Courtesy of Pasternack : Waveguide Frequencies and Geometries

Loss, whether due to radiation leakage or conduction resonance, is a common problem in RF microwave transmission lines, especially when high-powered frequency transmissions are involved. The solution? Waveguides.

Waveguide Basics

A waveguide is an electromagnetic feed line used for high frequency microwave signals in high-power transmitters and receivers and is used in radar equipment, in microwave ovens, in satellite dishes or in any RF microwave system where high-power transmission is needed. Waveguides are hollow metallic tubes or light carbon fiber composites constructed with high grade metals like copper, brass, or plated metals. Silver or other plating is used on the inside walls of a waveguide which acts to decrease the resistance loss by shielding and provides efficient isolation between adjacent signals. Transmission lines like microstrip, stripline, or coaxial cable may also be considered to be waveguides and they are usually referred to as dielectric waveguides with a solid center core. However, on high-powered microwave waveguides where the line may get too hot, air in the cavity may be pressurized, recirculated, or a Freon-like gas is used to keep the waveguide cool and also can prevent arcing.

While the disadvantages of using a waveguide include a high production cost, large size and mass of the guide, and the inability of running a DC current alongside the RF signal, the advantages in using a waveguide are that they are completely shielded, high-powered transmission lines that provide good isolation and very low loss that can bend without compromising performance.

Frequencies and Geometries

For the signal to propagate, waveguides need a minimum cross section relative to the wavelength of the signal; these cross sections can be either rectangular, circular, or elliptical. The dimensions of a waveguide determine the wavelengths it can support and in which modes. The lowest frequency range a waveguide will operate is where the cross section is large enough to fit one complete wavelength of the signal. In hollow waveguides, or waveguides using a single conductor, transverse-electromagnetic (TEM) mode of transmission waves are not possible, since Maxwell’s Equations demonstrates that an electric field must have zero divergence and zero curl and be equal to zero at boundaries, resulting in a zero field.

Comparatively, for two-conductor lower frequency transmission lines, like microstrip, stripline, or coaxial cable, TEM mode is possible. In rectangular and circular waveguides, the dominant modes are designated the TE₁₀ mode and TE₁₁ modes.

According to Maxwell’s equations, there are three rules that apply to waveguides:

1 )Electromagnetic waves are reflected by conductors,

2 )Electric field lines that make contact with a conductor must be perpendicular to it,

3 )Magnetic field lines close to a conductor must be parallel to it.

These rules allow for certain modes of propagation such that the TE₁₀ (transverse electric) mode is the mode in which energy propagates in rectangular waveguide. The mode with the lowest cutoff frequency is noted as the dominant mode of the guide.

Because waveguides are the transmission lines for super high frequency (SHF) systems, they must operate with only one mode propagating through the waveguide. Waveguide propagation modes depend on the operating wavelength, polarization, shape, and size of the guide.  Waveguides standards are based on rectangular waveguides and are designed with these characteristics:

one band starts where another band ends, with a band overlapping the two bands in order to allow for applications near band edges,

> the lower edge of the band is approximately 30% higher than a waveguide cutoff frequency thus limiting dispersion and loss per unit length,

>In order to avoid evanescent-wave coupling by way of higher order modes, the upper edge of the band is approximately 5% lower than the cutoff frequency of the next higher order mode,

>the waveguide height is half the waveguide width which allows a 2:1 operation bandwidth – having the height exactly half the width maximizes the power inside the waveguide.

For convenience, our waveguide calculator provides the cutoff frequency, operating frequency range and closest waveguide size for a rectangular waveguide.

Variations on the Waveguide

>The double-ridge rectangular waveguide is a type of waveguide used in RF microwave systems. The ridges in this waveguide design serve to increase the bandwidth but, on the downside, creates higher attenuation and lower power-handling capability.

>The single ridge waveguide is similar to the rectangular waveguide but noted for its large capacitive loading centered on its broad wall.  Compared to a rectangular waveguide, the single ridge waveguide has a lower cut-off frequency with a smaller cross section. However, when compared with the double ridge waveguide, the single ridge yields increased loss and reduced power handling capabilities.

