On Wed, Jan 26, 2005 at 01:33:29AM -0600, Tim Engel wrote: | If you're fussy, the correct antenna length depends upon the frequency. | | Wavelength = Speed of Light / frequency | In meters = 299, 792.458 / frequency in MHz | In inches = 11,802,852.6771654 / frequency in MHz
The problem here is that the speed of light is slightly lower in the antenna, which is usually described with a `velocity factor' or a longer `electrical length'. | Quarter Wave = Wavelength / 4 | R/C receiver antenna are typically quarter wave, so compute the wave length | using a formula above and then divide by 4. | | For example | 72.000 MHz frequency band generically, ignoring decimal | 1/4 Wave = 1.0409 meter (1040.9 mm) | 1/4 Wave = 40.982 inch The standard formula for a 1/4 wave wire antenna, taking into account the velocity factor is (234 / frequency) in feet. So for 72.00 mHz the answer would be 3.25 feet or 39 inches rather than 41 inches. | Note that the computed lengths are total, not just the portion | sticking out of the receiver case. Total length will include any | wire inside the case plus the length of trace on the PC board. It actually gets more complicated than that. There may be some sort of matching network (usually a loading coil or a capacitator) in the receiver, which will require an antenna that's shorter (or longer) than the value calculated to be used. For 72 mHz, the calculated length is right about one meter, and that's how long most of the antennas are, so 39 inches is probably very close to what the antenna ought to be, but to be sure, I'd just duplicate the original antenna length. For 75 mHz, the antenna would be even shorter. I imagine that the manufacturers may actually use stock 72 mHz antennas and then adjust it so it's resonant. Or they may not worry about it being resonant at all -- after all, how often do you drive an R/C car a mile away? For other frequencies (40 mHz, 35 mHz, 27 mHz, etc.) the antennas are still usually about a meter long, so there's almost certainly a loading coil of some sort in the receiver. To figure out the correct length, you'd need to measure the loading coil -- it would probably just be easier to duplicate the original antenna. And yes, R/C antennas usually *are* resonant antennas, though they're usually tuned pretty sloppily. There is certainly room to tune them better, but unless you know what you're doing, I'd suggest not adjusting the length of your R/C antennas. Repairing antennas is easy enough -- just make it the same length -- but making them longer or shorter is a great way to reduce your range unless you open up the receiver and see components the antenna is connected to. -- Doug McLaren, [EMAIL PROTECTED] A flashlight is a case for holding dead batteries. RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
