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velocity factor???
larry d clark wrote:
given that the length of a half wave dipole is calculated by 468 / freq in mhz when velocity factor is 1, ie 468 / 1.9 is about 246 ft. i'm sitting here wondering why folks with small city lots don't use (468 / freq in mhz) * velocity factor, to construct a much shorter antnenna, particularly on 80m & 160m? as an example, rg59 coax typically has a velocity factor of .66, so plugging to the formula, 468 / 1.9 is about 246 ft, * .66 is about 162 ft. why couldn't 162 ft of rg59 be cut in two, attached to a 50 ohm, have the remaining ends of the rg59 shorted together, and hoisted into the air? so what am i missing? there are no free lunches:-) larry kd5foy One wavelength in free space in feet is 984/F in feet (300/F in meters); a half-wavelength is 492/F. 468/F assumes a vf of 0.95 of the antenna radiating surface. The vf of coaxial cable is dependent upon the dielectric used. 0.66 is common for solid polyethylene. Foamed PE is typically near 0.80. As with many RG cables, RG59 is available both ways. In your example, the vf of the dielectric would not matter, since the majority of the current flows on the *outside* of the shield due to skin-effect. Thus, the formula for the length would be very close to (only slightly shorter than) 468/F. Since I^2*R losses would be lower, bandwidth would be better than a smaller-diameter wire. Bryan WA7PRC |
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