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Consumer HiFi salespeople sometimes jokingly brag that their most expensive audio gear’s bandwidth is DC to daylight. Sound is in fact pressure waves mechanically generated and propagated through a solid, liquid or gas. Humans can only hear from about 18 Hz to approximately 20 kHz. Some animal species can hear higher and lower frequencies. The sound of a ‘flat’ premium HiFi system is controlled more by how well the loudspeakers couple with the air than by wideband circuitry.

The electromagnetic spectrum covers from a few cycles (Hz) above DC to light and beyond. Like sound waves and light waves, radio waves are also affected by reflection, refraction, diffraction, absorption, polarization, and scattering. Nearly all electromagnetic medium wave (300 kHz to 30 MHz) radio waves begin as vertically polarized ground waves propagating parallel to the Earth’s surface.

Ionospheric Wild Card

The ionosphere is at an altitude of approximately 30 to 600 miles and gets its name because it is ionized by solar radiation. Ionization depends mainly on the Sun and its Extreme Ultraviolet (EUV) and X-ray irradiance which is affected by sunspot activity. Some ionospheric layers can attenuate radio waves while other layers can reflect them, all depending on states of ionization. On the AM broadcast band at night, ground waves can be reflected long distances by the ionosphere. This phenomenon is known as skip or skywave. Most overseas shortwave communications are between 3 and 30 MHz and rely on ionospheric skywaves for long-distance.

Sporadic E propagation (E-skip) is a more random form of radio skip that uses ionospheric ‘clouds’ of ionized metals from micrometeoroids temporarily appearing at low altitudes of about 50-100 miles that usually don’t refract radio waves. E-skip affects signals between approximately 20 to 150 MHz including FM and TV low VHF channels, and it can result in temporary reception using an average antenna of distant broadcast stations as far away as 1400 miles. E-skip is somewhat periodic and seasonal, peaking near the summer solstice in both hemispheres.

F2 propagation (F2-skip) is the occasional refraction of broadcast signals off the F2 layer of the ionosphere. It is rare compared to E-skip, but it can refract signals thousands of miles beyond their intended broadcast market, much farther than E-skip. F2-skip typically affects signals in the same frequency range as E-skip. Stations occasionally get reception reports from remarkably distant locations due to Sporadic E and F2-skip conditions.

M, MM & MHz

Frequencies can be described by wavelength or by time. Wavelength is the distance over which a wave’s sinusoidal shape repeats, typically between peaks or zero crossings, measured in meters or millimeters. Time is the frequency of the oscillation cycles of the current in an electromagnetic field measured in Hz, kHz, MHz, GHz, and THz. The THz band covers from 0.3 THz (300 GHz) to 3 THz. Wavelength is inversely proportional to frequency. Compared to longer waves, shorter waves require more radiated power to deliver equal signal strength over equal distances.

Usable Spectrum

The usable electromagnetic spectrum for radio communication is from about 20 kHz to 300 GHz. The ITU considers extremely high frequency (EHF) to be from 30 to 300 GHz, which is where an extensive amount of RF R&D is now funded and focused. EHF radio waves are wavelengths between ten and one millimeter called millimeter (mm) waves. Incidentally, a 1-millimeter amateur radio band is between 241-250 GHz. The 1 mm band world distance record is 71 miles. 1 mm amateur band record runners up in 2019 were 5.8 miles in the UK and 2.4 miles in Australia.

The mm band includes satellite communication bands such as the Ka Band from 18-27 GHz, V Band from 40-75 GHz, and W Band from 75 to 110 GHz. G band covers from 110 GHz to 300 GHz. Terahertz frequencies above 300 GHz (0.3 THz) are only useful in extraordinarily short distances or in a vacuum. Excessive air gap obliterates THz RF communications.

If you think millimeter waves are new technology, you should know that they were first investigated by Indian physicist Jagadish Chandra Bose in the late 1890s. He wanted to investigate the light-like properties of radio waves, but the long radio waves were difficult to closely observe and study. Bose managed to produce a wavelength of about 5mm by generating frequencies up to 60 GHz during his experiments, which were in progress about the same time Guglielmo Marconi was experimenting with spark gap transmitters.

Marconi’s first, crude, spark gap transmission across the Atlantic wasn’t until 1901. The transmission was three repetitive clicks that Marconi knew in advance to listen for in the static. The signal center frequency of the signal was about 850 kHz, based on the length of the antennas.

