LaserGecko
Senior Heliman
Location: Las Vegas, NV
| Well, without diving into an in-depth antenna theory lesson the likes of which I don't want to fathom, basically the length of any antenna needs to match the wave length of the radio frequencies being emitted or received. The more closely the "electrical" length matches the wavelength, the more efficient the transmission of your data and the safer it is for your transmitter. It doesn't have to match exactly, but there are certain ratios that will work (like 1/4, 1/2, or 5/8 wave) almost as well.
We're pretty lucky in RC to only have to deal with a narrow range of frequencies, so our antennas can be "tuned" to the center of the band more precisely. The external antenna on your FM radio isn't so lucky. It has to cover a frequency range of 88-108MHz. That's a whopping 20MHz of difference, so the antenna has to be very "broad" banded. Therefore, it's not as efficient.
(Here's a cool trick. If you have a distant FM station that you want to hear better, build an antenna specifically for that frequency. Divide 468 by the frequency in MHz to get the length of your perfect halfwave vertical antenna. Then, take a piece of coax antenna wire and strip back the insulation and shielding to exactly that length. For 97.1MHz, that length is 4.82 feet. Connect the conductor and shield to the antenna jacks on the back of your stereo and listen to the station BOOM into your radio.)
Higher frequencies have shorter wavelengths, therefore those antennas are shorter. In the ham radio world, we commonly refer to the bands by their wavelength rather than their frequencies. For example, when we talk about "two meters", we know that it's the 144MHz band.
If the antenna doesn't match correctly, part of the radio frequency energy (RF) will be reflected back into the transmitter or away from the receiver. This can be a very bad thing and is called the Standing Wave Ratio or SWR. (You've probably heard that term.)
With higher frequencies, the quality and type of connectors, transmission lines, and antennas becomes even more important since the incredibly low power of the transmitters means even a slight loss can have drastic results. Remember, the only difference between WiFi and your microwave is the power behind the RF energy!
All of the above is subject to correction by those who know more than I do, especially on four hours sleep and a day of playing radio for offroad racers.
Jason
KC0ERG |
The down side of moving forward, I guess.
