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For us inner city people who would like to operate the 160 meter cw contest,
a 160 meter dipole is out of the question. Even an 80 meter wire is not possible. Modeling a 14 gauge copper 68 foot dipole on 2 Mhz at 35 feet over real ground shows a feedpoint impedance of 7-j2700 ohms. This translates to a 3.11 dB total loss in a 50 foot long low-loss open wire feedline. If the 2700 ohm reactance can be eliminated, the total line loss becomes a more much acceptable .25 dB. This can be accomplished with a stub but that is good at only one frequency. If the antenna copper loss is excluded in the simulation, Z is 2.8-j2700 ohms and the feedline loss is still less than 1 dB after reactance cancellation. Use two parallel feedlines. Clip a 60 foot line onto the dipole. Connect a 30 microhenry high Q variable or roller coil at the station end and adjust it for +j2700 ohms at the antenna end. This acts as a variable stub. Voila! the reactance is cancelled out. Notwithstanding the lowered radiating efficiency of a short dipole or the match loss introduced by the antenna tuner, at least the line loss is minimized to an acceptable level. |
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