Radio Antennas

This invention relates to antennas for the transmission and reception of radio waves for telecommunications, broadcasting sound and television, radar, satellite communications and the like.

Known antennas usually have a single feeder connected to either a single conductor element of approximately half a wavelength, or to a single driven element within a group of parasitic elements as in the Yagi-Uda array. By means of added reactive components such as inductors, end capacitors, resonant traps and such, antennas have been constructed with somewhat smaller dimensions than the basic half wavelength element. Loop antennas are also known and are useful in direction finding. However most antennas of reduced dimensions have disappointing transmission efficiency due to the necessarily increased circulation currents which cause large conductor losses and or magnetic core losses.

The Poynting Theorem states that for any superimposed electric and magnetic fields there must be energy flowing in the medium and thus the phenomenon of radio wave propagation has been explained in the

presently accepted theory as the radiation of electromagnetic energy in the form of an electric field E and a magnetic field H in a cross-product Poynting vector E x H = S watts per metre squared. The perpendicular geometric relationship and the time synchronism implied by the above formula must be produced by any antenna which is to radiate efficiently. Presently known antennas are probably achieving the requirements in an uncontrolled or accidental manner.

Due to extended physical dimensions and high location above the ground, it is probable that there is fortuitously provided in the large volume of space a means of setting the necessary perpendicularity and simultaneity as well as a degree of rotationality for the fields, although the absence of these conjectures from the present texts ought not to be used to condemn the validity of the concept. From the large surrounding and lightly stressed volume the comparatively weak Poynting vector progresses outwards to infinity.

According to this invention there is provided a radio antenna in which the electromagnetic waves are synthesised or captured in a small volume by two separately fed electrode systems, one of which produces the high frequency electric field, and the other of which produces the high frequency magnetic field, the said

electrode systems each having a feeder conducting a part of the power to cross stress a common interaction zone of both fields in order to create an intense radio wave source from which electromagnetic waves radiate.

In the present invention the paramount objective adopted in the design is to synthesise and launch an intense Poynting vector from a very small volume which may be less than 1/lOOth of a wavelength in height or width or depth. Two separately controlled fields stimulated as radio frequency electric field E and an independent magnetic field H, driven by power from the same source but time phased so that across the interaction zone around the antenna there is E X H synchronism and Poynting vector synthesis occurs. Since it can be shown that the components of a radiated Poynting vector must have rotational E and H fields, then there is no absolute limit of reduction of size of antenna which will efficiently radiate a radio wave since very small but very intense Poynting vectors can, once synthesised, expand to infinity just like radio waves initiated by conventional antennas.

The invention is further described and illustrated with reference to the accompanying drawings, showing embodiments by way of examples. In the drawings:

Figure 1 shows schematically a plan view of an embodiment with a horizontal coil, Figure 2 shows the embodiment of Figure 1 in elevation, Figure 3 shows a phasing unit for feeding an antenna according to the invention, Figure 4 shows a further feeder unit, Figure 5 shows an embodiment for radiation of vertically polarised waves, Figure 6 shows a further embodiment using capacitive effect to produce the magnetic field, Figure 7 shows an embodiment similar to Figure 6 using cylindrical elements. Figure 8 shows an embodiment forming a ground plane construction, and Figure 9 shows the feed arrangement for an antenna similar to that shown in Figure 8. Figure 1 shows a plan view of an elementary form of twin feeder crossed field antenna according to this invention. The horizontal coil 1 is fed by feeder 2 via matching and isolating transformer 3 and carries a radio frequency current shown by arrows indicating an anticlockwise maximum in the cycle time. Thus upwardly directed in the centre of the coil there is high magnetic

field density H from J + D ^' = ^~ ^~ 7XE which returns downwards all around the periphery of the coil. There are two pairs of conducting plates 4 and 5, 6 and 7, with planes standing vertically which are insulated from everything else but are fed with antiphase voltage of the same frequency in pairs as shown, by power in feeder 8 via matching and isolating transformer 9. At the same instant in the cycle the plate pair 4 and 5 are electrically positive relative to the plate pair 6 and 7. Thus due to the very small dimension of the whole antenna, the propagation delay across the interaction zones marked X and Y is negligible and so the correct simultaneity, orthogonality and rotationality exists and Poynting vector synthesis occurs and radio power radiates away with the velocity of light in the directions marked S.

Figure 2 shows the same antenna in elevation. Detailed consideration of the phase requirement may be deduced as follows. Sinusoidal carrier waves are being applied and electric field E is in phase with the voltage across the plate pairs. The retardation due to size is negligible as is the magnetic field retardation around the coil. Thus the field H is in synchronism with the current causing it, that is the magnetic field is in phase with the current. Current in a coil is

however always lagging by about 90° relative to the voltage across the coil due to self inductance. So, in order to obtain phase synchronism of the fields interacting in the crossed field antenna, the feed voltage to the coil needs to be approximately 90° advanced on the feed voltage between the electrical plates- Assuming both transformers have identical phase characteristics, then the signal to feeder 2 requires to be phase advanced by 90° compared with the power in feeder 8. Cable lengths are only significant if different, so for a single frequency application an electrical quarter wavelength extra in feeder 8 would fulfil the phase requirement. If there were a power divider so that a single transmitter could supply approximately half the power to each of the twin feeders , the interaction zone will send out the total power in the synthesised Poynting vector. An antenna for general radio communications requiring many operational frequency changes will require to have a phase adjusting unit.

