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Old 04-22-2013, 11:58 AM
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DaveInDenver DaveInDenver is offline
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Join Date: Jun 2006
Location: Larimer County
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LMR400 isn't quite 1/2", it's 0.405" OD. That's standard for RG-8 and similar types. Knowing the true diameter is important if you order ferrite beads.

LMR400 has a solid center conductor, so it's very stiff. You can wind it into an ugly balun like Brian mentions but not in a tight circumference and so it does not lend itself to VHF and UHF baluns. Plus it's got a foam dielectric, which will melt if the conductors heat up. You hear of times when a solid center conductor will cut it's way through the dielectric and short out and winding the coax into a coil can make this chance higher. Running low power it's less likely, but happens a lot with 1kW stations. OTOH, the whole point of a choke is convert currents on the shields into heat, so it might be worth thinking about.

The diameter of the coil is important to the range of frequencies over which the ugly balun works. For VHF the size of the coil will have to be small, around 1.5". It wouldn't take many turns, probably 4 or 5, to present a very significant impedance to the currents on the shield. They are likely assuming you are using RG-58, which can be wound that tight.

An ugly balun winds the coax in an organized, tight coil (it's called solenoid winding, each turn lays next to the last one on the form, just like the proper way to wind a winch cable) and works better because the circulating current in the coil is traveling down the shield length through fields generated by currents in other winds. You want the fields to fight each other, creating the RF impedance. The size of the field depends on the frequency, so a 6" coil will not create a strong enough local field within the coil to work as a choke and just ends up becoming a bunch of extra coax length.

You could wind a choke from RG-58 and just insert it at the feedpoint. It would be cheap and easy, particularly if you have scrap RG-58 laying around.

The alternative is to slip a bunch of beads over the coax like the Palomar kits you link. It would be a good choke. If you already have the coax and connectors, ferrite beads in mix 31 or 43 are not expensive.

http://www.amidoncorp.com/round-cable-beads/

One thing puzzles me is why you need a balun at all for a J-pole. It should not be necessary. Curious. Can't hurt, though.
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