Transmission lines Part 2

This tutorial will cover waveguide and its properties as well as the proper use of 75Ω and 50Ω impedance cables.
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Figure 1. These curves, developed at Bell Labs, demonstrate the trade-offs between different impedances.

As discussed in the last tutorial, there are three important parameters to determining the impedance of a coax line, whether it’s flexible RG-59, helix or rigid coax: the ratio of the inner and outer conductor sizes, the dielectric of the material between them (gas or foam) and the resistance of the conductors. The impedance can be calculated with these three parameters, but why use 50Ω instead of 75Ω?

Included in the last tutorial was a graph that illustrated the benefits and trade-offs of various impedances. What this graph showed was that the best impedance for transferring power is 50Ω, despite some attenuation. This is why we use 50Ω transmission lines today, to be able to transfer as much power from the transmitter to the antenna as possible. On the other hand, if power is not a concern but voltage transfer, or low attenuation, is, then 75Ω is your best bet. When we use 75Ω for video cable, it’s because with only 1V (or 800mV in the case of SDI) you’re not transferring power, but you do want most of the voltage from the source to make it to the destination. (See Figure 1.)

Waveguide construction

Most waveguide is constructed in the shape of a rectangular pipe, with the width about twice its height. Waveguide comes in various sizes to accommodate different frequency ranges, and each size has a lower and upper cutoff frequency. The width of the waveguide is equal to one-half the wavelength of the lower cutoff frequency. Although the upper and lower cutoff frequencies mark the absolute passband of the waveguide, the actual usable frequency band is somewhat smaller. Waveguide can be made of almost any metal that conducts electricity, but it’s most commonly made of aluminum, brass or copper. Of course, for the larger sizes, aluminum is used to cut down on weight.

Rectangular waveguide comes in many sizes, and they are designated by the EIA with “WR,” for waveguide rectangular, followed by a number, from WR-0.65 at .165in by .083in that is used for 1.1THz-1.7THz to WR-2300 at 23in x 11.5in that is used for 320MHz-490MHz.

Some applications require a different shape or profile, and that is where circular waveguide comes in. Again, different sizes are used for different frequency ranges; the sizes used for UHF broadcast are 1350, 1500, 1700 and 1750. Circular waveguide has less attenuation than rectangular, which makes it more efficient. That comes in handy when you need to reach an antenna atop a 2000ft tower. Also, being circular, this waveguide presents much less wind loading to the tower, about 80 percent less than rectangular waveguide. Rigid coax has the same shape and, thus, the same wind loading factor, but it is much less efficient and would require a higher power output from the transmitter.

How it works

Waveguide theory can fill a book (and it does — many of them), so I will only touch upon it here. The RF signal is conducted along the waveguide’s length in the form of electrical voltage and magnetic fields. The width, or top, and bottom of the rectangular waveguide are made to be one-half the wavelength of the lower cutoff frequency for this particular-sized waveguide. The height, or sides, of the waveguide are half the size of the top and bottom, which means that the sides are a one-fourth wavelength of the lower cutoff frequency. The sides become one-fourth wave sections that present high impedance to the RF signal that is traveling down the waveguide between the top and bottom planes. The top and bottom planes act like two conductors separated by high-impedance insulators (the side walls).

The electrical field is referred to as the E-field, and the magnetic is the T-field. The highest voltages are in the center of the broad plane (in rectangular WG), and the voltage diminishes the closer you get to the sidewalls. When arcing occurs within a waveguide, the pits and marks are found down the center of the broad plane where the highest voltages are present. (See Figure 2.)

Because the wavelength determines the size of the waveguide, it is used exclusively at higher frequencies, such as microwave and the UHF band; you would never see it used on VHF or FM bands. As an example, a waveguide for the FM band would be huge, about 5ft across, a little too big to be practical. At higher frequencies, waveguide is very efficient and widely used because of its inherent low attenuation at these frequencies (practically none at microwave frequencies).

Power handling

Waveguide has no internal components such as coax; it consists of an empty box that transports RF from one point to another. Because of this, waveguide can handle much greater power levels than coax, up to several megawatts.

Power-handling capability is controlled by two factors: average and peak power. Average power slowly heats a transmission line over time. If the average power is too high, components within can be damaged and fail. In the case of rigid coax, that would be the spacers that keep the center conductor in place. But with waveguide, there is no center conductor. This is what allows waveguide to handle much greater power levels than rigid coax. (See Figure 3.)

Peak power is associated with peak voltages, which can lead to arcing within a transmission line. In any transmission line, the voltage level that leads to internal arcing is determined by the spacing and the dielectric between the conductors. In this case, the dielectric is air. With rigid coax, that is the spacing between the inner and outer conductors. In waveguide, that space is the internal height of the line; with WR-2300, that space is 11.5in — quite far for an arc to occur.

Pressurization

As with any transmission line, waveguide must be kept free of moisture. The best way to ensure a dry line is to pressurize it with nitrogen or dry air. As mentioned in the last tutorial, rigid coax is normally pressurized to about 5PSI, but it can go up to 25PSI in some cases. We can pressurize coax to these levels because it’s round and the high pressures will not deform it. But with rectangular waveguide, it’s another matter altogether.

Rectangular waveguide comes with either thick or thin walls, referring to the thickness of the metal it’s made from. Thin wall is used when the line is inside a building and does not require pressurization; thick wall must be used whenever the line will be pressurized. Rectangular waveguide can only be pressurized to around .5PSI-3PSI; after that, the rectangular shape will be deformed, causing a bowing outward to the top, bottom and sides. A safety valve is always installed to prevent any overpressurization at the top and bottom of a vertical run. Even the sun shining on the transmission line will heat the air within, causing it to expand. When it cools off, the internal pressure will be reduced, so it’s a constant balance between the valves reducing pressure and the nitrogen bottle or air dryer filling it back up.

Because the shape of the waveguide determines its frequency range, any deformation will effectively detune the waveguide and require its replacement. (See Figure 4.)

Why not use only waveguide?

Waveguide has many advantages, such as high power-handling capability and low attenuation, but it has its disadvantages as well. Waveguide only works practically at higher frequencies. It requires different sizes for different frequency bands (coax requires different sizes for different power levels); it’s harder to install; and more care must be taken when pressurizing it. But, if you’re using a high frequency and need low attenuation, waveguide may be just what you need.

Next time

The next tutorial will delve further into waveguide and how it is used in modern TV broadcasting facilities.