* Note to self: The time for a new post is long overdue but it is not as though I haven’t had other distractions to keep me occupied. Last week for example I had to chase the same bear out of camp three separate times during the night. The next morning it was determined that the bear had confiscated a roll of sausage, a stick of butter, a box of cookies and a bag of marshmallows.
Generally, any antenna that is used to receive RF (Radio Frequency modulation) is capable of adequately transmitting that same RF. Sprouting from the Italian word for the longest or central tent pole supporting a tent, “antenna” entered radio vernacular sometime after 1895 when Marconi (camping in the Alps) supported his radio’s aerial from the pole. Aerial and antenna are usually synonymous and both are simply transducers or implements which convert one type of energy into another. The word “aerial” however is sometimes used to refer to only a rigid vertical transducer.
* Antennae is a seldom used plural form of the noun – antenna, and might most frequently be encountered when discussing bugs. Depending upon the type of insect, antennae might be used to feel, hear, smell, or even to detect light. Apparently male mosquitoes employ their antennae to hear female mosquitoes from as far as ¼ mile (400m) away.
Radio antennas are thought of as being directional or omni-directional. A directional antenna will prefer to radiate in, or receive from one direction more than it will in any other. A vertical rod or isotropic radio tower supposedly radiates in all directions equally. No aerial is perfectly isotropic (omni-directional) however. In the case of a vertical tower there is a blind cone or null lobe straight up and another straight down where radiation is not sent or where reception is absent. In the same fashion, there is no antenna that is perfectly directional. A pictorial depiction of a directional antenna’s radiation pattern usually shows particular zones as being elongated lobes. There are main lobes, back lobes, side lobes and null lobes of radiation pattern.
Gain is a concept unique to directional antennas and is a measure of efficiency. Gain is the ratio of a directional antenna’s intensity relative to that of a hypothetically ideal isotropic antenna. A low-gain antenna sends or receives signals partially from several directions while a high-gain antenna is much more focused. Both types have their advantages. A high-gain antenna may need to be carefully aimed or pointed towards its target, to work. That achieved, a high-gain antenna has a longer range than a low-gain type. It’s a “conservation of energy”; less energy is wasted by radiating in useless directions. Modern household satellite dishes for TV reception are examples of high-gain antennas. Antennas on cell phones and Wi-Fi equipped computers however are low-gain types, which enables them to receive signals from many directions.
The parabolic shaped antennas used for satellite TV and radars, are usually associated with microwave frequencies. The first parabolic antennas were constructed however, over 120 years ago when Heinrich Hertz used them to prove the existence of electromagnetic waves. The dish or parabolic shaped element can be made of mesh, wire screen, sheet metal or mirror. The dish is only a passive device; a reflector that collects signals and bounces them towards the active (cable connected) feed. Monstrously huge parabolic antennas are used for radio telescopes. Radio telescopes can be used to determine the composition of molecular clouds in space because when excited, individual molecules rotate at discreet speeds and emit radio energy as they do so. Carbon monoxide likes to emit at 230 GHz for example. These telescopes can be used to study all sorts of things: black holes, radio-emitting stars, radio galaxies, quasars, pulsars, gamma ray burst, super novas and so on. They can be used to track satellites, do atmospheric studies or to receive radio communications from distant traveling spacecraft like Voyager 2.
* The VLA (Very Large Array) radio astronomy observatory is located in a remote area of N.M., just east of Pie Town, N.M. The array is made of 27 independent parabolic dishes that stand about 10 stories high (82’or 25m) and are visible from space as little white dots. Each independent dish weighs 209 metric tons (2,205 lbs x 209) and is mounted on a robust rail system (doubled – two parallel sets of standard gauge tracks) so that it can be moved. The rails are configured in a “Y” shape. To focus on an object or area in space the 27 dishes expand from a minimum of 600m at center to a maximum baseline radius of 22.3 miles. These antennas can listen to a large chunk of the radio spectrum (from 74 MHz to 50 GHz / wavelengths 400 cm to 0.7 cm). Computers are used to correlate the data from each dish into a single map; the VLA observatory itself is called an “interferometer”. Occasionally the VLA is brought online to link with other radio telescopes around the country to form an even larger (5,351 miles long) baseline called the VLBA (Very Long Baseline Array). These other antennas are located in Brewster, WA, Kitt Peak, AZ, Los Alamos, N.M, Owens Valley, CA, Fort Davis, TX, North Liberty, IO, Hancock, N.H, Mauna Kea, HI, and St. Croix, U.S. Virgin Islands. On occasions when radio telescopes in Arecibo, Puerto Rico, Green Bank VA, and Effelsberg, Germany join in the whole affair is called the High-Sensitivity Array.
