The Development of HF Broadcast Antennas
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Development of HF Broadcast Antennas FEATURES FEATURES Development of HF Broadcast Antennas the 50% power loss, but made the Rhombic fre - Free Europe and Radio Liberty sites. quency-sensitive, consequently losing the wide- Rhombic antennas are no longer recommend - The Development of HF bandwidth feature. The available bandwidth ed for HF broadcasting as the main lobe is nar - depends on the length of the wire and, using dif - row in both horizontal and vertical planes which ferent lengths of transmission line, it is possible to can result in the required service area not being Broadcast Antennas access two or three different broadcast bands. reliably covered because of the variations in the A typical rhombic antenna design uses side ionosphere. There are also a large number of lengths of several wavelengths and is at a height side lobes of a size sufficient to cause interfer - Former BBC Senior Transmitter Engineer Dave Porter G4OYX continues the story of the of between 0.5-1.0 λ at the middle of the operat - ence to other broadcasters, and a significant pro - development of HF broadcast antennas from curtain arrays to Allis antennas ing frequency range. portion of the transmitter power is dissipated in the terminating resistance. THE CORNER QUADRANT ANTENNA Post War it was found that if the Rhombic Antenna was stripped down and, instead of the four elements, had just two end-fed half-wave dipoles placed at a right angle to each other (as shown in Fig. 1) the result was a simple cost- effective antenna which had properties similar to the re-entrant Rhombic but with a much smaller footprint. Quadrant antennas may also be stacked to achieve a more directive vertical radi - ation pattern and a higher directivity gain. As in the classic HRR441 curtain array, the height of the lowest dipole above ground in wavelengths at the design frequency is used to select DX or Fig. 1 Corner Quadrant design nearer coverage areas. For near-range broad - (from ITU-R BS.705-1, courtesy of the ITU) casting, which requires high angle radiation, the By 1951, in order to achieve the required suspension height of the antenna above ground bearings at Bethany, three more re-entrant or fre - should not be more than 0.4 λ and, for reasons of quency-critical Rhombics were erected side-by- antenna gain, not lower than 0.25 λ. As the effect side resulting in 27 antennas. These antennas of ground conductivity also needs to be consid - were cheap to make and maintain and so, despite ered, a suspension height of 0.3 λ is typical. covering quite a large area, continued to be used These antennas were used by Deutsche at other Voice of America and post-war Radio Welle at Jülich and Wertachtal for transmitting An early 1930s AT&T Rhombic Antenna at Dixon, California The development of classic HF curtain arrays into or vertex. Each of the four sides is the same effective and versatile HF transmitting antennas, length. As the wires are horizontal to the opposite which we explored in WRTH 2019, continued with wire, the waves are horizontally polarised and modifications for many years, and they remain radiate off the end of the antenna in the opposite the backbone of HF transmission. They are, how - direction to the feedline. Typically they have a low ever, expensive to build and maintain and require elevation angle of 7° to 10°. and thus are DX a large plot of land. From the early 1930s various antennas. They have a high forward gain and a companies worked on designs that would be less wide bandwidth and can operate over a large expensive, have a smaller footprint and, in some range of frequencies. They are typically fed at one cases, be mobile. of the two sharper vertices, at the top or bottom One of these was masterminded by the of the diamond, via a balanced transmission line. American inventor and industrialist Powel The ends of the wires at the opposite vertex can Crosley. This design, the Rhombic Antenna, was be left open which makes the antenna bi-direc - first used by AT&T for radio transmission of tele - tional with, for example, beams on 114° and phone calls to islands in the Pacific but, following 294°. It was found that if a resistive load is con - Pearl Harbor, Voice of America used Rhombics nected to the wires at the opposite vertex then for HF transmissions to the US forces fighting the same antenna has a one-directional beam overseas. By the end of World War 2 the VOA site on, say, 114°. This increased the gain and pre - at Bethany had six senders and 24 re-entrant vented the RF going in the reverse direction, but Rhombic antennas. it