  For the shortwave listener, passive antennas -- no electronics-- typically are thought of as wire antennas. These include butare not limited to the dipole, a random wire, loop, vertical orBeverage. The dipoleExperimentation with antennas has existed since radio began. Oneof the basic antenna forms is a dipole, whose length is half thatof the wavelength it is to receive. The center of the dipole iscut so that a coaxial cable can be connected. At this length theantenna impedance best matches that of the connecting cable, namely50 ohms, and there is a maximum transfer of energy. The dipole may be mounted horizontally, as an inverted-vee(with the center higher then the ends), or as a sloper (one endis mounted higher than the other).  A dipole performs best on the wavelength for which it was cut.On other frequencies, reception will also be possible, but theimpedance is then much higher than 50 ohms, e.g. 1000 ohms, ormuch lower, e.g. 5 ohms. This leads to much poorer energy transfer. Three solutions to the energy transfer issue may be foundin the G5RV antenna (so named for R.L. Varney, an amateur radiooperator in Sussex, England), in the fan dipole and in the T2FDantenna. The G5RV antenna is a variant of the dipole and usesa specific length of 300- or 450-ohm twinlead and 50-ohm coaxialcable as the feedline to achieve a reasonable impedance matchacross the 3 to 30 MHz spectrum. The fan dipole is simply multiple¼-wavelength elements, one pair for each frequency rangedesired, connected to a common feedline; the wire elements arespread out in opposing directions as a woman's fan. The T2FD (TerminatedTilted Folded Dipole) antenna is a sloped two-wire antenna thatachieves a wide 1:5 frequency coverage; the formula to calculateits length in meters at the lowest design frequency is 100/MHz.A 20-meter long T2FD designed for 5 MHz will provide coverageto 25 MHz. A 10:1 balun is required to match the antenna to thecoaxial feed line. There are many solutions to help reduce this problem of mismatch.A longwire, 5 times the length of the desired wavelength, hasa more constant but higher impedance. Some communication receiverseven have a 600 ohm antenna connection to suit this type of antenna.But because coaxial cable cannot be used as the antenna lead-inwire, such a longwire antenna is more susceptible to local interferencesources (fluorescent lights, thermostats, etc.). Antenna tuners are often advertised as a way to match the antennato the receiver. But they work best when they're mounted as closeas possible to, or even in the antenna. There is little pointputting one at the end of a 50 ohm coaxial lead-in wire as themismatch has already taken place. The random wireAlso known as a longwire (a misnomer) or an end-fed wire, a randomwire is simply a length of insulated or uninsulated wire withone end connected to the receiver. To be sure, the antenna willpick up signals. If the antenna is brought into the near fieldnoise environment of the house, the unshielded wire will pickup these noises. The impedance of the antenna approaches a nominal50 ohms on those frequencies where the antenna length is approximatelyat the odd multiples of one-quarter wavelength. See the followingtable for measurements. The wire may be installed in the form of an inverted-L or bentas necessary to fit it into your property. The end-fed wire canbe an effective antenna so long as the appropriate impedance matchingnetwork is applied to the juncture of the wire and the coaxialfeedline. Other passive antennasThe loop antenna can take the shape of a square or a triangle.For best long distance reception and sensitivity to the low-anglesignals, these antennas should be mounted vertically and broadsideto the area of the world one wants to listen to.  The loop antenna is a full wavelength long. As the loop antennaexhibits a radiation resistance of approximately 100 ohms, a balunor matching section of cable is required. While more difficultto put up, the loop does exhibit gain over a dipole. The vertical antenna has the advantage of taking requiring relativelylittle space. The vertical is a quarter-wavelength of wire forthe intended frequency. Radials and a ground connection are required.You'll have to build your own antenna(s), as we note only commercialproducts intended for the radio amateur. The task is not difficult,nor is it expensive. Recommended reading is Edward Noll's Easy-UpAntennas for Radio Listeners and Hams. The Beverage antenna is one of the most intriguing antennas wehave used, but it requires a lot of space. It is a longwire oneor more wavelengths long mounted near to the ground and orientedin the direction of the desired reception. A nominal 6:1 balunis required at the juncture of the wire and the coaxial feedline.The far end is terminated with a nominal 600-ohm resistance. Theantenna is highly directional, responsive to low-angle signals,has little noise pick-up, and produces excellent signal to noiseratios. Our tests show it is most practical up to 7 MHz or so.  The antenna was invented in the early 1920s by Harold H.Beverage. It was first discussed in a paper titled "The WaveAntenna - A New Type of Highly Directive Antenna" writtenby Beverage, Chester W. Rice and and Edward W. Kellogg for thejournal of the American Institute of Electrical Engineers (Volume42, 1923, page 215 ff.). The paper discusses longwave (between25,000 and 7,000 meters -- 12-43 kHz) tests of antennas 11 kilometers(7 miles) long at Riverhead, Long Island, NY, and mentions "shortwave"tests around 450 meters (665 kHz) as a practical upper limit insubsequent experiments. While others have since written aboutthe antenna, if you can find a reprint of this original work ina research library, you'll find the paper is fascinating reading.In 1938, the Radio Institute of America presented Beverage withits Armstrong Medal for his work in the development of aerialsystems. The Beverage antenna, the citation said, was "theprecursor of wave antennas of all types." Harold Henry Beverage,Stony Brook, NY, USA, passed away on January 27, 1993, at the age of 99.
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