Four Egyptian MW Broadcast Crossed Field Antennas
Introduction
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.
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 distance l 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:
- it synthesises E and H fields to be in time-phase in the “near field”;
- it designs Zw to match space impedance Zspace.
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.
Section 2
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.
Basic CFA Design Principles
Figure 1 The basic operation of a GP CFA

Figure 1 shows the general concept of a GP CFA. Power from a transmitter is fed into a phasing unit from which two voltage feeds are taken to the respective electrodes. One feed is taken to what is called the E-plate, a hollow metal cylinder which produces curved E field lines to the GP. The other feed connection is taken to the D-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:
- both E and H are in time-synchronism; and
- the field strengths are such that Zw matches Zspace.
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 (Ampere’s Law = X H = J) or a displacement current D’ (Maxwell’s 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, Maxwell’s 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.
Two significant features of CFAs therefore arise from these design concepts.
- Wavelength independent antenna sizes
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% of l. 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.
- Minimal inductive field
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.
Ground-Wave Enhancement, Field Strength Measurements and Vertical Plane Radiation Field Patterns
Four Broadcast CFAs with Improved Ground-Wave Radiation
The standard GP CFA can be modified with the addition of extended conic sections to the E-plate (see Figure 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).

Figure 2 The addition of conic sections to the E-plate

Table 1 Details of 4 Egyptian Broadcast GP CFAs


Near Field Measurements
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.

CFA Vertical Plane Radiation Field Patterns
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.
Figure 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.


For Barnis in Figure 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.
Bandwidth Measurements and Extended Service Zones
Frequency Bandwidths
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.
Sorry about the poor quality images, but the original sitr only had low resolution files available. – MD0MDI





Extended Service Zones

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.
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:
- Increased broadcast service areas with useful signal strength
- Reduced transmitter power and capital costs thus long term reduced electricity costs – additional benefit includes longer life for transmitter components
- CFAs require no planning structure licence due to small height
- Reduced hazards for aircraft
- CFAs can be mounted unobtrusively on rooftops
- Different CFA antennas can operate in close proximity with no interference due to minimal coupling, i.e. CFAs are EMC friendly
- No tower construction
- Saving on tower maintenance such as lighting, upkeep, guys, insulators etc.
- Reduced insurance costs
- No large real-estate required
- Night-time broadcasts possible due to reduced sky-wave characteristics
- Improved safety due to lower voltage levels of CFAs
- High quality of received audio signal due to broadband characteristics
- Possible use of CFAs for higher data rate broadband digital transmissions at MW
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.
References
- Hately M C and Kabbary F M – US Patent No. 5155495, Radio Antennas
- Kabbary F M, Hately M C and Stewart B G – “Maxwell’s Equations and the Crossed-Field-Antenna”, Electronics and Wireless World, Vol 95, pp216-218, March (1989)
- Hately M C, Kabbary F M and Stewart B G – “CFA: Working Assumptions”, Electronics and Wireless World, Vol 96, pp. 1094-1099, December (1992)
- Kabbary F M, Khattab M and Hately M C
“Extremely Small High Power MW Broadcasting Antennas”, IEE International Broadcasting Conference (IBC), Amsterdam, 10-12th September (1997)
Acknowledgements
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.
Originally posted on the AntennaX Online Magazine by F M Kabbarya, M Khattaba, B G Stewartb, M C Hatelyc and A Fayoumia
Last Updated : 3rd May 2024