

For GP antenna

GP antenna

 ANTENNA DESIGN

  1. Ground pole Antenna(GP)

This Antenna is most widely used all over the world. For example, when you see a police car it has a transmitter with Ground Pole Antenna The body of car serves as ground). It accepts load from 50 ohm source and has larger power output than Half-Wave Dipole Antenna.The range would be 1 KM in radius. 

Radiator GP wire to Radical wire angle is 135 degrees.

 Dimensions are in CM

Frequency Radiator Wire Radiator GP wire
108MHZ 660 693
104MHz 684 720
100MHz 713 749
90MHz 792 819

 

 

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