| This site <http://www.dxing.com/frequenc.htm
| says that this is the formula for wavelength:
| wavelength = 300 / frequency in MHz
| 300 /
35 = 8.57 meters
| 300/ 72 = 4.17 meters
| So the one-meter antennas come close to 1/8 wavelength for
| 35 mHz and close to 1/4 wavelength for 72 mHz.
For the record, the it's 1/4 wavelength that you're usually looking
for in a whip antenna.
And your line of reasoning is correct, however if you look at the
`serious' R/C radio equipment around the globe, on all the bands
(except for the 900 or 2.4 gHz bands), the antennas are all about one
meter in length. I can't claim to have checked much stuff outside the
US, but in the US, 27 mHz, 50 mHz, 72 mHz and 75 mHz all usually has
antennas right about one meter in length.
So if the antenna is one meter in length, and the frequency is not 72
mHz, there's two possible explanations --
1) there's a loading coil or other matching network that makes the
antenna resonant at the chosen frequency, or
2) the designers have decided that the loss of range caused by
not having a resonant antenna is acceptable.
I think it's usually #1 for most of the good R/C equipment. (I'm not
talking about cheap Radio Shack-ish R/C `toys', or things like the
Hitec Feather.) They probably stick with the standard one meter
because 1) 3 meters for the 27 mHz band would be too long, and 2) it's
easier to stock one length of antenna than several different lengths.
| Truly optimizing a receiver antenna is beyond my skills and
| equipment. You'd have to put some test equipment on the receiver
| itself to see what is really happening.
It's not so hard to make a good effort. In the absence of a matching
network, the antenna should be about 95% of 1/4 wavelength. (The 95%
is to cover the electrical length of the antenna vs. it's physical
length.) The only real difficulty is determining if there is a
matching network or not.
As for determining range, you don't need fancy equipment for that
either, but it can make it easier and more accurate.
| KC2NEB (lowest HAM rating)
The level of your ham license doesn't really say anything about your
skills. Many `techs' know a lot more than many `extras' ...
Doug McLaren, firstname.lastname@example.org
If Barbie is so popular, why do you have to buy all of her friends?
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