Phased array radar antennas like the flat panel above actually house many small evenly spaced aerials. The phase of the signal to each individual aerial is logically controlled, resulting in a collective beam from all the little aerials that can be amplified and focused in a specific direction almost instantly. Quicker and more versatile than mechanically rotating antennas because they require no movement, phased arrays are also more reliable and require little maintenance. Limited phased array radars have been around for 60 years but recent improvements and affordability in electronics has made them more commonplace. Most new military radars being built today are phase array systems.
* RADAR is an acronym coined during WWII by the U.S. Navy, from “Radio Detection And Ranging”. Before that however, the British were calling the same thing RDF (Range and Direction Finding). The most common bands used for radar are microwave bands (at the upper end of the radio spectrum between 1 GHZ and 100 GHz – the L, F, C, X, Ku, K and Ka bands). Radars used for very long-range surveillance however might use longer VHF frequencies starting at 50 MHz or UHF frequencies between 300 and 1,000 MHz (1 GHz).
Omitting the simple aerial, some commonly encountered antenna shapes are shown above. The most basic antenna type perhaps is a “quarter wave vertical” (where the length of the aerial is ¼ of the wavelength targeted). The simplest and most commonly encountered antenna however is probably the “dipole” antenna. A dipole antenna is essentially just two elevated wires, pointing in opposite directions. A dipole is fairly omni-directional unless its axis is parallel to the target emission. A monopole antenna is formed when one side or one half of a dipole is replaced with a ground pane that is perpendicular or at a right angle to the remaining half. A whip antenna correctly installed on a car for example, uses reflected radiation from the automobile’s body (the ground plane) to mimic a dipole. In this instance the monopole will have a greater directive gain and a lower input resistance.
* Grounding provides a reference point from which changes in waveform can be detected. A radio tower that is constructed to transmit at AM frequencies for example must be grounded or be compensated for lack of ground, and its height or length of element is determined by the wavelength. Certain ground soils allow good grounding to earth but others do not. In the absence of a good ground an antenna can simulate a ground by adding drooping radials (additional elements hanging at 45°). A typical Marconi antenna is a perpendicular ¼ wave aerial with a proper ground (perhaps the soil is moist, marshy, full of iron ore or otherwise conductive). In this case the ground acts to provide more signal, adding the missing quarter to mimic a full half wavelength antenna. Often two or more quarter wave antenna towers will be seen in the same vicinity. Usually a group of similar towers like this is creating a directional array that transmits greater power in a certain direction. Since AM broadcast (US.) wavelengths range between 1,826 ft. and 909 ft. in length it would be prohibitively expensive to erect a desirable full length or even half length vertical transmitting tower to hold up the element. For economic reasons some large transmitting antennas therefore are laid out and polarized in the horizontal plane.
The folded dipole is a variation of the simple dipole. Folded dipoles are about the same overall length as a standard dipole but provide greater bandwidth, have higher impedance and can often provide a stronger signal.
Loop antennas are generally used to conserve space. The old TV set top “rabbit ears” often incorporated a loop in addition to the two telescoping, adjustable dipole elements. Loops respond to the magnetic field of a radio wave, not the electrical. A loop induces very small currents on each side of the loop and the difference between the two must be amplified usually, before any useful signal is fed to the receiver. Loop antennas are very inefficient. One useful property of the loop however is that is very directional, they pick up signals when positioned in one axis, but not another. Most direction finding radios incorporate a loop antenna. A loop by itself can determine the axis of a signal’s radiation but not forward from backward. Direction finding radios were/are used in aircraft and boats or ships at sea to navigate with. Modern civilian aircraft usually have an ADF (Automatic Direction Finder) box that is attached to a loop and sensing antenna combination. In earlier days the loop was manual (turned by hand) and not automatic. The non-directional, sensing aerial on a small aircraft might be a simple wire running from the tail, forward to the cabin. The ADF’s electronics compares the two antennas (directional and omni-directional) to determine the signal’s phase (+/-) and therefore forwards from backwards.
Loopstick antennas (using ferrite rods) found in many small AM radios are actually examples of loop antennas. Today “DX-ers” and radio hams might construct a shielded loop antenna, wrapping hundreds if feet of wire onto a spool. Such an antenna would have the advantage of containing a half-wave or even a full-wave element in a small space, but it would be directional and introduce a new set of technical complications.
