At any point in time, current flowing in one direction on one side of a ladder line is acted upon by an equal and opposite current flowing on the other line. Thus the plight of the Amateur Radio Operator.Ĭommon mode current is current that flows unimpeded by an equal magnitude current flowing in the opposite direction. What may be 1/4 wavelength for one frequency may very well be 1/2 wavelength for another. You can immediately see the problem with this if we're attempting to operate on multiple bands using the same antenna and feed line. This is one reason why ladder or window line should be measured out in odd multiples of 1/4 wave lengths for a specific band.
Common mode currents can run along ladder line just as easily.
Coax is not the only transmission line subject to common mode current issues. Great! But being that Radio Amateurs want to use more than one band on a single antenna and a single feed line, how do we abide by the 1/4 wave rule? Well.read on. Thus, energy from the center conductor and inside of the braid does not view the path back to the transceiver on the OUTSIDE of the coax as better than that of the impedance of the antenna itself. If our feed line is a 1/4 wave long, the impedance at the antenna end of the feed line would be significantly different and significantly higher than that of the transmitter end. If the feed line is exactly 1/2 wave length long, the impedance of the line, as viewed by the transceiver, is exactly the same at that of the far end connected to the antenna. How do we resolve this problem? Here's where the impedance transformation of the feed line comes into play. In a nutshell, common mode currents are complicated and there are many engineering papers that have been written on the subject. This often happens when feed line length is at or near 1/2 wave length for the current operating frequency. Depending on feed line length and signal frequency, the path back down toward the rig (and ground) may have a lower impedance than the radiation resistance of the antenna itself. RF power will flow down the outside of the coax braid IF the impedance at the feed line / antenna junction is MORE than the impedance of the outside braid looking back toward the radio. An imbalance in currents on each half of an antenna can be caused by many different factors, but even perfectly balanced antennas (dipoles) can have common mode currents flowing on the feed line due to near field coupling or a favorable path to ground through the feed line. So, how does current end up on the outside of the coax braid? The simple answer is that common mode currents are caused by imbalances in the antenna or through antenna to feed line coupling.
This is due to the fact that at HF and above, AC signals flow on the surface of the conductor and not through the center of the conductor. We all know that RF power flows out of our transmitters as an AC signal along the center conductor and the INSIDE of the coax braid. Did you know that coaxial cable supports 3 separate current paths? It does, and the image above illustrates those paths.
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