The development of classic HF curtain arrays intoeffective and versatile HF transmitting antennas,which we explored in WRTH 2019, continued withmodifications for many years, and they remainthe backbone of HF transmission. They are, how-ever, expensive to build and maintain and requirea large plot of land. From the early 1930s variouscompanies worked on designs that would be lessexpensive, have a smaller footprint and, in somecases, be mobile.One of these was masterminded by the
American inventor and industrialist PowelCrosley. This design, the Rhombic Antenna, wasfirst used by AT&T for radio transmission of tele-phone calls to islands in the Pacific but, followingPearl Harbor, Voice of America used Rhombicsfor HF transmissions to the US forces fightingoverseas. By the end of World War 2 the VOA siteat Bethany had six senders and 24 re-entrantRhombic antennas.
THE RHOMBIC ANTENNAIn this antenna, one to three parallel wires aresuspended above the ground in a diamond shapesupported by poles and insulators at each corner
or vertex. Each of the four sides is the samelength. As the wires are horizontal to the oppositewire, the waves are horizontally polarised andradiate off the end of the antenna in the oppositedirection to the feedline. Typically they have a lowelevation angle of 7° to 10°. and thus are DXantennas. They have a high forward gain and awide bandwidth and can operate over a largerange of frequencies. They are typically fed at oneof the two sharper vertices, at the top or bottomof the diamond, via a balanced transmission line.The ends of the wires at the opposite vertex canbe left open which makes the antenna bi-direc-tional with, for example, beams on 114° and294°. It was found that if a resistive load is con-nected to the wires at the opposite vertex thenthe same antenna has a one-directional beamon, say, 114°. This increased the gain and pre-vented the RF going in the reverse direction, butit created a problem for high power broadcastuse, however, as up to 50% of the hard-won RFpower can be lost in the load. The solution was toemploy a re-entrant system where a resonantlength stub of transmission line was used in placeof the resistance. This preserved the gain, saved
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The Development of HFBroadcast Antennas
Former BBC Senior Transmitter Engineer Dave Porter G4OYX continues the story of thedevelopment of HF broadcast antennas from curtain arrays to Allis antennas
the 50% power loss, but made the Rhombic fre-quency-sensitive, consequently losing the wide-bandwidth feature. The available bandwidthdepends on the length of the wire and, using dif-ferent lengths of transmission line, it is possible toaccess two or three different broadcast bands.A typical rhombic antenna design uses side
lengths of several wavelengths and is at a heightof between 0.5-1.0λ at the middle of the operat-ing frequency range.
By 1951, in order to achieve the requiredbearings at Bethany, three more re-entrant or fre-quency-critical Rhombics were erected side-by-side resulting in 27 antennas. These antennaswere cheap to make and maintain and so, despitecovering quite a large area, continued to be usedat other Voice of America and post-war Radio
Free Europe and Radio Liberty sites.Rhombic antennas are no longer recommend-
ed for HF broadcasting as the main lobe is nar-row in both horizontal and vertical planes whichcan result in the required service area not beingreliably covered because of the variations in theionosphere. There are also a large number ofside lobes of a size sufficient to cause interfer-ence to other broadcasters, and a significant pro-portion of the transmitter power is dissipated inthe terminating resistance.
THE CORNER QUADRANT ANTENNAPost War it was found that if the RhombicAntenna was stripped down and, instead of thefour elements, had just two end-fed half-wavedipoles placed at a right angle to each other (asshown in Fig. 1) the result was a simple cost-effective antenna which had properties similar tothe re-entrant Rhombic but with a much smallerfootprint. Quadrant antennas may also bestacked to achieve a more directive vertical radi-ation pattern and a higher directivity gain. As inthe classic HRR441 curtain array, the height ofthe lowest dipole above ground in wavelengths atthe design frequency is used to select DX ornearer coverage areas. For near-range broad-casting, which requires high angle radiation, thesuspension height of the antenna above groundshould not be more than 0.4λ and, for reasons ofantenna gain, not lower than 0.25λ. As the effectof ground conductivity also needs to be consid-ered, a suspension height of 0.3λ is typical. These antennas were used by Deutsche
Welle at Jülich and Wertachtal for transmitting
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Corner Quadrant Antenna at Moosbrunn_(Ulrich Eitler -CC BY-SA 4.0 httpscreativecommons.orglicensesby-sa4.0)
An early 1930s AT&T Rhombic Antenna at Dixon, California
Fig. 1 Corner Quadrant design(from ITU-R BS.705-1, courtesy of the ITU)
fixed-bearing designs whereas others are rotat-able; allowing great flexibility in both frequencyand bearing. Log Periodics generally have hori-zontal radiating elements but in some cases theyare mounted on a horizontal boom, with theundesirable result that the pattern of the now ver-tical elements shows an increased number oflobes as the operational frequency increases.Log Periodics were used by many stations by the1970s, including BBC Rampisham, but they weredropped as alternative designs such as the TCI611 extra-wide slewable array were developed.A novel rotatable Log Periodic was in use at
ORF Moosbrunn. In this instance, instead of theelements being driven in parallel and attached toa central transmission line, they were driven inseries with adjacent elements connected at theouter edges. The benefits of this are unclear andit was likely simply experimentation by the mana-ufacturer. ORF Moosbrunn can also lay claim toan early, totally rotatable curtain array where thesupport masts were mounted on a circular rail-way and could be moved around to any point toset the precise bearing required.