The Yagi- Uda antenna was invented by two Japanese scientists back in the late1920’s. Early airborne radar sets used in WWII night fighters used Yagi antennas and were employed by almost everyone except the Japanese. Yagi antennas have several parallel elements, some active (directors) and some not (reflectors). The unconnected multiple elements help to improve gain and directivity. The illustration shows a horizontally polarized, dual band antenna, once popular for analogue TV reception. The whole thing is a combination of three separate Yagi antennas. The longer elements are for VHF reception. The shorter, closely spaced elements on the left half of the antenna were for UHF reception. The shortest elements on the straight tail are directors and reflectors that act to improve the UHF gain and directivity. The next longest elements (mounted on the vertical “V”) are UHF half-wave dipoles. The longest elements on the right would be half wave dipoles, arranged in a “phased array” to pick up multiple channels. Wavelengths of the FM and VHF TV bands are somewhere between 11’ and 9’ long. The longest single element in this example would be about 5.5ft.
* Beware of salesmen selling snake oil. There is no such thing as a digital TV antenna. An antenna does not care how the wave is modulated; it does not distinguish between analogue and digital signals.
* Although as of 2009 UHF TV is gone in the US., someone else will now transmit in those UHF bands (probably AT&T or Verizon). The front half of these old antennas are still good useful for FM and HDTV reception if a local broadcaster is still transmitting on his legacy bandwidth. The FCC is eager to grab this bandwidth and sell it to cell phone companies.
Horn shaped antennas are commonly used at UHF and microwave frequencies. Parabolic antennas (where the dish itself is just a reflector) often use a horn as the ‘feeder’. Advantages of horn antennas include simplicity, broad bandwidth, fair directivity and efficient standing wave ratios. A few large horn antennas were built in the 1960’s to communicate with early satellites or for use as radio telescopes.
Radio-Frequency Identification (RFID) tags are growing alarmingly in popularity and in sophistication. This unregulated and potentially invasive technology broadcast identification and tracking information by using radio waves. RFID tags generally come in three types these days: active, passive and battery assisted passive. New technology has enabled the miniaturization of these devices to a point where individual ants can host their own personal transmitter. Many pets and livestock are either internally or externally tagged with RFID chips. At least one version of a subdermal microchip implant (RFID transponder encased in silicate glass) about the size of a grain of rice (11mm x 1mm) was manufactured for use in humans until the year 2010.
A passive RFID tag requires an external electromagnetic stimulus before it can modulate its radio signal. An active tag carries its own little battery and therefore transmits its signal autonomously. A biologist might harness some animal like a sea turtle or wolf with this type of tag, and it would only broadcast for a limited time but for a greater distance. A battery assisted passive (BAP / or semi-active) RFID tag sets dormant until stimulated, and its battery helps boost the range of the tag’s radio signal.
Even a simple, cheap passive RFID tag can hold up to 2 Kb of memory. These contraptions use a simple LC tank circuit (a resonating inductor and capacitor). Their antennas are designed to resonate within a certain radio spectrum. Usually a RFID transponder resonates anywhere between 1.75MHz and 9.5 MHz – with 8.2 MHz being the most popular frequency. Usually RFID chips work within traditional ISM (Industrial, Scientific and Medical) frequencies set aside for non-communications purposes. ISM occupies reserved niches in the LF, HF, UHF and microwave frequencies that RFID tags can and do exploit, often without the need for a license. The chip’s antenna picks up electromagnetic radiation from a reader or detector; converts that to electrical energy which powers the microchip which then reflects or broadcast any information held in memory-back over the same antenna.
* Passive tags, when used for electronic article surveillance are usually deactivated by frying the capacitor with an overload of voltage which is induced from a strong electromagnet at the checkout counter. Also a few seconds inside a microwave oven will destroy most RFID chips. Many retail items are “source tagged” at the point of manufacture, with the RFID device hidden within the packaging. Since every vendor does not employ the same type of EAS system (or perhaps none at all) alarms can go off when customers carry or wear these still activated tags into other stores. Some stores may deliberately not deactivate these tags; the motive of building a customer shopping database has been suggested.
Big & rare
Up until 2010 when a certain skyscraper in Dubai was completed, the tallest manmade structure ever built was a half-wave radio mast. Standing at 646.38 m (2,120.6 ft) above the ground and perched upon 2 meters of electrical insulator, this tower broadcast longwave radio (@ 227 kHz and later 225 kHz) to all of Europe, North Africa and even to parts of North America. It was used by Warsaw Radio-Television (Centrum Radiowo-Telewizyjne) from 1974 until it collapsed in 1991.
