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## [TowerTalk] W0IYH Feed line Choke Performance

 To: [TowerTalk] W0IYH Feed line Choke Performance ccc@space.mit.edu (Chuck Counselman) Wed Aug 20 08:40:10 2003
 ```At 9:55 PM -0700 8/19/03, Jim Smith wrote: >...if 500 ohms really is enough impedance to reduce the common mode >current to a negligible amount then none of this matters.... It depends on the "impedance" (i.e., the ratio of the voltage to the current) of the common-mode wave at the point on the common-mode transmission line where you insert the choke. (I put "impedance" in quotes 'cuz I'm glossing over the important facts that common-mode waves travel both ways on the line; the complex amplitude of the voltage at a point is the sum of the complex amplitudes of the voltages of the two waves; and the complex amplitude of the current at a point is the difference of the complex amplitudes of the currents of the two waves.) Typically the standing-wave ratio (SWR) of the common mode is high; in other words, there are significant maxima (peaks) and minima (nodes) of common-mode current and voltage along the line. One reason for the typically high SWR is that the characteristic impedance Zo of the common-mode transmission line is far from uniform. Consider, for example, the common-mode transmission line formed by the outer surface of the shield/braid of a coaxial cable, and "ground." The diameter of the shield/braid may be uniform from one end of the cable to the other, but the distance between this conductor and the effective "ground" or return conductor may vary radically. E.g., the cable may be be taped to one leg of a tower from the top of the tower to the bottom; then the cable may run through a 6-inch conduit from the bottom of the tower to the house; then the cable may have a lot of open space surrounding it on its way through the house to the shack; in the shack, the shield/braid connects to the rig, and the effective "diameter" of this conductor balloons by orders of magnitude; then the conductor becomes the AC power distribution network within the house; and so on. Perhaps even more important are the common-mode wave reflections occurring where the coaxial cable shield/braid is connected to lightning "grounds." If you inserted a common-mode choke at a voltage node where the "impedance" of the wave was, say, 25 ohms, then a 500-ohm choke could be effective; but you inserted a common-mode choke at a current node where the "impedance" of the wave was, say, 2500 ohms, then a 500-ohm choke would be utterly ineffective. If the SWR of the common mode were near unity, then the effectiveness of a choke would depend on the ratio of its impedance to the characteristic impedance Zo of the common-mode transmission line. A rough order-of-magnitude estimate Zo can be obtained from the well-known formula for Zo of a coaxial transmission line, Zo = [(138 ohms)/sqrt(eps/eps0)] log (b/a), using eps/eps0 = 1 for air, a = 0.005 m for the braid of RG-8-size cable, and b = 1 m for coax running through a house: Zo equals about 300 ohms. A 500-ohm choke would not be effective here. Bottom lines: 1. I wouldn't waste my time or money with a choke whose impedance was less than 3000 ohms. 2. If 3000-ohm chokes were available, I would use more than one of them, separated by a quarter-wavelength along the line. 3. In general, the best places to insert chokes are where you know the common-mode current is near-maximum: wherever the transmission line connects to a much fatter conductor, such as your lightning "ground" or your rig. 73 -Chuck, W1HIS ```
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