CB radio: windings on an antenna?
What's the point of coils on an antenna? I'd like to build a CB antenna. I would calculate it using the simple formula lambda = c/f with a gain factor of 0.95 for an RG58 cable and then tuned it with an SWR meter. However, I've also seen that some antennas have 7, 8, 9, or even 10 coils below the radiator. So, what's the point of these coils, and how do you calculate them?
Thanks 🙂
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They help the antenna to be shorter than it would have to be computerized. See it as a part of the antenna in rolled up form. Otherwise, with a Lambda half antenna, this would have to be about five and a half meters long, with a quarter lambda long antenna still proud about 2.75 meters.
Such a length wants the antenna to be in resonance with the frequency. If it does not fit the (electric) length, it is not tuned, and sending and receiving is bad.
The "winds" are a coil 😉 These serve to extend an antenna electrically, even if it is mechanically (physically) shorter. Coils extend an antenna electrically and capacitors shorten an antenna electrically.
Thus, for example, an antenna can be constructed for two frequency ranges which actually function in spite of different frequencies in both ranges due to the use of coils of possibly capacitors.
But maybe you start with the simplest antenna at all: The Dipole.
How to calculate the correct length and build things, for example, in this video:
https://www.youtube.com/watch?v=H39N2O09Jo4
If you have made the Dipol and have gained first experience with it and want to build even more antennas, I can definitely recommend only one specialist book: https://darcverlag.de/RothammelsAntennenbuch
Although the book is primarily aimed at radio amateurs, it also offers all the necessary knowledge needed for antenna construction in the CB radio.
The perfect antenna would be as large as the wavelength of the radio frequency. about 11 meters.
Aerial self-construction, I'd guess.