4-Element Yagi Antenna for PMR446  

Information supplied by Per, SM6ETH

PLEASE NOTE: FOR INFORMATION ONLY

DELBOY ENTERPRISES DOES NOT RECOMMEND OR ENCOURAGE THE MODIFICATION OF PMR446 RADIOS

Some people have replaced the fixed antenna on their PMR equipment with a coaxial connector which allows them to use external antennas instead. I will in the following describe a four element Yagi antenna I designed and built for the PMR band, it is a real low cost design and easy to put together. The goal was to make a design that require not to much experience and use of ordinary tools possible to find in almost every household, as an example, there is no need for a drilling machine.

The tools needed are as follow.

- Hammer

- Folding rule or equivalent

- Pencil

- Hacksaw

- Knife (to cut the coax cable)

- Soldering iron

- And a small amount of glue.

List of materials (All measurement in mm)

Number Type Dimension Length Remarks
1 Wood Strip 21 x 15 600 Boom
4 Wood Strip 21 x 8 100 Support
8 Nails      
1 Aluminum Tube  12 326 Reflector
2 Brass Tube 12 150 Driver
1 Aluminum Tube  12 271 Director 1
1 Aluminum Tube 12 261 Director 2
16 Plastic Strip   Min 120  

(RED Figures are IMPORTANT and MUST be accurate) 

Wood frame

The antenna consist of a wood frame, see figure 1. The dimension used is not critical, I used two different dimension of wood strip, boom dimension is 21 x 15 and the four supports is 21 x 8. The frame is glued and nailed together. The supports shall be mounted in right angle to the boom, see figure 1 and 2.

When you are ready with the frame it has to be painted. It is necessary, because it have not to take up any moister which can affect the performance of the antenna. You can use a metallic boom instead, but in this design is it necessary to isolate all the antenna elements from the boom.  

Antenna elements

The antenna element I used is aluminum and brass tubes with the diameter of 12mm. The brass tube is used for the driver because it is possible to solder the coax cable to it. All antenna elements can without problem be changed to brass or copper tubes with the diameter of 12mm, use tubes with a thin wall to keep the weight down. The reason using such fat elements as 12mm is because it gives the antenna wider bandwidth which makes the design less sensitive for variations in the built up. It is not possible to change the diameter of the antenna elements to a smaller size without recalculating the design, because the performance of the antenna will change.

Assembly antenna element

To attach the antenna elements to the Wood frame I used plastic straps, it is easy and fast, see figure 3, 4, 5.

When you assembly the driver element leave a gap of 2mm between the two element halves, see figure 4.  

Soldering the coax cable

In this text I will not describe how to split up a coax cable, you can find it some where else at the web.
IMPORTANT, do not forget to seal the coax cable after you have split it up, to avoid it to take up moister. Also the solder joints must be sealed, use epoxy, glue, paint or what else you have in house.

When you solder the coax cable to the two halves of the driver, heat up the tubes first, it takes some seconds for the brass tube to heat up. Apply a small amount of solder and let it melt before you solder the coax to the tubes. In figure 6 you can see the coax cable connected to the driver.  

Antenna performance

In figure 7 you can se the complete antenna. The performance of this antenna is rather good, wide bandwidth with a good match to 50 ohm coax.  

Gain: 9dBi
F/B: 11dBi

Best regards from Per, SM6ETH (Drawings not in scale)


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