rafaeld@teklabs.UUCP (Rafael De Arce) (10/07/85)
Their is a simple reason for the 0.95 factor involved in calculating the lenght of a dipole: the speed of light moves slower in air than in vacuum. The 0.95 factor is the speed shift. Second, that formula is an approximation. (a very good approx.) Most of us cut the lengths on the long side and tune (cut) it as close to 1:1 on the SWR. .
larson@sri-unix.ARPA (Alan Larson) (10/10/85)
Guess again. Perhaps your next guess could account for why a 3/2 wavelength antenna is (462*2.95)/freq. The .95 factor is for what is sometimes called 'end effect'. There are capacitive effects seen at the ends of the dipole that make it seem electrically longer than it is. The .95 is a correction for that, so that it will come out to the right frequency. Alan, wa6azp
karn@petrus.UUCP (Phil R. Karn) (10/11/85)
> The .95 factor is for what is sometimes called 'end effect'. There > are capacitive effects seen at the ends of the dipole that make it > seem electrically longer than it is. The .95 is a correction for > that, so that it will come out to the right frequency. The speed of light in air is neglegibly smaller than the speed of light in vacuum. Besides capacitance seen at the ends, dipoles are made from wire with finite diameter and this also has the effect of lengthening the dipole electrically. This effect is usually small at HF, but can be pretty significant at VHF and above. It also affects the feedpoint impedance. See the 1985 ARRL Handbook, page 17-3. Phil
CRC.DlosLV@Xerox.ARPA (10/14/85)
And the point 95 factor is only good at one element diameter. Other ratios of diameter to wavelength change the end-effect factor. ARRL antenna book is a good source of more info.. Chuck... W5USJ