created a problem for high power broadcast use, however, as up to 50% of the hard-won RF THE RHOMBIC ANTENNA power can be lost in the load. The solution was to In this antenna, one to three parallel wires are employ a re-entrant system where a resonant suspended above the ground in a diamond shape length stub of transmission line was used in place supported by poles and insulators at each corner of the resistance. This preserved the gain, saved Corner Quadrant Antenna at Moosbrunn_(Ulrich Eitler -CC BY-SA 4.0 httpscreativecommons.orglicensesby-sa4.0) 24 www.wrth.co m www.wrth.co m 25 Development of HF Broadcast Antennas FEATURES FEATURES Development of HF Broadcast Antennas Log Periodic Array at Yavne (Voice of Hope) Allis or Rigid Curtain Antenna Array over a radius of up to 1300 km (800 miles) to, for THE LOG PERIODIC ARRAY example, the United Kingdom and Ireland on Log Periodics are essentially Yagi antennas in fixed-bearing designs whereas others are rotat - can either be two towers with the antenna sus - 6MHz. Quadrant Antennas are band specific, but that a selection of the elements are the directors, able; allowing great flexibility in both frequency pended in between or a single rotating tower with have a small footprint and a minimal height which there is a resonant dipole at the specific frequen - and bearing. Log Periodics generally have hori - a substructure for the antenna. In this antenna a contributes to their lower costs. cy, and a reflector at the rear of the antenna. zontal radiating elements but in some cases they high-band HF assembly from 13MHz to 26MHz, The need remained, however, for wider fre - These antennas provide wide frequency cover - are mounted on a horizontal boom, with the and a low band 6MHz to 12MHz assembly, are fit - quency coverage in an antenna that retained the age with a gain that is comparable to that of the undesirable result that the pattern of the now ver - ted on either side of a substantial vertical tubular small footprint and compact design characteris - Rhombic antenna. The broad bandwidth allows a tical elements shows an increased number of shaft. An aperiodic screen is fitted between the tics of the Quadrant Antenna. In response to this single antenna to work from 6MHz to 26MHz, lobes as the operational frequency increases. two assemblies as the reflector. the Log Periodic Array, or Log Periodic Dipole with one design being from 2MHz to 26MHz. Log Periodics were used by many stations by the The number of dipole columns defines the Array, was invented by Dwight Isbell and Power handling can be up to 500kW and with a 1970s, including BBC Rampisham, but they were azimuth beamwidth. For a 2-wide dipole array, the Raymond DuHamel at the University of Illinois in gain of about 66% to 75% of a standard HRR441 dropped as alternative designs such as the TCI beamwidth is around 50° whereas for a 4-wide 1958, and marketed by the US company TCI. wire curtain antenna. Some Log Periodics are 611 extra-wide slewable array were developed. dipole array it is around 30°.The number of dipole A novel rotatable Log Periodic was in use at rows and the height of the lowest element above ORF Moosbrunn. In this instance, instead of the ground determine the elevation angle, and con - elements being driven in parallel and attached to sequently the distance of the service area. A 2- a central transmission line, they were driven in row high array has a typical take-off angle of 20° series with adjacent elements connected at the and is used for medium range communications, outer edges. The benefits of this are unclear and while a 4-row high array has a typical take-off it was likely simply experimentation by the mana - angle of 10° and is used for DX communications. ufacturer. ORF Moosbrunn can also lay claim to The folded half wave and rigid steel dipoles an early, totally rotatable curtain array where the are an integral part of the construction. The sup - support masts were mounted on a circular rail - port shaft incorporates motors to rotate the shaft way and could be moved around to any point to to any bearing. This antenna can be free standing set the precise bearing required. or, in the case of the 11 installations at Issoudun, a 500kW HF transmitter is contained in a building ALLISS OR RIGID CURTAIN ANTENNA incorporating the support shaft. It forms therefore ARRAY a self-contained, single transmitting station. In the late 1980s at Allouis and Issodun in France a revolutionary development was pioneered by CONCLUSION the French manufacturer Thomcast. Originally It is unlikely that further major developments in called Alliss, after the sites, it is now more com - HF antennas will occur.