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
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.
Folding rule or equivalent
Knife (to cut the coax cable)
And a small amount of glue.
|1||Wood Strip||21 x 15||600||Boom|
|4||Wood Strip||21 x 8||100||Support|
|1||Aluminum Tube||Ø12||271||Director 1|
|1||Aluminum Tube||Ø12||261||Director 2|
|16||Plastic Strip||Min 120|
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
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.
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
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.
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.
Best regards from Per, SM6ETH
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