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## Re: [TowerTalk] VERTICAL BALUN

 To: "KC1DI" , ,"K4PI" Re: [TowerTalk] VERTICAL BALUN "Tom Rauch" Tom Rauch Fri, 4 Nov 2005 09:01:53 -0500
 ```> Balun is really a misnomer Not really. Balance and UNbalanced in the context of antennas describes voltages, not currents. All two terminal feed systems, coaxial or parallel wires, have equal and opposite phase currents in each conductor when properly operating. The difference between balanced and unbalanced is in the voltages from each conductor to the environment around the conductor. Antennas that have less than perfect grounds have voltage from the shield terminal to "ground". That means they are not perfectly unbalanced. They are also not perfectly balanced. Most antennas are not perfectly balaunced or unbalanced, and we have no problem with using the term "balun" in those systems. The correct term and device is actually a "current balun", because we want to be sure currents are balanced (equal and opposite) at the two antenna terminals. > diameter. However that said.. this may not be needed depending upon the > feed method used .. you would not gain a lot of advatage in a shunt fed > system or a system where the vertical element is shunted to ground through > a coil and will already will discharge static currents and other noise > producing effect to ground. The problem has nothing to do with the feed system. The problem stems from the ground system. When the ground system is imperfect and has greater than zero common mode impedance, there is a potential difference between it and anything connected to that common point that is "grounded". This is true even if it is a shunt fed element. When we connect the coax, that potential difference between the shield and the radial system causes a current to flow over the outside of the shield. This is a particularly bad problem with small radial systems, be they elevated or NOT elevated. Consider a single radial and a single vertical. The common mode impedance of the single radial just above dirt might be 50 ohms. If the vertical element impedance is 35 ohms and one ampere is flowing, the radial end at the antenna must have 50 volts potential to an earth neutral voltage reference. The vertical element base voltage, with reference to neutral earth potential, would be 35 volts. These are the voltages required to maintain ALL current flowing in the radial to equal current flowing in the vertical. If we start adding radials, two may provide a common point impedance of 25 ohms. Now we have 25 volts for every 35 volts from the antenna base to earth. Four would be 12.5 ohms, and the voltage is lower. This effect is why systems using a few elevated radials, say a dozen or less, will actually lose efficiency when a current balun or common mode feedline choke (the terms are interchangeable) is omitted. I'm really very surprised so many people with small radial systems (including elevated radials) omit the current balun or common mode isolating device. I wonder what they are thinking. The very first articles about elevated radials indicated isolation of the feedline made an improvement in performance. I measured actual system (and although I initially though otherwise) measurements confirmed it was true! After thinking about it, I realized it made perfect sense. After all, I can't install a 1/4 wl groundplane with four radials hundreds of feet in the air without some form of isolation for the feedline shield or a judicious feedline length selection. Omitting isolation causes feedline radiation and a feedline interaction with SWR. As those radials are moved closer to a lossy media like earth things get even worse, not better. The real bottom line is the advantage of using a current balun on a Marconi antenna would depend on how many radials you have, not what the particular feed system is. If you have a small ground system with only a few radials, there is the potential for significant current flowing over the outside of the shield without a choke. This will add loss to the system, and can bring noise back to the antenna and RFI to other things around the system. If you have a large ground system, say twenty or more radials, a current balun would be significantly less important. It's the ground system that sets the requirement. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] VERTICAL BALUN, K4PI Re: [TowerTalk] VERTICAL BALUN, KC1DI Re: [TowerTalk] VERTICAL BALUN, Tom Rauch <= Re: [TowerTalk] VERTICAL BALUN, Ian White G/GM3SEK Re: [TowerTalk] VERTICAL BALUN, Keith Dutson Re: [TowerTalk] VERTICAL BALUN, Tom Rauch Re: [TowerTalk] VERTICAL BALUN, Ian White G/GM3SEK Re: [TowerTalk] VERTICAL BALUN, donovanf Re: [TowerTalk] VERTICAL BALUN, Tom Rauch