Hatfield & Dawson Consulting Engineers
Total Page：16
File Type：pdf, Size：1020Kb
HATFIELD & DAWSON BENJAMIN F. DAWSON III, PE CONSULTING ELECTRICAL ENGINEERS TELEPHONE (206) 783-9151 THOMAS M. ECKELS, PE 9500 GREENWOOD AVE. N. FACSIMILE (206) 789-9834 STEPHEN S. LOCKWOOD, PE SEATTLE, WASHINGTON 98103 E-MAIL [email protected] DAVID J. PINION, PE ERIK C. SWANSON, PE JAMES B. HATFIELD, PE CONSULTANT THOMAS S. GORTON, PE MICHAEL H. MEHIGAN, PE MAURY L. HATFIELD, PE (1942-2009) PAUL W. LEONARD, PE (1925-2011) 20 November 2013 Suzanne Bosman Whatcom County Planning & Development Services 5280 Northwest Drive Bellingham, WA 98226 Dear Ms. Bosman; This purpose of this letter is to provide a basic framework for understanding the electromagnetic spectrum and how radio signals work, and to illustrate some key characteristics of the differences between AM radio and other uses of the spectrum. Electromagnetic Spectrum Bands The federal government, through the Federal Communications Commission (“FCC”), coordinates and regulates all uses of electromagnetic spectrum in the United States. Every portion of the spectrum is organized into types of uses. See attached United States Frequency Allocation chart and Radio Frequency Spectrum table. AM radio operates in the Medium Wave (MW)1 band, and is different from FM which operates in the Very High Frequency band (VHF),2 TV which operates in both the VHF band and the 1 MW is Medium Wave which is 300 kHz to 3,000 kHz (3 MHz). 2 VHF is Very High Frequencies from 30 to 300 MHz. Hatfield & Dawson Consulting Engineers KRPI Site Selection and Radio Engineering Basics Page 2 Ultra-High Frequency band (UHF),3 and cellular which also operates in the Ultra-High Frequency band. Other communications uses of the electromagnetic spectrum include Amateur “Ham” Radio, military communications, and emergency responder communications. There are also industrial spectrum uses that include heat sealing machines (for retail plastic packaging), fluorescent lighting, infrared temperature sensors, television remote controls, laser bar code scanners, etc. All of these common, everyday devices utilize different portions of the electromagnetic spectrum. Each portion of the spectrum has different uses and each portion of the spectrum behaves in a different manner. Different portions of the spectrum need different types of equipment in order to be harnessed for a useful purpose, and each portion of the spectrum has unique properties. As an example of the quality of these differences, consider a comparison between steel and ice. Both are solid structures that can be shaped into a bar, but the physical properties are quite different. Their melting points are different; their ability to resist fracture is different; and they can’t be used for the same applications. Likewise, cellular spectrum has little in common with AM; and FM spectrum has little in common with AM. They are different frequencies and different spectrum uses, and for analytic purposes they should not be lumped together. 3 UHF is Ultra High Frequencies from 300 MHz to 3,000 MHz. Hatfield & Dawson Consulting Engineers KRPI Site Selection and Radio Engineering Basics Page 3 Radio Propagation (how signals travel) FM and TV signals travel using line-of sight paths, so VHF antennas must be placed at elevated locations to transmit FM and TV signals. In the central Puget Sound region, these signals are primarily transmitted from towers at West Tiger Mountain, Cougar Mountain, Queen Anne Hill and Capitol Hill. In the northern Puget Sound region, FM and TV broadcasters tend to locate on Mt. Constitution on Orcas Island. The overwhelming majority of FM and TV stations in Western Washington transmit their signals from areas higher than 950 feet above mean sea level, to provide line-of sight service to radio receivers. VHF signals essentially travel the same way whether it is daytime or nighttime. The FM signal you listen to in your car traveled to your car receiving antenna on a direct, line-of-sight path from the transmission antenna. Similarly, cellular signals also travel by line-of-sight paths; cellular antennas must be elevated above obstructions such as trees or buildings in order to function at an optimal level.4 In contrast, AM radio signals have unique propagation pathways. By analogy, if you placed an orange, sliced in half, wide (cut) face down on your desk, you’d have a little dome. Imagine the orange core is an AM antenna holding the dome together like a tent. This is similar to what an AM signal looks like if you could see it. AM radio signals have two components, a groundwave signal (the part touching your desk in all directions) and a skywave signal (all the surfaces of the orange not touching your desk). The skywave signal bounces back to earth at night when one portion of the atmosphere's properties change by being shadowed from the sun. The groundwave signal travels along the surface of the Earth, and is the primary pathway to enable in-car or in-home reception of an AM radio station. Groundwave signals depend on specific properties of soil and topography; AM radio transmitters cannot typically be located on, or next to, hills, because such features impede groundwave signal propagation. 