FOUR EGYPTIAN MW BROADCAST CROSSED-FIELD-ANTENNAS
|aEgyptian Radio and TV Union||bDept of Engineering||cHately Antenna Technology|
|Cairo||Glasgow Caledonian University||Aberdeen|
|Egypt||Glasgow, Scotland, UK||Scotland, UK|
(A Paper Presentation at NAB99 ~ Reprint by Permission)
Crossed-Field-Antennas (CFAs) are novel, small, broadband, high power antennas commonly less than 2 to 3% ofl in height. Currently there are a number of MW broadcast CFAs in service in Egypt. Information relating to four of these broadcast antennas is presented. The paper details: the basic CFA design principles which result in their novel size-wavelength independent nature; near field measurements showing the existence of minimal induction field; vertical plane radiation field patterns; evidence of strong ground-wave and diminished sky-wave radiation; input impedance and bandwidth evaluations of the four CFAs showing their broadband frequency characteristics; and finally, advantages and benefits of CFAs over conventional MW and/or LW antennas.
Crossed-Field-Antennas (CFAs) originated around 1988 at the Robert Gordon University in Aberdeen, Scotland. 1,2,3 These antennas derived from a research project, the main aim of which was to develop a technique to synthesise directly radiated Poynting vectors from separate E and H field sources. Over the past few years CFAs have been built and put into service for MW broadcasts by the Egyptian Radio and Television Union (ERTU).4 To enable an appreciation of the novelty, design and benefits associated with CFAs as compared to standard antennas it is helpful first to review some important features of conventional antenna theory.
Broadcast and antenna engineers will appreciate that effective medium and long wave transmissions are possible with tower antennas that arel/4 to l/2 in size. For the MW and LW bands this often results in antennas of significant height. For example at 1600kHz, a l/4 antenna tower is about 46m (150ft); at
600kHz, al/4 tower is about 125m (406ft). Not only are such towers expensive to manufacture, install and maintain, but they also introduce a significant hazard in relation to electromagnetic safety due to the substantial resonant voltages and currents flowing on the antenna structures.
A further issue relating to conventional antenna theory concerns radiated power. It is well known that radiated power from a dipole or tower antenna has low efficiency. The radiated power for these antennas occurs in the "far field" (generally thought of as the region extending beyond a distancel from the antenna). In the far field, the E and H fields are in time-phase, and the ratio E/H, often called the wave impedance Zw, matches space impedance Zspace = 377W. In this region the Poynting vector S = E X H produces real power radiation. Two key points also arise in this respect. Firstly, the strong E and H fields in the "near field" are 900 out of time-phase close to the antenna resulting in reactive or non-radiated power in the the near vicinity of the structure. Secondly, the E and H fields in the far field, which produce the radiated power, are much weaker than the reactive field components located in the near field. These details explain why conventional antennas possess large inductive fields and are not efficient radiators.
What then is the CFA? To put it simply, the CFA is an antenna which achieves the following features:
In other words, a CFA is fundamentally an antenna which is designed to move the radiated power production from the conventional far field region to the near field, thus saving land and minimising the reactive power or inefficiency problems associated with standard antenna designs.
The content of this paper is as follows. Section 2 introduces the essential design concepts of Ground Plane (GP) CFAs outlining the basic techniques underpinning Poynting vector synthesis. Section 3 discusses the improvement to the basic design of GP CFAs for ground-wave broadcasting purposes through the addition of extended cones, and details four MW broadcast CFAs currently in daily service in Egypt. "Near field" measurements on a broadcast CFA are presented showing the non-inductive capabilities of these antennas and thus indicating their high radiation efficiency. In addition, vertical plane radiation patterns for two CFAs are presented showing the relationship between ground-wave and sky-wave radiation. Section 4 presents wide-band input inpedance measurements of all four antennas and discusses the extended zone broadcast capabilities of CFAs. The final section, 5, presents a general summary of the advantages of CFAs over conventional MW and LW antenna towers.
The fundamental principle underpinning CFA design is that electric and magnetic fields are produced from separate field stimuli, or field electrodes, and crossed-stressed inphase within a small volume, called the interaction zone, close to the CFA structure.
Fig. 1 The basic operation of a GP CFA
plate, a circular metal disk, which in conjunction with the GP forms a parallel-plate capacitor. The time varying electric field lines between the D-plate and the GP produce H field lines around the capacitor as shown in Fig. 1. This induced H field now links with the E field from the cylinder to produce significant power radiation when the following conditions are met:
The fact that a time-varying electric field creates a magnetic field is a well known phenomenon. The 4th Maxwell equation, viz.
