Part 6
Welcome to Part 6 of our discussion.
Last time, we started to look at Baluns. Recall that their purpose is to help to match a balanced antenna (like a dipole) to an unbalanced feed line (like coax). They do this by choking off RF current trying to flow down the outside of the coax shield, forcing equal and opposite currents to flow on the center conductor and the inside of the shield. Baluns can be easy to make...
A very simple, but effective, balun can be made by coiling coax over a non-conductive core. PVC pipe is a popular choice. Something like this is easy to do, and just about free if you have extra coax:
To cover all of the HF bands, use about 25 feet of coax. It should be close-wound, with no overlapping turns. The number of turns is not critical, but make sure that the diameter of the form is large enough to accommodate your cable's minimum bending radius. If you don't, impedance can vary, and worse, the center conductor can migrate through the dielectric and short against the shield.
Another approach is to slip ferrite beads over the coax, or use split core beads that snap onto the coax.
In either case, you need to protect the beads after you assemble the balun by covering them with tape, heat-shrink tubing, or similar. About 18 inches of beads should cover 80-10 Meters.
Either of the above methods will provide a moderately effective reduction in unwanted radiation from the shield of the coax. However, in many cases it's not enough. Neither design gives sufficient "choking impedance" to provide adequate performance in many cases. They could if you wanted to greatly increase the length of the coax or the number of beads. That, of course, results in greatly increased weight and wind-load; it also gets expensive.
A much better solution is to combine the two methods by winding the feedline over a ferrite core, like this:
The balun above is mounted in a waterproof NEMA box -- not shown is the gasketed cover. It's conservatively rated at 5KW and provides much better performance than either of the two simpler methods. But a word of caution is in order here. It is very important to get a good quality model. Cheap ones are typically housed in PVC and look something like this:
There are several disadvantages to this type. First, they are sealed, so in the event of mechanical failure you can't fix it. All you have left is a paperweight. Second, because they use thin wire and a ferrite rod instead of a more expensive toroid, they are very much prone to saturation. When that happens, they overheat. That's your precious RF doing the heating, instead of radiating as a signal. The heating also affects the impedance -- increasing SWR and adding to losses. Do yourself a favor and stay away from these. Yes, I know, it's (supposedly) rated at 1.5 KW or more. Under ideal conditions? Maybe. Real world? Not even close. Spend a few extra bucks and get a good one.
That's it for this month. Next time we'll talk about some of the many different types of coax cable that are available -- and why more expensive coax is not always the best choice. Until then,
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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