| May, 2005 |  |
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| May, 2005 | |
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A 50 ohm COAXIAL FED antenna is a good radiator if fed directly with 50 ohm coax (NO BALUN). So, why use a balun? The radiation pattern of the antenna will not be like the text book because the system is now unbalance by the unbalanced coaxial line. For example, a dipole radiation pattern looks like a figure 8. It only looks that way when the system is balanced. In an unbalanced system, the pattern will be skewed. However, I have had good results using dipoles without a balun. Another reason for using a balun is to help eliminate RF feedback into the shack. RF feed back can be caused by unbalance in the antenna system. RF feed back can contribute to TVI. Therefore, a balun MIGHT help to reduce or eliminate TVI problems.
Note that ALL OF THE ABOVE is for an antenna fed with COAX. A matched system using coax is required for efficiency because coax can cause a lot of power loss if the mismatch is high.
Generally speaking, when LADDER LINE is used, a matched system can be ignored. Why? Ladder line is low loss even with very high mismatches. The antenna wire can be almost any length as long as a tuner is used to get the antenna system (antenna and feed line) to look like 50 ohms to the transmitter. However, a longer wire is a better radiator than a short wire. The tuner in this case transforms (tunes) the antenna and feed line combination to look like 50 ohms for the transmitter. The ladder line can be connected directly to one side of a tuner as long as the tuner is designed to accept balanced feeders. The transmitter connects to the tuner's coaxial unbalanced input. The tuner, in this case, is doing a similar job as the balun we used in the coax fed dipole above. It transforms the unbalanced transmitter output to the balanced ladder line and antenna combination. At the same time the tuner is tuning the antenna and ladder line combo to look like 50 ohms at the transmitter. The transmitter wants to see 50 ohms or it is unhappy.
So why did I use a balun outside for my Windom? I find it easier to run a short length of coax into my house to my tuner than ladder line. Loss in the coax is not severe because it is only about 6 to 8 feet in length. For my system, the tuner does not act like a balun because it uses the unbalanced coaxial transmitter input and the unbalanced coaxial output.The balun, outside my window, transforms my balanced ladder line to the unbalanced coax.
Why did I use a junction box for my balun? My balun is homemade and needs to be protected from the elements. I placed my balun inside the junction box and connected the ladder line and coax to connectors that I attached to the box. If I had a commercial balun, I might have gotten away without using a junction box.
Why do hams use a 4 to 1 balun. Sometimes the impedance of an antenna system is so high that it is difficult or impossible to get the tuner to tune it out. High SWR can also mean very high voltages on the transmission line. One way to get the impedance to a lower level is to use an impedance transformer. Any balun greater than 1 to 1 is an impedance transformer. Therefore an antenna system with a very high SWR can sometimes be transformed down to a reasonably low SWR that the tuner can handle.
Sometimes, a 4 to 1 balun offers a better match for certain antennas on several amateur bands. I'm not certain without looking it up, but I think the G5RV uses a 4 to 1 balun to obtain a good match on several bands while transforming the unbalanced coax to the balanced antenna.
Ron K3PF (previous page)
| May, 2005 | |
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