Correct me if I am wrong, but all RF light waves are created via a changing electric and magnetic field. But when looking at all kinds of radios and RF circuits, this light wave is always made with a simple wire antenna. Why are coils not used instead? Wouldn't a coil of the same length create a stronger magnetic field thereby creating a more efficient 'antenna' that produces RF waves?

I know that there has to be a reason why coils are not used, but I cannot find an explanation. It makes more sense to me to have a coil creating a fluxuating electric/ magnetic field rather than a piece of wire.

Thanks in advance!


Take a look at this picture of a dipole antenna: -

enter image description here

Both electric AND magnetic fields are produced by the antenna.

Magnetic fields are produced by straight wires with current flowing in them i.e. the wire doesn't need to be wound into a coil to produce magnetism.

Now, the physics (well, a bit of it): -

Space has an impedance of approximately 377 ohms and an antenna has to produce an E field in the right proportion to the H (magnetic) field to maximize convsersion of electrical energy flowing to the antenna into EM power.

The ratio of E field to H field is 377 ohms so trying to produce a bigger H field than is necessary is a waste of time because the impedance will be wrong. See this wiki article for extra reading.

  • \$\begingroup\$ This is a good start! So are you saying that a coil would not create a fluctuating electric field like an antenna? The impedance of space and proper ratios makes a lot of sense, given that most antennas are 1/2, 1/4, 1/8... wavelengths. \$\endgroup\$ – Takide May 18 '15 at 14:22
  • 1
    \$\begingroup\$ @Takide "respect the ratio"! E and H fields need to be of the right ratio to maximize RF propagation. Who needs a coil when the simple dipole does the job and creates the fields in the right ratio. \$\endgroup\$ – Andy aka May 18 '15 at 14:25
  • \$\begingroup\$ This makes a lot more sense. Thank you for your answer! So just out of curiosity, does this mean that a coil could be made to create a directional antenna? Im guessing that it would just be less efficient than a dipole? \$\endgroup\$ – Takide May 18 '15 at 14:37
  • \$\begingroup\$ Coils used for receiving are fine as directional antennas because they convert the H field (mainly) into the microvolts needed by the circuit. Coils used as transmitters tend not to be as good (generalism alert) for the reasons stated in my answer. See this: en.wikipedia.org/wiki/Loop_antenna - it's about loop antennas and that is the nearest thing that fits to your description. The article explains in simple terms that a coil needs to be E-field resonant (i.e. big enough) to be effective as a transmitter. \$\endgroup\$ – Andy aka May 18 '15 at 14:42

While a coil will create a stronger magnetic field, that field is mainly contained within the coil so that a coil does not make a strong radiator so it doesn't make a good antenna. Note that the strength of the magnetic field is not an indicator of efficiency. An efficient antenna radiates most of the energy fed to it. A wire antenna, if it is long compared to a wavelength, is an efficient radiator of electromagnetic energy. By the way, it is incorrect to refer to RF light waves. RF stands for radio frequency and generally refers to frequencies measured in MHz and wavelengths in meters. Light waves are electromagnetic waves but have much higher frequencies (greater than teraHertz) and much shorter wavelengths (measured in millionths of meters).

  • \$\begingroup\$ Doesn't this mean that a coil would still work as a directional antenna? Thanks for the response by the way! \$\endgroup\$ – Takide May 18 '15 at 14:24

E-M waves are very different from a simple magnetic field. In an antenna both electric and magnetic fields are generated. They are inseperable and interact with each other constantly. They form a wave that is radiated, not just a simple field. Yes, coils have been used as antennas in certain cases and, with the exception of a single winding (which acts more like a regular antenna), almost always for reception, not transmitting. The efficiency of the antenna is much higher than a multiple turn coil would be. The multiple turn coils have other properties that make them useful sometimes, but almost always a regular antenna is a better alternative. For transmitting you want the energy to radiate, not stay contained in a small area. For reception you want a big "capture area". It is also easier to tune it in practical situations. The receiver needs a transfer of power to be able to received signals. How much power will a magnetic (only) field from a coil transfer at distances of several thousand miles? None? An antenna can transfer a surprising amount. That leads to good signal to noise ratios and good, comprehensible reception.

  • \$\begingroup\$ Most coils used as receiving antennas are bidirectional often discribed as having a "figure 8" pattern if used vertically, and neary omnidirectional if used horizontally. \$\endgroup\$ – Keith Martineau Jul 23 '15 at 1:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.