Figure 3 shows a simple phasing unit with which the said phase adjustment could be provided. The transmitter power is split partly into the upper capacitive path and partly into the lower inductive path. Setting the capacitor 10 to some value will give 45° advance; setting the inductor to another value will

result in a corresponding 45° delay which will ensure that after stimulating the two fields the radio wave will be correctly synthesised in the interaction zones.

Figure 4 shows a more sophisticated form of phasing unit which will provide phasing for any kind of twin feeder crossed field antenna under almost any circumstances over a wide frequency range. A switched auto transformer 12 is connected to feeder output 88 and is preceded by phase adjustment arrangements switchable into either sense by switch 14, of which coarse settings are provided by the dual gang switch 13A, 13B and a selection of cable lengths 15, and a fine adjustment by the variable capacitor 16.

A more complex phase adjustment system, (not shown) would have a series of two-pole change-over switches able to connect any total combination of delay cables selected from a sequence of lengths incremented in a 1/8 1/4 1/2 1 2 4 8 16 32 metre system. Such a scheme would allow a user to correct the phase of the feed to a crossed field antenna so well that a single device could be radiating successfully at any frequency in the whole HF spectrum.

An alternative twin feeder crossed field antenna which will radiate vertically polarised waves instead of horizontal, is shown in Figure 5. The antenna consists of a narrow vertical coil 17 fed from cable 2C via

matching transformer 18 , and two conducting plates 19 and 20 fed by feeder 8C via matching and isolating transformer 21. A widespread electric field E is created in arcs from the top plate to the lower plate and produces a cross-product with the magnetic field H rotating in the directions indicated and thus synthesises intense Poynting vectors S which radiate outwards in broad azimuthal angles to space. The said antenna having several advantageous features namely a reduced number of components and also a larger interaction volume than has the first type according to Figures 1 and 2. The first feature reduces costs and simplifies the structure. The second advantage gives enhanced signal voltages when used in the receive mode. Furthermore, since any one of the four input terminals (two plates and two coil terminals) may be connected to earth it will be optimal to have the lower plate earthed for safety as well as providing an opportunity to bond the screens of the coaxial feeders thereto.

It is possible for transformer 21 to be dispensed with, and direct feed from the inner of feeder 8C to be connected to the upper plate 19 with the screen remaining connected to plate 20.

As a further development of the twin feeder crossed field antenna types which use a coil to generate the

magnetic field, a further arrangement is proposed called the Maxwell type, in which the magnetic field is produced from an electric field displacement current located within a capacitor. It is an arrangement which has many advantages theoretically and practically, and allows the construction of a truly omnidirectional vertically polarised antenna. Examination of the Maxwell law D ¹ = ^~ XH where D ¹ = oD/ot shows that a changing displacement field causes a rotational magnetic field. As the displacement current density is simply related in space (or in air) by the formula D' = £ E' where E is the electric field intensity and is the dielectric constant, it is easy to calculate that this will be a very useful technique for HF crossed field antennas of small size. Also it can be seen that as before, the S = E X H relationship of the Poynting vector demands geometric perpendicularity synchronism and rotational form to both fields. The differentiation with respect to time within the Maxwell law again inserts a 90° phase change but in this type it is of the opposite sign. There is a 90° advance of magnetic field relative to the voltage gradient and so there must be a 90° delay in the voltage fed to the plates of the said capacitor. The Maxwell type of crossed field antenna requires two separate electric field stimulator plates; one pair as in the first type to initiate the E field, and the other

pair to initiate the magnetic field by the Maxwell law. The second pair are called therefore, the D plates. In total there are four phases of electric potential within the antenna structure:- 0° and 180° of the E plates; 90° and 270° of the D plates.

^" Figure 6 shows a basic form of the Maxwell type of twin feeder crossed field antenna. Two flat plates 22 and 23, standing vertically are insulated from other electrodes and ground and are fed by coaxial cable 26 via matching and isolating transformer 27 , thereby producing the electric field E shown in the downwards phase. Two insulated flat elliptical plates 24 and 25, disposed horizontally are also insulated from earth and other electrodes and constitute the capacitor within which a large displacement current density D ^' is produced by radio frequency power arriving from feeder 28 via matching and isolating transformer 29. The rapidly changing displacement current is then the origin of the considerably curved H around the whole antenna in the direction shown. In the wide interaction zones at mid height, in front of and behind the structure, copious field crossing is present and so considerable Poynting vector power density is generated and radio waves propagate away at the velocity of light in the directions shown S. The waves are vertically polarised; the

horizontal polar diagram is a figure of eight. The lower plate may be earthed and the screens of the coaxial feeders bonded to it. The transformer 27 may be dispensed with and a direct connection made between the inner of the feeder 26 and the plate 23.

