What is the purpose for coiling wire antennas?

Question

I wonder why do some antennas are coiled and some are not....Is there any explanation to be benefits of having a coiled antenna wire rather than a striaight wire

Answer 1

I agree with Numbat.  The main purpose of coiling , or inductively loading an antenna is to shorten its physical length with respect to the wavelength of the frequency being transmitted or received.

Antennas of actual wavelength, or integral fractions of a wavelength will work best.

Take an 80M Amateur radio antenna.  That is approximately 3.5 to 4.0 MHz in frequency.  A half wavelenth dipole antenna is approximately 135 feet long, or a 1/4 wave vertical antenna would be over 60 ft tall.  These antenna sizes are difficult to find space for, and basically impossible to use in a mobile or vehicle mounted scenario.  Adding inductors, in essence, coils, increases the antennas inductance and essentially matches or "fools" the system into being tuned at the required frequency, even though the length of the antenna might only be 4 ft tall.

Efficiency of coupling the signal through the air is reduced relative to using a full sized antenna, but the the convenience of useage is greatly enhanced.  This is a good example of an engineering design tradeoff, a compromise to make a solution practical.

Answer 2

Coiling increases the inductance but also reduces the length of the antenna. This has side effects which are undesirable (which is why they don't coil every antenna. However, on the lower frequency (longer wavelength) sometimes a shorter antenna is the only one you can use and so the disadvantages are tolerated.

In general, the best antenna is the full length one.