Commercial antennas for remote control 27/47 MHz links

Hi,

As far as i know from my elementry antenna theory knowledge, The size of antenna become quite big at low frequencies..and become impractical...But i have seen very small antennas for 27/47MHz remote control applications (used in toy cars) ...They are simply a peice of wire not more than 1 or 1.5 feet long...Why is the case...What kind of antennas are these...and how such small antennas work on such low frequencies...

thanks

These are monopoles, which need some ground plane on the PCB. Usually, the monopole length would be 1/4 wavelength. To create a resonance condition for these short wires, an inductor is placed in series with the antenna. This lossy inductor reduces the antenna efficiency.

well as u said the antenna length should be 1/4 of wavelength so for 27MHz, 1/4 of wavelength is around 2.77m which is still quite big...commercial antennas at 27MHz are not more than 1-1.5 feet.. So does the inductor placed in series with these short antennas make the operation possible at 27MHz

Yes, exactly.

how i can design my own antenna of such kind then..how many turns of inductor will i have to have to have desired frequency...are there any formulae...

Sorry, I have no easy equations ready. I would measure or simulate the antenna input impedance and then use an RF circuit simulator to calculate the matching component values.

Here's an introduction to electrically small antennas: http://www.highfrequencyelectronics....7_tutorial.pdf

In that document, a short dipole is discussed, but the idea is the same for a short monopole.