VHF Omni-range navigation system antenna

An antenna which is suitable for use in a very high frequency omni-directional range (VOR) navigation system for aircraft. The antenna is driven to radiate reference and variable phase signals which provide flight bearing information to an aircraft which enters the radiated field. The antenna comprises a cylindrical radiator which is formed with four orthogonally disposed longitudinally extending slots, and each slot is backed by a separate cavity which extends into the cylinder. Each cavity has an effective depth which is greater than the radial or, more usually, the diametral dimension of the cylinder, and all four cavities are configured so as to locate wholly within the cylinder.

FIELD OF THE INVENTION 
This invention relates to a cylindrical antenna which is formed with cavity 
backed slots. The antenna has been developed primarily for use in a very 
high frequency omni-directional range (VOR) navigation system and the 
antenna is herein described in the context of such application. However, 
it is to be understood that the antenna may have application in other 
systems, in particular as a localiser antenna element in an instrument 
landing system (ILS) for aircraft. 
BACKGROUND OF THE INVENTION 
The VOR system as such is employed extensively throughout the world and it 
is operated to provide an aircraft with flight path bearing information. 
Two signals are radiated by a VOR antenna to produce a rotating field in 
space, one signal being referred to as a reference phase signal which is 
radiated omni-directionally and the other signal being referred to as a 
variable phase signal which has a phase which varies linearly with azimuth 
angle. Bearing information is derived by detecting the phase difference 
between the reference and variable phase signals as received by an 
aircraft flying toward or from the VOR site. 
The reference phase signal is generated as a radio frequency (r.f.) carrier 
which has a frequency falling within the region 108-118 MHz and which is 
amplitude modulated by a 30 Hz frequency modulated 9960 Hz subcarrier. The 
variable phase signal comprises a portion of the r.f. carrier from which 
the modulation is eliminated and, when radiated, is space amplitude 
modulated at 30 Hz. The space modulation is achieved by feeding the 
radiating antenna so as to produce a field which rotates at 30 Hz. 
The bearing information is derived and indicated by a receiver within an 
aircraft. After processing in the r.f. stage of the receiver and 
subsequent detection, the received (audio) reference and variable phase 
signals are processed in separate channels and are applied as separate 
inputs to a phase comparator. Bearing information relative to the VOR site 
is indicated by the phase difference between the reference and variable 
phase signals. 
Antennas which currently are employed for radiating VOR signals are: 
1. An arrangement of four or five closely spaced Alford loops. When five 
loops are employed a central one is driven to radiate the reference phase 
signal and the four surrounding loops are driven to radiate the variable 
phase information. When a four-loop arrangement is employed the reference 
and variable phase signals are combined in simple bridges and fed to the 
four loops. 
2. The so-called AME slotted cylinder antenna which incorporates four 
orthogonally disposed longitudinally extending slots located within the 
peripheral wall of a cylindrical radiator. All slots are excited with the 
reference phase signal and respective pairs of the slots are fed with sine 
and cosine signal components of the variable phase signal. 
3. An antenna which is known as the Thomson CSF antenna and which comprises 
four cylinders and two Alford loops. The four cylinders are terminated by 
common (upper and lower) metal end plates, are disposed parallel to one 
another, are arranged with their longitudinal axes centered on apices of a 
square and are excited to radiate the variable phase information. The 
Alford loops are located one above and the other below the end plates and 
are fed with the reference phase signal. 
All of the abovementioned prior art VOR antennas have recognised 
deficiencies. 
The arrangement which incorporates four or five Alford loops has a large 
octantal error. Octantal error is a bearing error which is cyclical in 
azimuth with a half-period of 45.degree. and which increases in magnitude 
with increasing diameter of the complete antenna. The Alford loop 
arrangement has an inherently large diameter and, indeed, produces an 
octantal error which is unacceptable to regulatory authorities in 
Australia, although this can be overcome by precise but difficult to 
achieve control of drive currents. Moreover, the Alford loop arrangement 
is not very suitable for use in a multi-stack antenna array due to mutual 
coupling effects. 
The AME slotted cylindrical antenna is an extremely difficult antenna to 
set-up and maintain because of inherent internal coupling between the 
slots and, due to the fact that it tends to have a narrow bandwidth, it is 
subject to environmental drift. Also, the antenna produces different 
radiation patterns in the vertical plane for reference and variable phase 
signal excitations, because the slots have different current distributions 
for the reference and the variable phase signal excitations. This is an 
undesirable feature when the antenna is located on difficult (i.e. short 
ground plane) sites and is a particularly undesirable feature in a 
multi-stack array. 