The notorious ‘Woodpecker’ radio signal interfered with the world wide commercial and amateur communications and international broadcasting stations for about 13 years. Transmitting with about 10 megawatts of power from an antenna that was about 50 stories high and 1/3 rd of a mile long (150m tall x 500m wide) the original Duga-3 antenna was nicknamed “Woodpecker” for the interfering sound that it made. It was using protected frequencies set aside for civilian use. Operating from 1976 to 1989 the Woodpecker now resides within a 30 kilometer diameter region of exclusion surrounding the Chernobyl power plant. The Chernobyl disaster occurred in April 1986 but apparently the Woodpecker continued to operate for another three years.
Their has been varied speculation about the purpose of the Duga-3 broadcast, including intentional broadcast interference, mind control experiments and weather manipulation. These speculations are not without precedent. The most plausible explanation of the Woodpecker signal however, is that it was simply a Soviet over-the-horizon radar (OTH) intended to detect ICBM’s at long range by bouncing itself off the ionosphere. Apparently the Woodpecker was arrayed with other OTH systems like Duga-2 (also in the Ukraine) and a second Duga-3 built in eastern Siberia which points toward the Pacific.
A couple of videos filmed at this antenna which should provide an appreciation for scope and scale.
Climbing up the Russian Woodpecker DUGA 3 Chernobyl-2 OTH radar
Base jumpers sneaking into the ‘Zone of Alienation’ to jump from the antenna.
* During the ‘Cold War’ the term “International broadcasting” described broadcast pointed at or intended for foreign audiences only. For 60 years now, RFE/RL (Radio Free Europe (RFE) and Radio Liberty (RL)) have been spreading anti-communistic propaganda and psychological warfare behind the ‘iron curtain’ using shortwave, medium wave and FM frequencies. It would stand to reason that the Soviets might have wished to retaliate or block such popular broadcast. Although mind control by radio signal seems very far-fetched, the Soviets are accused of having for many years focused microwave radiations toward the U.S. embassy in Moscow. Perhaps the Soviets were attempting to slowly cook the Americans. A more feasible explanation is that the microwave energy was being used to stimulate passive covert “bugs” hidden within the embassy. In 1952 such a covert listening device now known as a passive cavity resonator was discovered inside the U.S. Ambassador’s Moscow residence. This infamous creation known as “The Thing” was designed by the Russian engineer and physicist Lev Sergeyevich Termen and preformed its espionage, unnoticed for 6 or 7 years.
* Weather manipulation using radio is theoretically feasible and supporting information will be included shortly.
Extremely low frequency (ELF) is an electromagnetic radiation range with frequencies from 3 to 30 Hz and wavelengths between 100,000 to 10,000 kilometers (62,137 miles to 6,213 miles) long. Since ELF frequencies can penetrate significant distances into the earth and seawater they have been used by the U.S., Soviet/Russian and Indian navies to communicate with submarines at sea. The British and French apparently also apparently constructed and experimented with ELF antennas. Because of the extreme wavelengths, sending antennas need to be very large and the few examples that do exist are buried in the ground. ELF transmissions were or are limited to a very slow data transmission rate (just a few characters per minute) and are usually just one way transmissions due to the impracticality of a submarine being able to trail an aerial behind it which was long enough to send a reply. The U.S. Navy transmitted ELF signals between 1985 and 2004 from one antenna located in the fields of Wisconsin and another located in Michigan. Due to environmental impact concerns involving everything from farmers concerned over their livestock’s behavior to disoriented whales beaching themselves en masse, the U.S. Navy abandoned its ELF effort. They use something better now anyway.
* Miners and spelunkers can use technology called through-the-earth communications which utilizes the (higher than ELF) ultra-low frequency (ULF) range between 300–3,000 Hz.
Plasma is conductive, ionized air or gas. Using an array of antennas attached to powerful radio transmitters ionospheric heaters are used study and modify plasma turbulence and to affect the ionosphere. Several of these ionosphere research facilities already exist (in Norway, Russia, Alaska, Japan and Puerto Rico) and are operated by organizations like SPEAR (Space Plasma Exploration by Active Radar), EISCAT (European Incoherent Scatter Scientific Association) and HAARP (High-frequency Active Auroral Research Program). By heating or exciting an area of the ionosphere, air can be made to rise or to act as a reflector from which other radio transmissions can be bounced. Theoretically then ionospheric research could, should or already does allow for enhanced radio communications, surveillance, long distance communications with submarines, weather modification and perhaps eventually even the transport of natural gas from the artic without the use of pipelines. The feasibility of altering the course of the jet stream or of steering the course of a hurricane seems very real. Readers wishing to learn more about this subject can find some information on the Internet. They could start by following these two links: