| There's also an interesting tidbit familiar to all hams and radio
| engineers called 'velocity factor', the speed of charge in a
| conductor. It's a little less than the speed of light in free space
| ('c'). Depending on the material, it's generally about 80%). So
| assuming a velocity factor of.8c, an antenna's electricaly-resonant
| length would be 8/10 (or 4/5) the free-space wavelength.
Yes, but in the case of most antennas, the dielectric is air, so the
velocity factor is far closer to 1.0 c. 0.95 c is the figure usually
used for antenna work, as it's the usually used velocity factor for
(In the case of a transmission line like coax, the velocity factor is
much lower. But with ladder line, where the dielectric is mostly air,
it's close to 1 c.)
| In practical terms, whazzat mean in relation to our 72Mhz Rx
| antennas (which are ALREADY cut to approx. 1/4 wavelength)?
... but we're talking about antennas, and so relevant term is usually
`electrical length' (though of course they're all related) --
And the usual formula for the length of a 1/4 wave antenna, taught to
all beginning hams in the US, is `234 feet / ( frequency in MHz)', and
for 72 MHz, that gives us a value of 39 inches. (Outside of the US,
they probably go for meters ...)
If you calculate the wavelength of a 72 MHz signal in a vacuum, it's
just shy of 41 inches.
You are correct that the length of each leg of a dipole is a tad
shorter than that of a 1/4 of the wavelength it's meant to detect, but
the difference is much smaller than 20% -- it's more like 5%.
| Simplty this? Free-space wavelength (1/4 thereof) is 1.04 meters,
| or about 40 inches. Assuming a velocity factor of .8, that means
| the antenna's electrical length will be 32 inches (!).
Sorry, but that's wrong.
| Apparently the mfrs. have standardized on 39 inches a compromize. It
| sure seems like a randomly-selected compromize beween electrical
| length and 'capture area'. So the notion of 39 inches as 'critical'
| is pretty much urban legend.
Well, a resonant antenna is more important for a TX than a RX, because
any signal that's reflected back can damage the TX. But in the case
of a 1 watt transmitter, it's not that difficult to make sure that it
can handle the reflection of the entire signal (which would come
pretty close to happening if you took the entire antenna off) for a
long period of time. They wouldn't want their transmitters frying
just because somebody left them on with the antenna down, would they?
| The same generalities can be extrapolated to 35Mhz
| antennas, which are already at half the capture area.
I would guess that 35 MHz and other bands (75 MHz, 50 MHz, 27 MHz, 40
MHz, etc.) generally just use a matching network to make their
antennas approximately resonant, both on the TX and RX ends. Perhaps
for 75 MHz it's not a big issue, but for the other bands the
difference is large enough that they'll want to adjust for it rather
than lose that much range.
But I've seen little evidence that most R/C manufacturers carefully
tune their radio gear -- they just cut antennas and such that are
about right, and leave it at that.