The major deficiency of the Thomson CSF antenna flows from its use of 
completely separate antenna elements for radiating the reference and 
variable phase signals. As abovementioned, the variable phase signal is 
radiated from the four-tube arrangement, which has an excellent broad band 
frequency characteristic, but it is fundamentally not possible to excite 
the same four tubes with the reference phase excitation. To accommodate 
this problem the reference phase signal is fed to the two Alford loop 
antenna elements (at the top and bottom of the four tubes), but the Alford 
loop antennas have a very narrow bandwidth and the vertical pattern of the 
radiated reference phase signal rarely matches that of the variable phase 
signal, particularly on difficult short ground plane sites. 
At this point it is mentioned that a recent development has been made in 
VOR systems for use at sites which have a limited counterpoise and which 
requires the use of multi-stack antenna arrays. Reference can be made to 
Australian Patent Application No. PE 4821, dated Aug. 1 1980, for 
particulars of such system. However, when employing multi-stack arrays it 
is necessary or, at least, desirable that the reference and variable phase 
radiation patterns should match in the vertical plane and this can be 
achieved only if the reference and variable phase excitations are added 
electrically to drive each of the stacked antennas. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide a slotted cylindrical antenna which 
is suitable for use in a VOR system, which is suitable for radiating both 
reference and variable phase signals when used in a VOR system, which is 
constructed to avoid or minimise internal coupling between the slots, 
which can be employed as a single element or in a multi-stack array, and 
which can be constructed to provide for an acceptably low octantal error. 
Thus, the present invention provides an antenna which comprises a cylinder 
having at least two slots formed within the peripheral wall thereof. The 
slots extend in the direction of the longitudinal axis of the cylinder and 
are spaced-apart around the periphery of the cylinder. Each slot is backed 
by a separate cavity which has a depth extending into the cylinder from 
the slot. The depth of each cavity is effectively greater than the radial 
dimension of the cylinder and the cavities are configured to locate wholly 
within the cylinder. 
The cylinder preferably has a circular cross-section, although it might be 
formed for example with an elliptical, square or polygonal cross-section. 
The number of slots provided within the peripheral wall of the antenna will 
depend upon the intended application of the antenna. For example, when 
employed as a localiser element in an ILS application, the antenna may be 
formed with two slots, for radiating 90 Hz and 150 Hz sideband signals, or 
it may be formed with three slots for radiating ILS carrier and sideband 
signals. 
When the antenna is employed in a conventional VOR system, the cylinder 
will be provided with four orthogonally disposed longitudinally extending 
slots, all such slots being excited equally with a reference phase signal 
and respective ones of the slots being excited with components of the 
variable phase signal. Thus, diametrically disposed slots which form one 
pair of slots are excited with a sine component of the variable phase 
signal and the other pair of diametrically disposed slots (which are 
orthogonal to the first pair) are excited with a cosine component of the 
variable phase signal. The diametrically disposed slots of each pair are 
excited in phase opposition with the variable phase signal components so 
that, effectively, a rotating figure-of-eight variable phase field 
component is radiated by the antenna together with a circular reference 
phase field component. 
The maximum diameter of the cylinder will be determined largely by the 
maximum octantal error allowable in any given application of the antenna 
(the magnitude of octantal error being determined by the maximum diameter 
of the antenna, as hereinbefore mentioned), and the longitudinal length of 
the slots is determined by the VOR system frequency, this normally being 
in the range of 108-118 MHz. Thus, in operation as a half-wave antenna, 
the slot would need to have a length of approximately 0.5.lambda..sub.c 
meters, where .lambda..sub.c is the wavelength in the cavity, although the 
total length of the antenna would normally be made somewhat greater than 
this dimension to permit on-site adjustments to the slot length during 
tuning of the system. The depth of each cavity is determined as a function 
of the slot length and width and, when the antenna is employed in a VOR 
system, each cavity would normally have a depth which is effectively 
greater than the diametral dimension of the cylinder. Each cavity is 
"folded" to follow a non-linear path so that it may fit within the 
available space. Various ways in which folding of the cavity might be 
effected will be hereinafter described and illustrated. 
Each slot is preferably fitted with at least one shorting bar or other 
suitable device for the purpose of adjusting the effective length of the 
slot and matching the slots. 