=X H = J + D
indicates that a magnetic field is created from either a charge current J (Amperes Law= X H = J) or a displacement current D (Maxwells Law = X H = D) or from both J and D together (note that D = e0E, and represents time derivative). To help appreciate the magnetic field production nature of a time-varying D field creating an H field, Maxwells 4th equation (omitting the charge current component) may be expressed in the reversed Maxwell Law form: 2
i.e. D creates an H field such that the curl of the H field is equal to D. The function of the D-plate is now self-evident. In addition, from the Maxwell Law, when a sinusoidal voltage is applied to the D-plate, the created H field close to the plate is 900 phase advanced from D field. To achieve radiated power the D-plate voltage must therefore be 900 phase advanced from the E-plate voltage for time synchronism of the fields and for outward S = E X H to occur.
With the above information the role of the phasing unit now becomes clear. Firstly, it provides the 900 phase difference between the voltage feeds on the E and D-plates to provide E and H in time-phase within the interaction zone, and secondly, it controls the voltage levels on the plates in order that Zw is able to match Zspace. When these condition are met then effective Poynting vector synthesis, i.e. of S = E X H, is achieved and radiated power flows from the interaction zone outward into free space.
It is important to emphasise that as a consequence of this design methodology, i.e. Poynting vector synthesis, CFAs are not resonant antennas like conventionall/4 or l/2 antennas.
Two significant features of CFAs therefore arise from these design concepts.
Firstly, the synthesis of E and H does not depend critically on CFA size thus CFAs can be made extremely small in comparison with the desired radiated wavelength. As will be seen below it is not uncommon for CFA heights to be less than 2 or 3% ofl. In other words size of the CFA is not wavelength dependant as conventional antenna theory stipulates. The only stipulation on size arises as a consequence of the power requirements of the CFA as necessitated by power engineering criteria.
Secondly, when the time-phase and space impedance conditions are satisfied, there is minimal inductive field around the CFA. The reason is obvious - the field energy in the interaction zone has been designed directly to provide radiated and not reactive power. This can be contrasted with the significant inductive fields from standard conventional antenna structures.
The standard GP CFA can be modified with the addition of extended conic sections to the E-plate (see Fig. 2)4. These extensions have the effect of confining the curved E field lines in the interaction zone to low angles, such that Poynting vectors produced from the interaction zone are now limited to lower radiation angles. The intended outcome of this arrangement is to produce a significant increase in ground-wave radiation accompanied by a highly desirable decrease in sky-wave radiation.
Four main broadcast CFAs with extended conic sections are now in daily operation in Egypt. Table 1 details basic broadcast information for these antennas, including their location, power level, frequency, CFA height and also the CFA height as a % of the radiated wavelength. Photographs of the two Tanta CFAs and the Barnis CFA showing the extended conic sections are given in Figs. 3 and 4. It may be noted from Fig. 3 that the two Tanta CFAs have been positioned on the rooftop of the same building separated by about 6m (19.5ft) (see later).
Fig. 2 The addition of conic sections to the
Table 1 Details of 4 Egyptian Broadcast GP CFAs
Fig. 3 The 100kW and 30kW Tanta CFAs situated
Fig. 4 The 100kW Barnis CFA
3.2 Near Field Measurements
To investigate near field characteristics of broadcast CFAs, field strength measurements (at reduced power) were taken at near ground level on the 30kW Tanta CFA. These measurements were obtained with a Potomak field strength meter over distances from 25m to 300m. The results are shown in Fig. 5. For comparison, the effective 1/r2 field strength values expected from inductive fields is also plotted on the same figure. The CFA shows approximate 1/r proportionality in the near field there is no sign of the inverse square law proportionality within the firstl/p as associated with the inductive field of a classical dipole antenna. The CFA therefore exhibits very little inductive field in its close proximity.
The significance of this result has resulted in the ERTU recently constructing the 100kW Tanta CFA and positioning it approximately 6m (19.5ft) from the 30kW CFA on the rooftop of the same building as pictured in Fig. 3. There is no evidence of inductive coupling between these antennas, and both operate independently and efficiently without interference.
Measured voltages on the E and D-plates of CFAs also show that voltage levels are about 1/6th of those on conventional broadcast antennas carrying the same input power. This feature is again indicative of the non-resonant like behaviour of CFAs. These reduced voltage levels also provide a safer environment near CFA structures.
Fig. 5 Near field CFA measurements on the 30kW Tanta
Measurements of the vertical plane radiation field patterns of the 30kW Tanta and the 100kW Barnis CFAs have also been taken. Fig. 6 shows the relative vertical plane radiation field pattern of the Tanta CFA. Measurements were taken at a distance of about 610m (1980ft) (using a nearby tall TV tower) utilising an RF meter. Fig. 7 displays the relative vertical plane pattern of the 100kW Barnis CFA, measured at a distance of about 70m (228ft) to a height of around 37m (120ft) using a kite floating a battery powered RF meter. Unfortunately vertical elevation angles of less than about 300 were not measured at Barnis as a consequence of the limited height restrictions on the kite. However, the plot shows expected interpolated values (dotted line) consistent with what might be expected in relation to the nature of the Tanta CFA pattern.
