as told in world radio......
Important Notice; WHY USE A 4:1 BALUN
Krusty Olde Kurt is now going to repeat himself. Why? Because the same
question keeps coming up over and over. And he wants everyone to get it right.
"I'm feeding my dipole with 600-ohm line. At the station end I need a balun
to convert to 50-ohm coax. I need a 12:1 balun, right?" Wrong! A 4:1
balun would be better.
Why is that? If your dipole is up, let's say, 35 feet then on 80 meters it
will probably have a resistance at resonance of about 40 ohms. The actual
resistance depends on the height above ground in wavelengths.
If the dipole is 40 Ohms then what do you see at the transmitter end of your
600 ohm line? If the line is a half-wave long (120 ft on 80 Meters) you'll
see 40 ohms. Remember, a half-wave line repeats what it sees at the other end.
But if it is a quarter-wave long you'll see 8500 Ohms! At other line
lengths you'll see impedances somewhere between these two extremes.
So you are not going to see 450 ohms at the end of your 450-ohm line. That
only happens if you have a 500-ohm antenna hooked onto it. With such a
variation in impedance at the trans­mitter end of the line there is no one
balun
transformer that will match it. Most of the time the impedance will be above
the 50 Ohms of your coax so a high impedance balun would be desirable.
Unfortunately high imped­ance baluns don't work well when not matched.
Experience has shown that 4:1 baluns work best in this service. They are
more rugged and will take bad mismatches especially if they are wound on an
iron powder core. So stop searching for that 12:1 balun. Use a 4:l BALUN and
your system will work great.
You can read Kurt N. Sterba âAERIALSâ column in World Radio Magazine.
don-kd9mf
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