One thing I noticed about the calculations is that when finding a 1/2 wave, 492 is the 'magic' number being used. Nothing wrong with that as long as you remember that the diameter of that conductor isn't being taken into consideration. For average sized wire dipoles, the 'magic' number is much closer to 468 than to 492. That takes the diameter of average sized wire into consideration. That diameter thingy is commonly referred to as 'k', as in (492 x k)/F(in Mhz) = feet formula. Two ways of finding that 'k'. One is by doing the calculations referred to in the Amateur Hand Book Antenna section. The other way, as cited on that web site is by cutting an antenna till it's resonant at a particular frequency. Then, divide 492 by that frequency. Divide the experimental length frequency by the '492' length and you have 'k' for your resonant antenna's positioning, height, environment, etc, etc. The resulting 'magic' number will be less than 492 or 468, probably. That 468 number is really a good one cuz' it allows some fudge factor in the resulting lengths, a little extra, which is better than a little too short, sort of. A very rough 'ROT' (rule of thumb) is that for the lengths specified on that web site, if you multiply them b y 0.95, it's will result in a length closer to what you normally see for the length of a particular antenna. Hey, it ain't 'perfect', but it's close enough, sort of.
- 'Doc
That '492' number deals with the perfect, 'free-space' type antenna length. That '468' number deals with 'down-here-on-earth' type antenna lengths for 'average' sized wire antennas. Has to do with velocity factor, speeds of an electron in space and in air and in metals (it's a secret thingy, don't tell anybody!).
[The only B.S. in all of that is the last 7 words in parenthesizes.]