Fig. 6 shows that a significant proportion of the radiated power goes into ground-wave radiation. For example, the field strength at an elevation angle of about 200 is approximately 0.32 that of the ground-wave strength, indicating that the radiated power at this angle is close to 10% (i.e. 0.322) of the ground-wave power. At higher elevations, the radiated power is seen to be less than 10%. The Tanta CFAs broadcast to residential populations across a region of 100km 250km over land based soil, which produces little attenuation of the ground-wave. These service areas are therefore constantly provided with strong signal strength broadcasts.
Fig. 6 Tanta 30kW relative vertical planeradiation field pattern measured in the vertical direction at a distance of 610m
For Barnis in Fig. 7 at elevation angles greater than 300, the sky-wave radiated power is less than about 40% of the ground-wave power. There is clearly a difference in the radiation patterns between the Tanta and Barnis CFAs, arising for a number of reasons. For example, different heights and angular conic sections on the antennas plus different sizes and separations of the D-plate. These contribute to a variation of the interaction zone field geometries and thus variations in the radiation field patterns. It should also be commented that the Barnis CFA is situated in a region of dry desert, which introduces attenuation on the ground wave thus resulting in what may be expected as a different radiation characteristic pattern than Tanta in the extreme far field.
The bandwidth of an antenna is usually presented in terms of input impedance and/or SWR measurements. A fascinating feature of CFAs is that the input impedance to the antenna can always be adjusted to match any desired input impedance at the required broadcast frequency. Using an HP Network Analyser attached to the input of the phasing unit, Smith Charts were obtained for the four broadcast CFAs and these are presented in Figs. 8-11. Table 2 details the bandwidth frequencies and % frequency bandwidths (i.e. bandwidth/broadcast frequency) assuming an SWR of 2:1 side-band down points. It can be seen that all CFAs show remarkable broadband characteristics. In all cases the bandwidths accommodate easily the AM audio spectrum and beyond. If the SWR were to be extended to a conservative 3:1 then it will be obvious that the bandwidths will increase beyond that which can be determined from the Smith Charts presented here. For all CFAs, % bandwidths based on this premise will extend well beyond 10%. In addition, due to the large bandwidth requirements of digital transmissions, these results indicate that it should be possible to transmit higher data rate digital signals at MW using CFAs.
Fig. 8 Smith Chart for the 30kW Tanta CFA
Fig. 10 Smith Chart for the 100kW Barnis CFA
Fig. 11 Smith Chart for the 7.5kW Halaieb CFA
Table 2 SWR 2:1 CFA Bandwidth evaluations
4.2 Extended Service Zones
Table 3 BBC reception reports on 30kW Tanta
An interesting feature is that the signal strengths were reported fair-to-strong during both morning and daytime. These simple checks further evidence the significant radiated ground-wave radiation and
diminished sky-wave radiation possible with broadcast CFAs, and thus show the extended broadcasting capabilities of these antennas. The fair signal strength report in the evening and the associated poor/fair reception report is due to interference from other broadcast stations operating on 1161kHz e.g. Moscow (at a power of 1MW) and Sofi (power 600kW). It is important to note that these stations have no influence during the daytime.
Recent reports have also indicated it is possible to receive reasonable audible signal levels from the 7.5kW Halaieb CFA during daytime in Khartoum, a distance of approximately 600km (375miles) south from Halaieb over desert and land.
5.0 CONCLUSIONS AND ADVANTAGES OF CFAs
CFAs are small, compact, high power radiation antennas. The construction of these antennas is radically different from conventional antenna techniques due to the fact that by their very nature they are designed to synthesise radiated power in a small interaction zone surrounding the antenna structure. CFAs appear to have minimal induction field, as measured and also evidenced by the fact that two CFAs located 6m (19.5ft) apart on the same rooftop do not interfere. They also possess superior bandwidths in relation to conventional MW antennas, and show vertical plane radiation patterns which exhibit strong ground-wave and reduced sky-wave characteristics.
Taking all the above features into account a number of distinct operational benefits and advantages of CFAs may be summarised as follows:
As a consequence of the success of CFAs, the ERTU have now decided to replace all conventional MW and LW broadcast antennas with CFA systems over the years ahead.
US Patent No. 5155495, Radio Antennas
"Maxwells Equations and the Crossed-Field-Antenna", Electronics and Wireless World, Vol 95, pp216-218, March (1989)
"CFA: Working Assumptions", Electronics and Wireless World, Vol 96, pp. 1094-1099, December (1992)
We would like to thank Miss Heba Said Mohammed for her help on the field and antenna measurements and also the BBC for their assistance in securing the reception checks at Nicosia in Cyprus.