RogerTango.com - Build a 4:1 voltage BALUN
|Build a 4:1 voltage BALUN|
Posted on Tuesday, 4/22/2008
You know what, a doublet is a fantastic antenna! To best feed it you need a good 4:1 BALUN. I have 3,
ranging in price from $29 to $89. All 3 are different. All 3 give different results. The home brew
BALUN I built using Romex worked pretty good last time I tested it out, so I decided to build another,
but more practical one and use stranded #14 wire this time. So, I build it... a nice 4:1 voltage BALUN,
and guess what... the doublet came alive with better efficiency and far better tunability than before!
I am running a 90ft doublet fed with 300ohm RadioShack twinlead (the foam core type), with the previous
BALUN I could not get below 2.5:1 SWR on 80m and 20m. With the new one, I am getting 1.5:1 or less
on ALL bands, 80m-10m. So lets take a look at how it is done!
Some of the hardware I got from Northwest Hardware for the construction. I already have the PVC and wire.
The PVC is 1 1/4" OD and 1/4" thick wall.
Using 2 colors of wire makes the job a snap, here is #14 stranded.
I position and drill to mount the terminals for the balanced feed line.
The balanced line wires are fed through some holes. Take a good look at how I drilled
then. A wire tie will hold the top of the coil, you will see how later.
Once the wires with lugs are in place, go ahead a wire up the terminals and tighten the bolts.
The top part is finished, we will start to wind the coil.
Pay attention: The red wire that goes right is the balanced terminal wire. It is about 8"
long and will attach to the SO-239 later. The red wire that goes left and starts the coil
process with the black wire is about 6 feet long, it is a DIFFERENT wire.
Mess up here and the whole project is wrecked.
The top of the coil is wire tied in place, 2 wraps on the coil are finished, we will have a
total of 12 coil wraps. Now you can see why the holes were drilled in the PVC the way they were.
All 12 coil wraps are done, count he red wire wraps. Trimming of the PVC will be
done now, then we will trim the wires. Not knowing how much is needed, I started
off with more PVC and wire that I needed. I would rather come up long than
come up short.
A hole is drilled at the bottom and the coil is tied off (later an additional is added to the
opposite side). The red wire at the bottom is tied to the red wire from the balanced line
terminal and will become the + lead wire in the BALUN.
Solder the red wires together, and into the center of the SO-239.
Drill out a mounting hole of the SO-239 for one of the bolts for the ground.
Getting ready to add the final wires. Eyeball for length and trim.
The remaining red wire from the top (off the coil) and the black wire from the bottom
(off the coil) are tied together and grounded onto the SO-239.
Congradulations! The BALUN is finished, just a little dress up and your ready to deploy!
Abt 4.5" long including the electrician's terminals.
A few wire ties to dress it up. Lets go try it out!
The BALUN will connect the balanced feed line (twinlead) to the unbalanced
feedline (coax). It will also provide a 4:1 matching in ohms.
The electrician's terminals are a blessing, connection of the twinlead
is very easy compared to other methods I used in the past.
Here it is in operation. I was totally pleased with it's performance!
Total invested: Abt $6 in hardware.
Here is what it is feeding, a home brew doublet.
I hope you found this information to be useful and that you are able
to put it to use. I encourage others to think about building before
buying, it is much more rewarding and the results are often better.
I expect the stranded wire version will handle a full KW, should run 600w
key down all day. I also have the prototype made from #10 Romex that is
estimated at 1.5kw. Just substitute the wire for what power you need to
run. Smaller wire could be used to build a QRP unit! Go experiment, you
should be pleased with it!