Many variants of the Maxwell type are conceivable and they constitute a generic family of twin feeder crossed field antennas disclosed herein. For instance the form described in Figure 6 could be turned through 90° and it will then generate horizontally polarised waves and have a radiation polar diagram which is a figure of eight in the horizontal plane.

Two further antennas of this family will be described as they are important in having a robust structural shape as well as a vertically polarised omnidirectional radiation which is often required in broadcasting and communicating to mobiles.

Figure 7 shows the cylindrical form of Maxwell type crossed field antenna. The downwards electric field E is initiated by voltage between the hollow cylindrical conducting electrodes 30 and 31 which are fed from feeder 32 via matching transformer 33. The lower cylinder may stand safely on the ground or could be formed as a flat plate on site. The displacement current D ^' is stimulated upwards at the same time in the cycle by

feeding the appropriate phase voltage between the two horizontal disc conductors 34 and 35 (having their central area removed for space to mount transformers, feeders etc.) using feeder 36 via matching and isolating transformer 37. Should there be a requirement to reduce weight or wind resistance, the said electrodes and conductors may be made with alternative materials such as conducting wire mesh, or a conducting surface applied to a plastics or other non¬ conducting structural component. Figure 8 shows a ground plane (or half symmetry) form of the cylindrical twin feeder crossed field antenna of the Maxwell type. The downwards electric field E is produced by applying a voltage between the hollow conducting cylinder 37 and the large conducting earth plane 38 with the upwards displacement current D' from the said earth plane to the circular conducting plate 39 with a central missing area marked 39a in order to create the required rotational magnetic field H to interact with the said E field and synthesiεe the Poynting vector S radiating all round to space.

In a practical construction for the frequency range 3.6to30 MHz, the cylinder 37 has a height of 25 cm and a diameter of 20 cm with the base spaced 10 cm from the plate 39. Plate 39 has a diameter of 40 cm and is positioned coplanar to and 5 cm distance

from plane 38. The parts may be mechanically connected by insulating pillars or foamed plastics blocks.

The feed arrangement is shown in Figure 9 and this has the E-field feeder 90 connected between ground plane 38 and cylinder 37 and the H-field feeder 91 terminating in toroidal ferrite coupling transformer 92 feeding between ground plane 38 and plate 39. It is important that the outer conductor of feeder 91 is not electrically connected with any part of the structure. For weatherproofing the structure may be encased for protection but in a preferred embodiment a louvred or apertured screen is used in conjunction with a top cover to provide air through flow.

Twin feeder crossed field antennas of the above forms or other forms may be made almost as small as desired. With correct time phasing, the power radiated from the interaction zones can be made as large as desired and is limited only by the necessary voltages at the electrodes and the ultimate possibility of corona discharge. However since the plates are large in area compared with the surface areas for wire antennas the problem is of comparative insignificance. Antennas of these types only 1/200 th of a wavelength in length (and less in diameter) have been able to radiate 400 watts on HF with no perceptible problems of electrode distress.

Calculations show that for the magnitudes of voltage used in wire antennas, teraWatt capabilities will be possible with crossed field antennas. There are no large circulating currents in any conductor since nothing is in resonance. It is a major advantage of the twin feeder crossed field antenna system that it is broadband, and low Q. For any given antenna radiating efficiently because it is correctly phased, the bandwidth is very broad, firstly because of the phase-sense of frequency change acting by the Maxwell Law is the same sense as change due to a wave on the delay cable, secondly because the two fields are both originated from capacitor stimulus and also change in the same phase sense, thirdly the two fields interact in such a way as to provide a lower input impedance in each capacitor and therefore self-optimise the synthesis. Thus an antenna which is say 1/400 th of a wavelength height may be expected to have a small depreciation of efficiency by a frequency change of about plus and minus 15%. Many of the electrical properties of the system described are uncritical. For instance the adjustments need in the phasing unit to produce a low VSWR in the common feeder leading will be found in practice to be self- optimising. The magnetic field generated around the displacement current capacitor is in the direction of

curvature to reduce the impedance experienced by the electric field generator since the synthesised Poynting vector takes away power from the radio wave continuously, and at no part of the cycle does the E field find its path as impedant as normal space; it is always presented to the field lines as a power sink as long as the magnetic field H is synchronous. For the same reasons, the H field lines find that they are flowing into a low reluctance interaction zone of a similar power sinking nature due to the cross-curved E field in phase at all times. Only in the unproductive zones around the antenna do the fields experience the normal path impedance and reluctances. The crossed field antenna system is almost an efficient "open frequency" antenna. It will also receive radio signals and so may be used in two way-radio systems.

In fact the new device is such a small sized source that many techniques not before possible are now within easy achievement. When used in a reflecting or phasing arrangement, the crossed field antenna allows perceptible directivity to be attained in either transmit or receive modes even when the waves concerned are much larger than the reflector or array diameter.