Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Which size Balun?

P

Peter Walker

W9WDX Amateur Radio Club Member
Feb 23, 2011
119
0
26
58
What determines which Balun you need? I have seen 1:1, 4:1, and 12:1 Balun's mentioned in a number of articles. Is there advantage to one over the other? Is there a formula to figure it out? Could someone please help?
 

Very basically, you need to know the input impedance of the antenna. Divide that input impedance by the feed line impedance and you have the ratio for the balun required. Since antenna input impedances can be almost anything, the most common way of determining which 'size' balun to use is to select the closest one to the impedance ratio between antenna/feed line. If it's close it should make things usable. It'll probably never be exactly 'right'.
- 'Doc
 
W5LZ said:
Very basically, you need to know the input impedance of the antenna. Divide that input impedance by the feed line impedance and you have the ratio for the balun required. Since antenna input impedances can be almost anything, the most common way of determining which 'size' balun to use is to select the closest one to the impedance ratio between antenna/feed line. If it's close it should make things usable. It'll probably never be exactly 'right'.
- 'Doc
Click to expand...


Huh? If you are using a typical T network tuner into balanced line the feedline impedance is not being matched. You are matching the antenna system inpedance. The actual feedline surge impedance only equals the antenna system impedance when the SWR is 1:1. Since most balanced wire fed dipoles are built to take advantage of low feedline loss at medium to high SWR levels, the feedline impedance will not be matched in almost all cases.
 
Kamikaze said:
Huh? If you are using a typical T network tuner into balanced line the feedline impedance is not being matched. You are matching the antenna system inpedance. The actual feedline surge impedance only equals the antenna system impedance when the SWR is 1:1. Since most balanced wire fed dipoles are built to take advantage of low feedline loss at medium to high SWR levels, the feedline impedance will not be matched in almost all cases.
Click to expand...

The original question was just about the balun and didn't mention a tuner at all. And of course the "tuner" doesn't do anything to the feedline impedance; all it does is provide an impedance very close to 50 +/-j0 for the transmitter. If the "tuner" is sitting in the shack next to the transmitter, everything downstream from the "tuner" remains just as it was before.

Since a BalUn transforms a BALanced system into an UNbalanced one (as in going from parallel feeders to coax), the BalUn's ratio needs to be carefully selected so as to provide the "tuner" with a ratio or span of ratios into which the "tuner" can work without difficulty.
 
Beetle said:
The "tuner" doesn't do anything to the feedline impedance; all it does is provide an impedance very close to 50 +/-j0 for the transmitter. If the "tuner" is sitting in the shack next to the transmitter, everything downstream from the "tuner" remains just as it was before.
Click to expand...

This is excellent review for those who might need it.
 
I don't see anything at all about balanced feed lines in the original post. Or about tuners for that matter. So, if a (balanced to unbalanced) 'balun' is required then I would assume that coax is the feed line being used. Wouldn't you??
- 'Doc
 
W5LZ said:
I don't see anything at all about balanced feed lines in the original post. Or about tuners for that matter. So, if a (balanced to unbalanced) 'balun' is required then I would assume that coax is the feed line being used. Wouldn't you??
- 'Doc
Click to expand...

Pretty safe to assume unbalanced on one end and balanced on the other.

Usually someone asking for optimum ratios has been cruising the WWW and seeing the QRM on the subject.
 
Kamikaze said:
Pretty safe to assume unbalanced on one end and balanced on the other.

Usually someone asking for optimum ratios has been cruising the WWW and seeing the QRM on the subject.
Click to expand...
I can't make that assumption because of so many people believing that winding a few turns of coax close to the feedpoint constitutes a "balun", when all it is, is a choke for common-mode current. It's still coaxial cable, and therefore UNbalanced, at the end that connects to the antenna.
 
When one looks at a 1:1 balun like a black box and lists each and every thing it does, one can substitute a coaxial choke with the proper amount of turns for the frequency of operation into that black box and realize it is indeed a balun. If the coaxial cable continues for any length past the coil all bets are off.

Check out Walt Maxwell's description of his balun and how it was designed. Notice his is coax cable with ferrite beads. Still comes out coax.

Mods, nuke this if links aren't allowed.
The Labor Pains and Birth of the W2DU Ferrite-Bead Current Balun
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back