# RF Wireless World

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## difference between waveguide and coaxial line

Both waveguide and coaxial line is used to carry electromagnetic waves of different frequencies. This page compares functions, modes, advantages and disadvantages of coaxial line and waveguide and mention difference between waveguide and coaxial line.

Figure depicts typical coaxial line. As shown it consists of center conductor and outer conductor. The fundamental or dominant mode wave in a coaxial line is TEM. So there is no concept of cutoff frequency in the coaxial line cable. But as the frequency increases, the wavelength will become comparable to the dimensions of the coaxial. As a result, higher order non TEM mode starts propagating. These waves are not desired as they cause larger attenuation. They also share power of the dominant mode.

The lowest non-TEM wave starts propagating from wavelength given by following equation:
λ = π/(a+b)

There are two types of strutures used in the waveguide circular and rectangular. As depicted, waveguide consists of single metallic walls acting as conductor. There is no center conductor in the waveguide. Hence TEM wave can not propagate via it. As a result there is no conduction current. In waveguide, energy transfer takes place using TE or TM modes. Refer TE vs TM mode to understand difference between TE and TM waves. One can also refer TEM mode vs Quasi TEM mode for basics of TEM and quasi TEM modes.

Following are the major difference between waveguide and coaxial line:

•  In case of the waveguide as there is no central or inner conductor and usually it is air filled. Hence it is easy to manufacture.

•  As waveguide is air filled there will be less loss compare to coaxial line. In waveguide, no power is lost through radiation and even dielectric loss is negligible.

•  Waveguide can handle higher power compared to coaxial cable. This is because inner portion in the waveguide is filled with air as dielectric and air has breakdown voltage of 30 KV/cm. This increases power handling capacity of the waveguide.

•  As the outer wall of the waveguide is metallic, it is bulky,heavy and expensive also. While coaxial line is smaller in size and lighter in the weight. Hence coaxial is in use for many microwave applications.

•  In the waveguide wall is not perfectly conducting, so some power loss as heat will occur in the wall of the waveguide.

•  The bandwidth of waveguide is smaller while the coaxial line is used for broadband application.

•  In the waveguide uniform cross section need to be maintained which is not the case in coaxial line. This is because, if the dimension changes then mode conversion will take place or higher order mode may also get generated in the waveguide.

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For types of waveguide viz. rectangular & circular, waveguide propagation modes, impedance matching of waveguide read following waveguide tutorial.
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Manufacturers of waveguide components such as waveguide to coaxial adapter, coupler, flanges, gasket, tees, termination, shorts, isolator, circulator, horn antenna, attenuator, feed-thrus, bends, loads, twists, straights, pressure windows, transformers, sliding short circuits, fixed short circuits, sliding mismatches etc. visit following link.
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Rectangular waveguide dimensions as per USA standard including frequency, broad dimension and narrow dimensions, visit following link.
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