>The slotted waveguide, generally used for radar and similar applications, serves as a feed path with each slot acting as a separate radiator. This antenna structure can generate an electromagnetic wave transmission in a specified narrow and targeted direction.

Courtesy of Pasternack : An Array of Antenna Arrays

An antenna array, or phased array, is a set of two or more antennas whose signals are combined in order to improve performance over that of a single antenna. An antenna array is used to increase overall gain, provide diversity reception, cancel out interference, maneuver the array in a particular direction, gage the direction of arrival of incoming signals, and to maximize the Signal to Interference plus Noise Ratio (SINR). An array antenna is usually made up of more than one dipole but it may be composed of driven elements. As these antennas elements radiate individually and while in array, the radiation of all the elements sum up, to form the radiation beam, which has high gain, high directivity and better performance, with minimum losses. Similar to the dipole, a driven element can function as a transmitter or a receiver. When connected to the transmission line, a driven element gets power directly from the transmitter or, as a receiver, transfers the received energy directly to the receiver. Applications of array antennas include satellite communications, wireless communications, radar communications, and in the astronomical study.

Types of Arrays

Arrays can be described by their radiation patterns and the types of elements in the system. When placed close enough to the driven element to permit coupling, a parasitic element will produce the maximum transmission radiation from its associated driver. When a parasitic element reinforces power from the driver, it is referred to as a director. When a parasitic element causes maximum energy to radiate towards the driven element, the element is called a reflector. An array antenna is known as a driven or connected when all of the elements in an array are driven. Interestingly, if one or more elements in the array are parasitic, the entire system is said to be a parasitic array. Multi-element arrays are usually associated with their directivity, for example, a bidirectional array radiating in opposite directions or a unidirectional array radiating in one direction.

Driven arrays

Collinear array

Unidirectional, high-gain antennas designed with two or more half-wave dipoles placed end to end and seated on a common line or axis making them parallel or collinear. The main purpose of this array is to increase the power radiated and to provide high directional beam by avoiding power loss in other directions. Advantages of collinear array antennas include increasing directivity with a reduction in power losses.

Broadside array

Bidirectional array used to radiate electromagnetic waves in specific direction to enhance transmission. The design elements include two or more half-wave dipoles of equal size and equally spaced along a straight line or axis forming collinear points with all dipoles in the same phase from the same source. The broadside array antenna has a radiation pattern that is perpendicular to the axis with a narrow beam radiation pattern and high gain.

End-fire array

Similar to the broadside array and uses two half-wave dipoles spaced one-half wavelength apart with a bidirectional radiation pattern with narrower beam widths, lower gain, and higher directivity than the broadside array. The direction of radiation is along the plane of the array and perpendicular to the elements which radiates to the end of the array, hence the name.

Parasitic arrays

 Yagi-Uda array

The most common type of antenna for home TV reception with high gain and directivity. In this antenna, several directors are positioned to increase the directivity of the antenna. The disadvantages of Yagi-Uda antennas are that they can be prone to noise and atmospheric effects.

Log-periodic array

An array antenna whose impedance is a logarithmically periodic function of frequency. Similar to a Yagi-Uda, the advantage of this antenna is that it maintains constant characteristics over a desired frequency range of operation with the same radiation resistance, SWR, and gain and front-to-back ratio are also the same. Types of log-periodic antennas include the planar, trapezoidal, zig-zag, V-type, slot and the dipole or LPDA (log-periodic dipole array).

Turnstile array

Basic construction is two identical half-wave dipoles placed at right angles to each other and fed inphase. Several turnstiles can be stacked along a vertical axis for higher gain called a bay. The polarization of the turnstile antenna depends on their mode of operation, Normal and axial, where in normal mode, the antenna radiates horizontally polarized waves perpendicular to its axis and in axial mode, the antenna radiates circularly polarized waves along its axis.

Super-turnstile array

Also known as the Batwing antenna, the dipole elements in turnstile are replaced by four flat sheets where 1 to 8 bays can be constructed on a single mast. The advantages of this design are high-gain and better directivity than the regular turnstile but with some power losses.