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Related articles at Balun Test contains model of "perfect" dipole currents. Sleeve Balun shows how a sleeve adds impedance, useful for VHF and higher baluns Receiving Common Mode Noise shows how lack of a balun can contribute to system noise (it applies to transmitting antennas as well) Balun and Core selection for transformers and baluns Transmitting baluns on testing transmitting baluns RF in the ShackMany people assume that RF in the shack or (worse yet) RF burns are tied to problems from poor station grounds. Bringing a longwire or some other single wire feeder directly into the shack will bring high levels of RF into the shack, but not two wire lines like coax or open wire. With properly operating coaxial feedlines or balanced feedlines, the operating position should have minimal RF even absent a shack ground! There is one exception to the above. Direct radiation from the antenna into station wiring could cause high levels of RF to appear on equipment or wiring, but even in that case the station ground is not the problem or cure! Contrary to popular belief that only dipoles need baluns, verticals and longwires can require baluns (more correctly called common-mode chokes or isolators in this application) also. This article shows why baluns (or more properly common-mode chokes) might be required, and what the choke can do. What causes common-mode current?Current flows because there is a voltage difference between two parts of a system along with a return path for the current flow. The path can be "special currents" without actual electrons flowing called "displacement currents", or it can be actual charges moving through conductors. Displacement currents flow through the dielectric of a capacitor, between a vertical or single wire fed antenna and the "ground" for that antenna, or currents that flow between a mobile antenna and a car body. Displacement currents commonly complete the entire current path in antennas. They are the sole reason current in physically large coils can vary from end-to-end, and are especially problematic in mobile antenna installations. They are the reason an open-ended antenna like a dipole, longwire, or vertical is able to have current flow out to the end of the antenna, even though the end just hangs out there in the air with nothing around it! When we force charges up into a Marconi vertical or longwire antenna (making current flow), we have to move an equal number of charges out of some ground system or counterpoise into the feedpoint. The ground can be a single conductor or many dozens of wires, and it can (and often does) involve equipment in the house and/or the coaxial feedline shield. The bottom line is we always must have the same current coming back to the feedpoint as the current that moves up into the antenna! There is no way around that rule, and this requirement for equal currents flowing into the ground and up into the antenna creates two problems:
Another way to view this is feedlines or feedpoints of our Marconi (end-fed) antennas must have something to push against to force current into the antenna. It is very much like physically pushing a car. If we have very poor footing, our feet will move and slide as we push. We not only waste energy that could be used to move the car, we have movement or motion where we don't want movement. The same is true for a ground system, as feedline power "forces" current up into the antenna the other terminal of the feedline has to be held steady. We waste energy that COULD be in the antenna, and we have RF movement where we don't want it....on the outside of the coax shield and on anything connected to that shield. The FeedlineIf you are not familiar with how coaxial cables work, you might want to look at a simple explanation on this site or one of the ARRL Handbooks. In order for a conductor like the outside of the shield to not have current flow at Radio Frequencies, it must have the same electrical potential and phase all along the length. If it has a high series impedance (common mode impedance) or if the potential difference along the conductor is low, very little current will flow. As seen in coaxial cable operational descriptions, any coaxial feedline can have unwanted common mode currents. Does a vertical or longwire present high common mode voltages to the feeder that can cause common mode currents? You bet it does! The only vertical (or longwire) that would not cause such problems is one with a very good or nearly perfect ground system, and that means something that looks like a large infinite groundplane. Even then, the cable must exit below that groundplane to be "shield current free". Aren't Four Elevated Radials Perfect?Here is a model of a groundplane with four radials:
EZNEC ver. 3.0 We can see significant current flows over wire 2, which would be the coax shield, a mast, or both. There is a trick with Eznec. By inserting an additional source in the mast or feedline and setting current to zero, we can observe the radial common point to earth voltage required across a balun to force current to zero. In this case the voltage across the balun would be: Source 2 Voltage
= 145.5 V. at 67.97
deg. Amazing isn't it? At 1500 watts the ground common point for the radial system actually wants to have 145.5 volts to earth to prevent current flow along the outside of the shield!! If we elevate the common point to 145.5 volts at 68 degrees phase angle, we now have the following currents at 1500 watts: Wire No. 1: 6.4 A How many times have we been told four resonant carefully tuned radials make a perfect ground? Too many! Obviously it isn't true. Even with four radials the antenna is NOT perfectly unbalanced, and not able to be connected to coax without causing appreciable current to flow over the shield. Other SystemsWith fewer radials the situation becomes much worse! (As a matter of fact, this is a good reason to use as many radials as we can even if the radials are resonant.) Claims that four elevated radials form a "perfect ground better than 120 radials" are obvious nonsense! If it was a perfect ground, there would be no potential difference to earth and no common mode current flowing to "real ground" ! If the antenna has a high base impedance, it will have less current at the feed connection. (Sorry, but we cannot do this by using a folded unipole!) Longwire and Windom antennas really aren't much different than verticals. They are a form of Marconi antennas, and require a counterpoise or ground of some sort. As with verticals, common mode current flowing into the antenna must be balanced by current flowing into a ground system. Instead of bringing the longwire directly to an antenna tuner, a better solution is using an RF ground system independent of the station safety ground, and keeping that ground isolated from the station safety ground. That can be accomplished by adding a good heavy-duty 1:1 choke or current balun a few feet from the tuner, and connecting the RF ground to one output terminal and the antenna to the other terminal. With a two-wire feed Windom (really an OCF dipole), the two wire should connect to the choke balun. The balun MUST be a current balun, rather than a voltage balun. The balun should be at or near the base of the antenna. In tough situations a second choke some distance away will help. SummaryThe cure for common mode problems caused by less-than-perfect grounds is inserting a 1:1 choke balun in the system at the antenna feedpoint. The coax should also be kept away from the radials as it exits the area of the radials and the antenna. An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly). There is no exception to this! For comparisons of baluns see balun test. |
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