ALLISS OR RIGID CURTAIN ANTENNAARRAYIn the late 1980s at Allouis and Issodun in Francea revolutionary development was pioneered bythe French manufacturer Thomcast. Originallycalled Alliss, after the sites, it is now more com-monly known as the Ampegon Rigid CurtainAntenna Array. It combines the advantages ofclassic wideband curtain arrays with the ability tofully rotate the system, and a small footprint. Thesupporting structure for a rotatable curtain array
can either be two towers with the antenna sus-pended in between or a single rotating tower witha substructure for the antenna. In this antenna ahigh-band HF assembly from 13MHz to 26MHz,and a low band 6MHz to 12MHz assembly, are fit-ted on either side of a substantial vertical tubularshaft. An aperiodic screen is fitted between thetwo assemblies as the reflector. The number of dipole columns defines the
azimuth beamwidth. For a 2-wide dipole array, thebeamwidth is around 50° whereas for a 4-widedipole array it is around 30°.The number of dipolerows and the height of the lowest element aboveground determine the elevation angle, and con-sequently the distance of the service area. A 2-row high array has a typical take-off angle of 20°and is used for medium range communications,while a 4-row high array has a typical take-offangle of 10° and is used for DX communications.The folded half wave and rigid steel dipoles
are an integral part of the construction. The sup-port shaft incorporates motors to rotate the shaftto any bearing. This antenna can be free standingor, in the case of the 11 installations at Issoudun,a 500kW HF transmitter is contained in a buildingincorporating the support shaft. It forms thereforea self-contained, single transmitting station.
CONCLUSIONIt is unlikely that further major developments inHF antennas will occur. The rotatable RigidCurtain Antenna Array is probably the apogee ofHF transmitting antennas, as it combines all theelements that broadcasters have been searchingfor since the advent of the classic wire curtainarray at Daventry, UK in 1934.
over a radius of up to 1300 km (800 miles) to, forexample, the United Kingdom and Ireland on6MHz. Quadrant Antennas are band specific, buthave a small footprint and a minimal height whichcontributes to their lower costs.The need remained, however, for wider fre-
quency coverage in an antenna that retained thesmall footprint and compact design characteris-tics of the Quadrant Antenna. In response to thisthe Log Periodic Array, or Log Periodic DipoleArray, was invented by Dwight Isbell andRaymond DuHamel at the University of Illinois in1958, and marketed by the US company TCI.
THE LOG PERIODIC ARRAYLog Periodics are essentially Yagi antennas inthat a selection of the elements are the directors,there is a resonant dipole at the specific frequen-cy, and a reflector at the rear of the antenna.These antennas provide wide frequency cover-age with a gain that is comparable to that of theRhombic antenna. The broad bandwidth allows asingle antenna to work from 6MHz to 26MHz,with one design being from 2MHz to 26MHz.Power handling can be up to 500kW and with again of about 66% to 75% of a standard HRR441wire curtain antenna. Some Log Periodics are
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A TCI Type 611 array at the BBC/VOA site at Woofferton for 9, 11, 13, 15 and 17MHz (Jeff Cant)
Log Periodic Array at Yavne (Voice of Hope)Allis or Rigid Curtain Antenna Array
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FEATURES Development of HF Broadcast Antennas