Radio Propagation

Marconi’s vision was focused on trans-oceanic wireless telegraphy. His early radio experiments were all conducted during the daytime, and the strength of his 850 kHz transmission predictably diminished with distance. He later discovered skywaves were regularly absorbed by the ionosphere in the daytime and refracted back to earth at night. When nighttime skywave propagation patterns were identified and understood, progress in long-distance wireless communications accelerated.

Early radio signal transmissions were typically specified in meters, referring to the length of the radio wave, and wavelength was also the origin of the radio terms long wave, medium wave, and shortwave. Portions of the radio spectrum reserved for specific purposes continue to be referred to by wavelength to this day, such as the mm band, amateur radio bands, and international broadcast bands known as 41, 31, 25, and 19 meters, for example.

Antennas work best when they measure a simple fraction of the wavelength to be transmitted or received. Tuned antennas are typically half-wave or quarter-wave in length. For example, the wavelength of an AM radio broadcast signal at 1 MHz (1000 kHz) is 299.792 meters, which is the reason most AM radio broadcast ‘quarter wave’ transmitting antennas are about 250’ tall.

Ham Radio Advantage

My father’s friend was the ham radio operator who got me interested in radio. I was hooked the moment I saw the violet glow of the mercury vapor rectifier tubes in his transmitter get brighter when he spoke into his mic. I got my Novice Amateur Radio License when I was 14. Shortly thereafter I got my first and last RF burn. Ham radio was an excellent RF learning experience because hams like to help one other and the common denominator is RF. Nearly all the engineers at the first TV station that hired me were amateur radio operators.

One of the first things I learned about radio was signal propagation. All I knew about RF propagation before becoming a ham was that I could hear distant stations on AM radio at night, and that the TV antenna or rabbit ears must be critically adjusted for minimum ghosting.

Listening and experimenting taught me the propagation characteristics of the amateur radio bands of 80, 40, 20, 15, 10, 6, and 2 meters, as well as the 220 MHz and 440 MHz ham bands. The propagation on each band between 80 and 10 meters and the international shortwave bands in between was predictable and varied between skywave and groundwave by band and time.

Ubiquitous in broadcast TV operations is the Global Positioning System (GPS). All GPS satellites operate at the same two frequencies, 1.57542 GHz (the L1 signal) and 1.2276 GHz (the L2 signal). Most TV transmitter exciters and some digital and IP gear require a GPS reference signal to operate. A climbing weed that grows to cover a simple outdoor GPS antenna mounted on a pole can knock a TV transmitter off the air.

Band Properties

Radio waves at different frequencies behave differently at different times during a 24-hour cycle, mostly due to ionospheric conditions. Long wavelengths at frequencies below approximately 2 MHz are generally considered ground waves except when the ionosphere refracts or reflects them at night.

Transmission and reception of distant signals between approximately 3 MHz to 30 MHz is controlled by the ionosphere. Different bands in that range are most active with distant signals during different times of the day and night. For example, distant signals from Japan or Australia may appear and then disappear on the same band that also temporarily carries signals from South America at another time during a 24-hour cycle.

Shorter wavelengths above 30 MHz don’t bend with the Earth. Higher frequencies rarely reflect off the ionosphere except during Sporadic E propagation and are nearly always limited to line of sight. Above about 500 MHz, moisture in and on trees and leaves attenuates signals. The higher the GHz the more a signal is attenuated by metal, walls, skin, and eventually atoms in the atmosphere.

BAS Bands

Broadcast Auxiliary Service (BAS) authorization is available to broadcast station licensees and to broadcast or cable network entities. Certain entities involved in television and motion picture production activities are also eligible for Low Power Broadcast Auxiliary authorizations. BAS bands are protected from interference because transmission requires a license and broadcasters are the local frequency coordinators.

BAS stations are typically used to relay or backhaul broadcast aural and TV signals. It is mostly used for studio-transmitter links (STL), transmitter-studio links (TSL), ENG and live EFP feeds and backhauls. BAS also includes mobile TV pickups and remote pickup stations which relay signals from a remote location back to the studio when a multiple hop is the only way to get a stable signal to the studio.

Broadcast Auxiliary Services bands are 1990-2110 MHz and 2450-2483.5 MHz (2 GHz), 6875-7125 MHz (7 GHz), and 12.7-13.25 GHz (13 GHz). Licensees must get a waiver of the rules to transmit with digital modulation in these bands. Digital modulation is allowed in the 6425-6525 MHz, 17.7-19.7 GHz and 31.0-31.3 bands. More information is available from the: Federal Register :: Broadcast Auxiliary Service Rules.