The invention will be more fully understood from the following description 
of a preferred embodiment of a VOR antenna, the description being given 
with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIGS. 1, 2 and 2A of the drawings, the antenna 10 has a 
cylindrical peripheral wall 11 which is constructed from a conductive 
material such as copper or aluminium. Four longitudinally extending, 
orthogonally disposed slots 12 are formed within the peripheral wall 11 
and respective ones of the slots are backed by cavities 13. The cavities 
are separated from one another by spiral form metal partitions 14 and, 
therefore, each cavity 13 may be considered as being folded as a spiral 
within the body of the antenna. This arrangement provides for a compact 
antenna construction, with each of the cavities having a depth a (see 
FIGS. 2A, 5 and 6) which is greater than the maximum outside diameter of 
the complete antenna structure. 
A metal plate 15 is fitted to each end of the antenna 10, whereby, but for 
the slots 12, the cavities 13 are closed, and a central support shaft 16 
extends through the complete structure in a longitudinal direction. 
Two longitudinally moveable metal bridges (i.e. shorting bars) 17 and 18 
extend across each of the slots 12 and interconnect the side walls of each 
slot to define the upper and lower limits of the resonant magnetic dipole 
length of each slot. The upper bridge 17 is selectively positionable to 
set the frequency of radiation of the antenna and sufficient adjustment 
scope is provided to accommodate a frequency shift over the range 108-118 
MHz. The lower bridge 18 is selectively positionable to permit matching of 
the four slots at a selected frequency. 
The bridges 17 and 18 provide for "coarse" adjustment of the radiation 
frequency and slot matching, and "fine" tuning is provided by the 
positioning of vane elements 17a and 18a which are located within each of 
the cavities 13 at the rear of the respective slots 12. 
As shown in FIG. 2B, the vane elements 17a and 18a are carried by 
concentric tubes 17b and 18b which are located in each of the cavities 13. 
The tubes are formed from a dielectric material, they extend for the full 
length of the slots 12 and, although not so shown in the drawings, the 
tubes are supported in bearings and project from the lower end of the 
antenna so that they might be rotated manually or mechanically. 
The vane element 17a is formed from metal and it extends arcuately around a 
portion of the periphery of the upper region of the outer tube 17b. The 
vane element 18a is formed in a similar manner but it extends around a 
peripheral portion of the lower region of the inner tube 18b. 
Both of the vane elements 17a and 18a can be selectively positioned with 
rotation of the supporting tubes 17b and 18b to present a variable area of 
metal to the passage of electromagnetic fields in the respective cavities, 
but, even when exhibiting a maximum area of metal across the width of the 
cavities, the vane elements do not make electrical contact with the walls 
of the cavities. 
Typical dimensions of the antenna structure as shown in FIGS. 1 and 2 are: 
Length (X)=1.80 meters 
Diameter (Y)=0.46 meters 
The antenna 10 may be constructed in various ways in order to obtain a 
desired depth a of the cavity behind each of the slots 12, and three 
alternative configurations are shown in FIGS. 3A to 3C. In each case, the 
peripheral wall 11 of the antenna is formed with four longitudinally 
extending slots 12 and each slot is backed by a folded cavity 13. The 
cavities are separated by partitions 14 and the respective cavities are 
defined by walls 19. 
Characteristics and parameters which are relevant to the construction and 
operation of the antenna are now described. 
The overall height (X) of the antenna is determined predominantly by the 
required length (l) of the slots 12 and the slot length (approximately 
0.5.lambda..sub.c) is determined by the operating frequency. The 
wavelength .lambda..sub.c (&gt;.lambda. free space) is the wavelength in the 
cavity 13. 
Then, the maximum diameter of the antenna is determined by constraints 
imposed on the maximum octantal error allowable in any given situation, 
this normally being specified by regulatory authorities. In this context 
FIG. 4 shows a plot of peak octantal error against radial dimension of an 
antenna and it can be seen that, in order to satisfy the Australian 
regulatory requirements for a peak octantal error not greater than 
1.5.degree., the maximum radial dimension of the antenna should not exceed 
0.12.lambda.. This corresponds with an antenna diameter of approximately 
0.60 meters at a transmission frequency of 118 MHz. 
The width w of the slot 12 is critical only to the extent that it affects 
the Q-factor of the antenna. It is desirable that a low Q-factor should be 
obtained in the interest of avoiding a too-narrow bandwidth and, 
therefore, the slot width should not be made too small. The slot 12 might 
typically have a width in the order of 5 to 15 mm. 
The depth a of the cavity 13 is determined as a function of the width w and 
resonant length l of the slot 12, and the width b of the cavity is 
determined by the power transmission requirements of the antenna. In 
practice, the power transmission requirement of a VOR antenna is 
relatively low and the width b of the cavity will be determined by 
structural factors or manufacturing techniques rather than by electrical 
factors. 
