Posted: 8/19/2008 4:35:35 AM EDT
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did some seaching, both on arfcom and the internet...haven't had much luck. 1:1? 4:1? I'm looking at making a makeshift dipole. Mostly for receiving on HF, but some transmitting too. I'm a no-code tech, so my HF is extremely limited. It's been a while since i read up on Baluns, and am having a hard time finding the info again. What type balun for what purpose? Odds are i'll be doing a dipole of some flavor with 300 ohm twin lead. Might experiement with a random length wire at some point. I don't have a lot of space at my QTH, so i'll have to do some playing. |
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If your using 300ohm twin and a tuner, you won't really NEED a balun. I've run this setup for years with no issues. If you want to run 50ohm coax, I'd suggest getting a 12" piece of 1.5" PVC and a couple caps. Using RG58, wind about 8-10 feet on the PVC and when your done winding one PL-end will need to be at the end of the tube and this will attachto the antenna feed point. There are several setups like this on the web for reference. We use this on field day here. link 73 |
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Baluns typically do 2 things: 1) Convert between BALanced and UNbalanced lines. 2) Transform impedance (for matching purposes) So a 1:1 balun JUST does #1. Good to use for an unbalanced line (coax) and balanced antenna (1/2 wave dipole) that are already "close enough" in impedance (50 ohm line and feedpoint and 75 ohm antenna). A 4:1 balun does #1 & #2 and is generally used when you have an antenna system impedance drastically different than your radio's feedpoint (i.e. by a factor of "4 to 1" approximately). Most common application is feeding a dipole or similar antenna using a 300 or 450 ohm feedline (TV Twinlead or Ladder Line). You can't "plug in" the high-impedance balanced line into the SO-239 jack on your radio because a) it won't physically connect and b) it's outside of your radio's rated operating impedance. So throw a 4:1 balun in-line. You tie each side of the balanced line to the wire leads or terminals at one end and screw a normal coax jumper on the other end. The connection points solve your connectivity problem and the internals of the balun will convert your mismatched impedance into a range the radio can work with (still needs to be fine tuned by either trimming your antenna or using an antenna tuner). For anyone wondering why you'd want to feed a dipole with balanced feedline, it's because you can tune it for multiple bands with very low feedline power loss. See QST July 2008 p.69 "Your Second HF Antenna" for a great article on a a multiband dipole. |
see also this excellent article on OCF ("off center fed") dipoles: www.radioelectronicschool.net/files/downloads/ocfdipole.pdf and also www.reflections.co.nz/index.php/archives/48 and also www.reflections.co.nz/index.php/archives/43 ar-jedi |
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Quoted:
did some seaching, both on arfcom and the internet...haven't had much luck. 1:1? 4:1? I'm looking at making a makeshift dipole. Mostly for receiving on HF, but some transmitting too. I'm a no-code tech, so my HF is extremely limited. It's been a while since i read up on Baluns, and am having a hard time finding the info again. What type balun for what purpose? Odds are i'll be doing a dipole of some flavor with 300 ohm twin lead. Might experiement with a random length wire at some point. I don't have a lot of space at my QTH, so i'll have to do some playing. A dipole in freespace is about 70 ohms, but closer to the gound is about 50 ohms can can be fed directly with 50 ohm coax. But you can get currents running up and down the shield. For a dipole connected with 50 ohm coax, a 1:1 balun. Look up via google "Ugly Balun", which is a 1:1 choke balun. This can be formed by wrapping about 20' of coax around a form 3"-5" in diameter. I used LMR-200, 17 turns close wrapped around a short piece of 4" pvc, holding the wraps in place with ty-wraps. 
If the dipole is OCF (offset center fed) then it is closer to 300 ohms, and a 4:1 or better 6:1 balun should be used. Now here's a question... low impedance balanced AND shielded cable is used for microphones. Very long runs can be made with little or no hum picked up. This cable has two parallel wires for the balanced feed, and an outer shield which is grounded. Is there such a thing for antenna feed? If not, why not? |
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4:1 balun takes a higher impedence balanced antenna and converts to a lower impedence (50ohm) coaxial cable at a 4 to 1 ratio... 200 ohm antenna to 50 ohm cable.
Probably the most common antennas for these would be folded dipoles and delta loops or other loop antennas. |
| My first and only balun was THIS one and recommend anyone to give it a try. (It's a 4:1) |