A Practical Guide To RF In Broadcast: The Principles Of RF Radiation

In 1865, James Clerk Maxwell published “A Dynamical Theory of the Electromagnetic Field.” It postulated that electric and magnetic fields travel through space as waves moving at the speed of light in the same medium, leading to his prediction of radio waves.

The first radio antennas were built in 1888 by Heinrich Hertz. In 1895 Guglielmo Marconi began development of antennas for long-distance wireless telegraphy for which he was awarded a Nobel Prize in 1909.

Transmission antenna design depends on resonance to maximize the radiation of electromagnetic radio waves. One of the simplest and most widely used transmitting antennas is the ½ wave dipole. It consists of two ¼ wave elements, oriented end-to-end, and center-fed by the transmission line. In each ¼ wave element, current from the feed point will change phase by 90° by the time it reaches the end of the element. The end reflects the signal by 180° and it is delayed another 90° as it returns along the element back to the feed point. This process creates a standing wave in the element with the maximum current at the feed point. At any instant, one of the elements is pushing current into the transmission line while the other is pulling current out.

Figure 1 – A simple dipole is the most common transmission antenna design.

Figure 1 – A simple dipole is the most common transmission antenna design.

The ¼ wave elements create a series-resonant electrical circuit due to the standing wave. At the resonant frequency the standing wave has a current peak and a voltage minimum at the transmission line feed point. The radiation pattern of a ½ wave dipole is perpendicular to the elements and weak in the axial directions. The problem with a single ½ wave dipole is that it is omnidirectional and therefore has no gain.

Antenna Gain

Antenna gain is produced by concentrating radiated power into a particular solid angle of space (called lobes) at the expense of power reduced in undesired directions (called nulls). It is not the same as amplifier “gain” which implies an increase in power. Antenna gain is produced by focusing the RF radiation using multiple antenna elements and phasing. Antenna gain benefits both transmission and reception.

All antennas must interact with electrical ground to work. For example, one side of a ½ wave dipole is always grounded. A monopole antenna, also known as a whip or rubber ducky, commonly used in walkie-talkies, produces a weak, omnidirectional signal that inductively interacts with the ground beneath it.

An AM broadcast tower is often the antenna, isolated from ground by a ceramic insulator. The ‘ground plane’ at many non-directional AM broadcast towers is often buried ½ wavelength 6” copper ground straps, extending radially from the tower base, designed to direct the radio waves towards the horizon.

TV Transmitting Antennas

The choice of TV RF transmission system components is a function of fully covering a DMA at licensed full power while avoiding interference with other stations. Selection of RF hardware is a scientific process of elimination that typically requires an experienced engineering consultant to design the best and most compliant RF system possible. Every TV transmission facility and project is unique.

Figure 2 – Polar plot shows the lobes and nulls of a highly directional, high-gain, Yagi UHF receiving antenna. Most broadcast TV signal lobes aren’t nearly as intense.

Figure 2 – Polar plot shows the lobes and nulls of a highly directional, high-gain, Yagi UHF receiving antenna. Most broadcast TV signal lobes aren’t nearly as intense.

Nearly all TV transmission antennas are variations of the dipole. A turnstile or crossed-dipole antenna consists of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature, making the two currents applied to the dipoles 90° out of phase. When mounted horizontally the antenna looks like a pedestrian turnstile. The antenna can be used in two modes. In normal mode the antenna radiates horizontally polarized waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis.

Specialized normal mode turnstile antennas used in TV broadcasting are often called super-turnstile, or batwing antennas. Axial mode turnstiles are widely used for broadcasting in both the VHF and UHF TV bands for circular polarization propagation. They are also popular for satellite earth station antennas because circular polarization is not sensitive to the orientation of the satellite antenna in space.

A bowtie antenna, aka butterfly antenna, is a dipole with triangular or arrow-shaped elements with the feed point where the triangles meet. The triangles can be cut from sheet metal with solid metal centers, or they can be created by two ¼ wavelength wires with their far ends connected outlining the shape of a bowtie.

Slotted coaxial antennas have many advantages over traditional broadband panel antennas including much smaller size and wind load, higher reliability and a greater degree of azimuth and elevation pattern flexibility. Many side-mounted slotted antennas are designed for single channel operation.

High power, top mounted, slotted coaxial broadcast antennas can be used for broadband multi-channel applications. Slotted antennas provide a lower cost, lower wind load and more reliable alternative to panel antennas. This is accomplished with phase cancelation through multiple feeds. The effect of external transmission lines, used for the multiple feeds, on the circularity of the azimuth pattern can be minimized with parasitic tubes near the surface of the aperture.

Top mounted, top-mounted stacked arrays, side-mounted, side-mounted center-fed, delta wing (dual batwings) panel antennas for UHF and are commonly used in horizontal, elliptical, or circular polarization with up to 14 dB gain. Beam tilt can be electrical or mechanical.

Transmission Line Types

Typical coaxial transmission lines for TV range from flexible and semi-flexible to rigid, from 7/8” to 12” diameter, as well as rectangular waveguide for high power UHF stations. Waveguide has no center conductor. Rigid and waveguide doesn’t bend. 90° elbows and couplers are used to turn corners.

Typically, TV transmission lines are kept free of moisture with some gentle positive pressure from a regulated nitrogen bottle, a nitrogen generator, or a dehydrator. Moisture in a transmission line can increase the VSWR.

Filters, Combiners & Directional Couplers

VHF Harmonic Filters, UHF Harmonic filters are designed for indoor use and provide high rejection of harmonic products outside the 6 MHz TV band. Waveguide filters are built for a specific operating frequency. Tunable standard coaxial bandpass filters are used between the transmitter output and antenna system to suppress undesirable frequency products. Coaxial filters are tunable with many tempting physical adjustments, but never try it yourself. If a filter develops a problem that you get a complaint about, send it back to the manufacturer for retuning or repair.

This hanging mask filter is connected to the transmitter output and the antenna transmission line. Note the probes on the filter input and output to monitor power and VSWR.

This hanging mask filter is connected to the transmitter output and the antenna transmission line. Note the probes on the filter input and output to monitor power and VSWR.

Single channel combiners are used to combine two or more transmitter PAs to create the total power output (TPO). Standard channel combiners are used to combine two or more separate channels on one antenna system. Types available include a constant impedance filter, a branch or starpoint combiner which uses a bandpass filter per transmitter, and a manifold combiner that combines multiple channels with a minimum spacing of two channels each.

Standard coaxial directional couplers with single or multiple probes are used to measure forward and reflected power (VSWR) before and after the filter. Motorized switches for most transmission line types can be used to switch between antennas or a dummy load.

Towers

TV towers are tall because broadcast TV signals are line-of-sight. Also, hills and valleys can block a TV RF signal by causing a ‘shadow’ that can only be overcome by raising the receiving antenna.

Towers are a broad topic, best addressed by professional broadcast TV consultants and manufacturers and entirety is beyond the scope of this chapter.

However, the most practical TV tower information is the maximum distance an omnidirectional full power TV signal will travel. Line-of-sight is equal to the square root of the height of the antenna (or eye) above average terrain times 1.225. For example, standing at sea level you can see 3 miles. The visible distance from a height of 100’ is 12.25 miles. At 500’ its 27 miles, at 1000’ it is 39 miles, and 2000’ is about 54 miles.

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