The cavity is illustrated in a developed (i.e., unfolded) form in FIGS. 5 
and 6 of the drawings, and the rectangular box structure as illustrated 
may be considered as a very short waveguide cavity which operates in a 
kind of "dominant mode". This cavity satisfies the boundary conditions on 
one side of the slot which allows it to radiate totally into the opposite 
half plane, the radiation from the slot effectively being equivalent to 
that of a one-sided magnetic dipole, with the maximum H-field emanating 
from each end of the slot. The cavity backed slot radiates almost all of 
its energy into free space at the operating frequency and has a low 
Q-factor typically in the order of 50. The lines of H-field do not form 
closed loops within the "waveguide", this contrasting with the more usual 
form of waveguide cavity in which the H-field lines are completely 
contained within the cavity limits and which usually demonstrate a high 
Q-factor in the order of 3,000 to 10,000. 
As above mentioned, the depth a of the cavity 13 is determined as a 
function of the length l and width w of the antenna slot, and FIG. 7 
illustrates the relationship of the various dimensions for a typical VOR 
antenna. Thus, for an antenna having a slot resonant length l of, say, 1.9 
meters and a slot width w of 5 mm, the cavity should have a depth a in the 
order of 0.62 meters. 
Each cavity backed slot unit as shown schematically in FIGS. 5 and 6 
constitutes one quarter of a VOR antenna, and a complete antenna is 
obtained by joining four such units and compacting them in the manners 
shown by way of example in FIGS. 2 and 3 to reduce the octantal error to 
an acceptably low level. 
FIG. 8 shows a developed view of the internal peripheral wall 11 of the 
antenna 10 (with the cavities 13 being omitted) and electrical connections 
to the four slots 12(1) to 12(4) are shown in the figure. The electrical 
connections are made by coaxial conductors 20, with the inner conductor 
being soldered to one side of the respective slots and the outer conductor 
being soldered to the other side of the respective slots. 
Employing the bridge arrangements 20a, b and c shown in FIG. 8, the 
reference phase signal component of the VOR signal is fed to all four 
slots, a cosine component of the variable phase signal is fed to the slots 
12(1) and 12(3), and a sine component of the variable phase signal is fed 
to slots 12(2) and 12(4). Slots 12(1) and 12(3) are fed in phase 
opposition, as are slots 12(2) and 12(4), whereby a rotating 
figure-of-eight variable phase field component is radiated together with 
an omnidirectional reference phase field. 
The bridge arrangement as shown in FIG. 8 is preferably housed within the 
body of the antenna structure at the lower end thereof. 
Reference is now made to FIG. 9 of the drawings which shows a schematic 
implementation of a VOR system which employs a two-stack antenna array. 
The two elements of the array, indicated by numerals 10(1) and 10(2), are 
identical and each element of the array may be constructed in the manner 
as hereinbefore described with reference to FIG. 1 of the drawings. 
The VOR system includes a conventional VOR signal generating arrangement 21 
which comprises an r.f. generator 22, a reference phase signal generator 
23, a variable phase signal generator 24 and a sine/cosine function 
generator 25. Such arrangement in its various possible forms is well known 
and is not further described. 
The reference and variable phase signals are fed to the lower element 10(2) 
of the two-stack array and, via an amplitude attenuator/phase shifter, to 
the upper element 10(1) of the array. The feed circuitry 26, 27 and 28 for 
the reference phase signal and for each of the (sine/cosine) variable 
phase signals each include a two-bridge arrangement, with a line stretcher 
being incorporated in one line between the bridges to permit amplitude 
adjustment of the feed signal. Also, a line stretcher is located in the 
output of each circuit to permit phase adjustment of the signal. 
The two-stack antenna array as shown schematically in FIG. 9 would normally 
be mounted to the roof of a VOR transmission station 30 in the manner 
indicated in FIGS. 11 and 12. Thus, the antenna units 10(1) and 10(2) are 
mounted to support shafts 16(1) and 16(2) which are joined by a coupling 
31, and the lower support shaft 16(2) is connected with the building 
structure 30. A fibreglass base module 32 provides a lower weathershield 
for the structure and two fibreglass radomes 33 and 34 provide protective 
enclosures for the two antenna units 10(2) and 10(1) respectively. A 
fibreglass spacer module 35 separates the two radomes, and a weather cap 
36 closes the upper radome. Access hatches 37 are located in the two 
radomes and in the spacer module, and the total structure is guyed by 
wires 38. 
The arrangement which is illustrated in FIGS. 10 and 11 is exemplary only 
of many possible arrangements.