4 For AM radio, the entire length of each AM structure acts as an antenna. This contrasts with cellular signals, which are transmitted from panels or rods attached to another, non-transmitting structure, such as a monopole, light standard or building roof. Hatfield & Dawson Consulting Engineers KRPI Site Selection and Radio Engineering Basics Page 4 After sundown, AM signals also travel using “skywave” propagation. Skywaves are radio waves that are reflected back to Earth by the ionosphere, and thus can travel much farther than do groundwaves. The ionosphere is the upper region of the earth’s atmosphere located approximately 30 to 250 miles above the surface of the earth. Sunlight changes the physical properties of the ionosphere. During daytime AM signals are absorbed by the ionosphere, but at nighttime AM signals are reflected back to Earth. Tuning through the AM band at nighttime it is possible to experience skywave propagation. There are a number of radio stations from locations far away that can be received in this area. In particular, there are several San Francisco Bay area stations that have good coverage up and down the coast during nighttime hours. A good example of this phenomenon is radio station KFBK, which operates on 1530 kHz in Sacramento, CA. KFBK cannot be received in the Puget Sound area during the daytime but at nighttime it is received quite well. The propagation change between daytime and nighttime often requires that a station use different antenna configurations and/or power levels during daytime and nighttime, in order to minimize interference to other stations.5 Most stations use multiple antenna structures in order to reduce skywave interference, which affects the size of property needed for an antenna system. KRPI uses five structures in different combinations to change its signal. At least one AM station has 12 structures to shape its signal. Radio Physics (basic properties) 5 Interference is defined as unwanted signals degrading the performance of reception of a desired signal. Interference can be from several sources. These include: co-channel interference which is a signal broadcast on the same channel as the desired signal; adjacent channel interference which is a signal broadcast on a nearby channel; and electrical noise which is static that can come from electrical sources (hair dryer, flourescent lights, other electrical motors, power lines, etc.). Hatfield & Dawson Consulting Engineers KRPI Site Selection and Radio Engineering Basics Page 5 Imagine a wave. In the common range for cellular frequencies of 300 – 3000 MHz, if you could see a cellular signal, the wavelength would be in the range between approximately four inches and approximately three feet long. If you could physically see an FM signal, the wavelength would be approximately six feet long. To transmit a signal, the ‘wave’ needs an appropriately sized antenna to send the wave on its way to your radio. This is why cellular signals commonly use three to six foot long white rectangular panel antennas attached to towers or buildings. FM signals are transmitted from a bigger antenna element attached to a tower, as well. Cellular and FM only use the tower as a structure to elevate the antenna. In contrast, for AM radio, if you could physically see an AM signal, the wavelength would be 635 feet long at KRPI’s frequency of 1550 kHz AM, and over 1,200 feet long at KGMI’s frequency of 790 kHz AM. AM radio signals require structures that range in size from 150 feet tall to 750 feet tall. The entire piece of steel is the AM antenna. The KOMO 1000 antenna structures are each 500 feet tall to make the KOMO signal that you hear in your car. In addition, about half of an AM antenna system is buried in the ground. FM, TV, and cellular antennas don’t operate that way. AM radio antennas require 120 copper wires around the base of each radiating element to develop the signal. These wires are the diameter of #2 pencil lead. The length of the wires is tied to the wavelength of the signal—for example, for KRPI 160 feet—and extend in a multidirectional pattern out from the above ground antenna structure. This takes up additional land. FM, TV, and cellular don’t need large tracts of land for signal propagation in the way AM radio does. Hatfield & Dawson Consulting Engineers KRPI Site Selection and Radio Engineering Basics Page 6 AM radio is also different from FM, TV, and cellular in another significant way. Many AM stations (including KRPI) must use a directional antenna at night to reduce potential interference to other stations on or near the same channel because of skywave propagation.