Why don't we use lenses for RF?

Question

Radio waves are a class of electromagnetic waves. Light also is a class of electromagnetic waves.

By shaping a material in which the speed of light changes, we can bend the propagation direction of light, we call this a lens, and we call the speed change rate refractive index \$n\$.

Also for RF we can define a speed change rate as \$\frac{1}{\sqrt{LC}}\$ or \$\frac{1}{\sqrt{\epsilon_r\mu_r}}\$.

We also use the exact same destructive interference in the same way for radomes and Anti reflective coatings.

So here comes the question: why don't we use lenses and ̶m̶i̶r̶r̶o̶r̶s̶ for RF, for example for focusing RF beams instead of using complicated directive antennas?

EDIT: yes, we actually use mirrors

Answer 1

In optical engineering, the choice between lenses and mirrors often comes down to aperture diameter: less than a few inches and lenses can be made cheaply and with high accuracy. Larger and costs increase exponentially, so even 6" diameter systems usually work better reflective.

At RF frequencies, a 6 inch lens is on the order of a wavelength, and so not useful for focusing. It isn't until you get towards the edge of the microwave spectrum that the wavelength gets short enough for lenses to start to become practical.

Of course if you don't care about cost, and you don't mind it being extremely heavy, you could build a lens to use with a WiFi antenna. It just doesn't make much practical sense.

Answer 2

i don't know of any rf lens usage case

That can be changed. They are quite common for ku-band and up. Think satellite communications, radar, point-to-point links where you want high but can't use a dish, e.g. for weather reasons.

Look at this nice lens antenna:

Or these nice insets to convert an open waveguide to an actual antenna

Answer 3

There is the Luneburg Lens, which can be used for various applications, from optical to RF. Luneburg Lenses are generally spherical, made of concentric shells of material with a stepped refractive index for practical purposes, but ideally the refractive index should be continously varying.

(image from www.rfwireless-world.com)

Applications are radar reflectors, microwave antennas and laser collimation.

Answer 4

You can make lenses out of metamaterials - see this Phys.org story

A three-dimensional self-supporting low loss microwave lens with a negative refractive index Journal of Applied Physics 112, 073114 (2012); https://doi.org/10.1063/1.4757577 Isaac M. Ehrenberg, Sanjay E. Sarma, and Bae-Ian Wu

Answer 5

Another well-known example of RF lenses are Fresnel zone antennas. They are based on the same principle as the optical Fresnel lenses: The focusing effect is achieved via the phase shifting property of its surface rather than volume, which allows for compact or arbitrarily sized antennas (e.g. built into a curved surface).

Answer 6

RF mirrors are quite common.

RF lenses are possible but much less common.

The short explanation is cost. Cost is almost always part of engineering. If there are two ways to do something, and one costs less for adequate performance, then that is the better engineering solution to the problem.

An array of wires to form a directive or reflective antenna element is cheaper to build and maintain than a solid lens structure for most applications.

Answer 7

Lenses can be used in the millimeter wave region. See, for example, https://doi.org/10.1007/BF01014036

Which describes use of Rexolite lenses for 90GHz military applications.

Answer 8

Heinrich Hertz used paraffin wedges, in the 1890s, as part of his 60MHz RF communication link. The wedges were crucial to proving WAVES were being emitted and collected and detected.