Building a better radiator
Mar 1, 2003 12:00 PM, By John Battison, technical editor, RF
FM antennas by virtue of their transmission technology require medium to tall towers. AM antennas vary in height according to frequency requirements. Both suffer from public dislike. As public rejection of tall towers has increased, radio engineers have been paying more and more attention to AM antenna design requirements in an effort to develop shorter AM antennas that are as efficient as their taller brethren.
The Kinstar test antenna on 1,680kHz.
It is unfortunate that the antenna structure � the last item in the transmission chain � can have the greatest influence on coverage. All the other stages of the chain are controlled by the broadcast engineer, but today John Q. Public is putting in his ten cents worth of problems and frequently tries to control the height and location of broadcast antenna structures. Luckily, there are some choices for a broadcaster who wants to install or modify an AM antenna structure. While almost any metallic structure can be made to radiate if properly driven, good engineering practice requires specifically designed radiators.
The quarterwave tower is probably the engineer's favorite. When used as a folded unipole, series fed or shunt fed, it offers good efficiency, easy mathematics and a reasonable price. Often lighting is not required. It requires a ground system whose diameter is about twice the height of the tower. Quite often the ideal place for a tower is unavailable because of a tower height restriction or a ground system requirement that calls for too much space.
If, for engineering design and station coverage reasons, higher radiation efficiency is required a tower might go as high as 5/8 of a wavelength. The days of building a new Franklin tower or other anti-fading design are over because opposition from anti-tower factions flares up at the thought of a tower that might be one wavelength tall. This is another factor that has to be considered when seeking maximum ground-wave coverage with freedom from fading.
Detail of the EH antenna phase-shift network.
The tower itself can affect signal quality. Efforts to use a smaller cross-section tower in an attempt to reduce visibility and appease the anti-tower crowd could result in a comparatively skinny tower with a high Q, narrowband response and consequently poor audio. In the case of a directional station, these effects could be more pronounced, especially in areas of tight nulls.
Efforts have been made to develop shorter, efficient AM radiators. Over the years many engineers have worked on the problem of reducing the size of AM antennas and also attempting to control skywave radiation. A number of years ago, the late Oggie Prestholdt, PE, who was CBS's top RF engineer, retired and joined George Adair's firm of consulting engineers. He designed and built an experimental controlled skywave antenna. Unfortunately, this did not quite perform as planned, possibly due to the vagaries of skywave transmission.
There have been other attempts to reduce the physical size and skywave radiation of AM antennas, but not a great deal of success has been achieved. More recently the CFA and the EH antennas have received a considerable amount of publicity, but so far neither has received full FCC approval. Elaborate plans were made to test the CFA antenna at a site in Shropshire, England. Ben Dawson, PE, and several other professional engineers had planned to make comprehensive field test measurements of this antenna's performance. Unfortunately construction of the test site has been indefinitely delayed. However, Dawson said that another higher power site on the Isle of Man, which had been initially denied, has now been approved. With hope, these tests will eventually be made.
The latest development in the area of low-profile antennas is the Kinstar, invented by Dr. James Breakall of Pennsylvania State University, and is being developed by Tom King of Kintronics. The design has been tested at scale frequencies of 1.3GHz, 440MHz and 52MHz. A full-scale model was erected on 1,680kHz at the Kintronics antenna test site, and the results showed excellent agreement with theory and the scale-model tests.
The EH antenna as it was being assembled.
Preliminary tests showed that the antenna's efficiency was close to the commission's requirement of 313.6mV/m for a quarterwave antenna. King said that the vertical radiation characteristic is the same as that of a quarterwave antenna and that the Commission has accepted the Kinstar for broadcast use. At present, a proof-of-performance similar to that for a DA proof is required.
The Kinstar concept is really quite simple and I'm sure that a lot of engineers are kicking themselves � myself included � for not thinking of it themselves.
The dimensions are frequency-sensitive; how-ever, even at the low-end of the AM band, a height of only 144 feet is required. This should also eliminate lighting requirements in most cases.
Four telephone poles, placed in a square with 200ft sides, support a cage consisting of four vertical and four horizontal elements. A fifth telephone pole in the center of the square supports the intersection of the four horizontal elements. There are four vertical radiators, each with top loading sections that intersect, but are insulated from each other at the center of the square.
The vertical elements are insulated from the supporting pole and are series fed. The purpose of the poles is merely to suspend this cage. Presumably a metal mast could be used in each corner, which could also form the vertical radiating element, and the horizontal top loading conductor could be attached directly to the top of this mast.
Each of the four radiators is excited in phase. A quarterwave section of regular transmission line is used to obtain the necessary phase shift and impedance transformation The whole assembly forms a radiator with a low Q, broad bandwidth and excellent VSWR. The radiation pattern is similar to that of a quarterwave monopole. The fields produced by the vertical radiators add in phase, and the currents in the horizontal top loading elements are out of phase and cancel out.
For the purpose of the test, a standard 120-radial, quarterwave ground system was used.
Other forms and functions
A new AM station in Ireland that has experienced problems obtaining clearance for a relatively short tower is considering the use of a Delta antenna. This is supposed to result in reduced height and provide adequate base operating impedance. The proposed antenna consists of two short masts supporting a horizontal cage antenna. Each end of the cage is connected to a sloping cage. These two cages meet at an insulated point on the ground midway between the two supports. This forms the driving point for the antenna. Unfortunately, I have been unable to obtain further information at this time on the application.
Ted Hart, the inventor of the EH antenna, is about to conduct tests on his antenna design with a radiator on 1,520kHz in Eatonton, GA, about 85 miles southeast of Atlanta. The construction of this antenna is interesting. In some shots it looks almost like a vertical dipole � and in fact it is.
The inventor states that a ground system is not required and that ground wave radiation is a function of the height of the antenna and that more power is required as the antenna's height is reduced. With a proper phase-shift network between the antenna elements, the familiar Hertz dipole antenna will function as an EH antenna. This requires a 90-degree phase delay between the current and voltage applied to the antenna so that E and H fields are in phase.
Hart will have more information available on his website at www.eh-antenna.com as his tests continue.
Up the dial
FM station towers are subject to the same harassment as AM broadcasters. Fortunately, the actual FM antenna does not require the support to be part of the radiator. Therefore, it is possible to mount an antenna on an existing tall building, or even add a mast to such a building, and obtain the required height.
There is a wide selection of FM antennas. The wind loading produced by various types frequently plays a large part in antenna choice. Antenna location, gain, directivity, side lobes and sometimes vertical beam width usually determine the choice of antenna.
The FM panel antenna is versatile and probably produces the best circular pattern when properly installed. It can also be useful when directional patterns are required. In many locations multipath has to be dealt with and antenna choice can become critical. Vertical beam width, null fill and antenna gain are interactive and great care is required in balancing these factors.
With the deployment of IBOC taking its first steps, a station's antenna will move to a higher level of importance. The tests on new AM designs show promise of a reduction in the physical space required. On the FM side, issues of using separate antennas have been discussed and so far disallowed by the FCC. A middle ground has been explored by feeding analog and digital carriers in a panel-antenna combiner with good results.