Radio antennae

Compact radio antennae are disclosed which comprise an extended conductor, such as a wire, arranged to form an elongated structure. The conductor is arranged in a plurality of windings comprising windings of opposite senses positioned coaxially in proximate relation. In one form the conductor is arranged in a series of helical windings all coaxial, to form a cylindrical structure, and successive windings in the series are alternately left-handed and right-handed. In another form the conductor is arranged in a first helical winding forming a cylindrical structure and a second helical winding of opposite chirality to the first, but with the same number of turns and longitudinal extent and coaxial with it. A flux-concentrating core may be longitudinally disposed within the cylindrical structure. Antennae are particularly described for the HF, VHF and UHF bands.

This invention relates to radio antennae. 
Portable radio sets operating in the VHF frequency band, such as 
walkie-talkie transceivers and domestic VHF receivers, generally use a 
whip type of aerial as antenna. While a VHF whip aerial is electrically 
satisfactory it is physically inconvenient because of its length and 
vulnerability to damage when extended. Since many VHF transmissions, 
particularly those intended for domestic reception, are horizontally 
polarised, whip aerials have to be extended almost horizontally to receive 
these transmissions, and this further aggravates the inconvenience already 
inherent in the length of the aerial. The awkward length and vulnerability 
of VHF whip aerials is particularly inconvenient for members of 
fire-fighting, police and other security services who may need to keep 
their radio equipment operating while moving quickly within buildings or 
in other congested situations. 
The UHF frequency band is commonly used for public television 
transmissions, and while, because of the shorter wavelength, conventional 
UHF dipole or monopole antennae are reasonably compact, the recent 
development of small portable television receivers makes it desirable to 
have a more compact UHF antenna. 
Whip aerials for use in the HF band are known and are generally even larger 
and more unwieldy than VHF whip aerials. Helical HF whip aerials are more 
compact, but they suffer from low band width and strong multiple 
resonances up to a high order. 
A Japanese patent specification, publication No. 44-18967, in the name of 
Matsushita, discloses a helical antenna having an electrical length of 
about 1.6 wavelengths for radiating linearly polarised waves. The antenna 
comprises two helical antenna elements which are coiled in opposite 
directions and are combined superposedly around the same axis and which 
require balanced feed inputs. A phase delaying element is included in the 
inputs to obtain linearly polarised waves. 
We have discovered that quarter wavelength antennae of particular 
counterwound and double-wound configurations have good characteristics 
over a wide band of operation and do not require complex phase delay feed 
arrangements for efficient operation. 
According to the present invention there is provided a radio antenna 
comprising an extended conductor having an electrical length of one 
quarter of the design wavelength, said conductor being disposed in a 
plurality of insulated windings of opposite senses (i.e., opposite 
"chirality") and of substantially equal numbers of turns arranged 
coaxially in proximate relation. 
The windings may be spiral and arranged in coaxial pairs, with the windings 
of one pair not necessarily coaxial with the windings of another pair. 
Preferably, however, the windings are all arranged coaxially to form a 
cylindrical structure and preferably the windings are helical. 
In one form of the invention the windings form a series extending along the 
length of the structure, successive windings in the series being adjacent 
and of opposite sense. The windings then may be of a fractional number of 
turns, but preferably comprise one or more turns. An approximately 
integral number of turns is convenient and about one turn is particularly 
so, since with just one turn per winding, if the turns are wound closely, 
the windings in the series can be brought close together, thus increasing 
the interaction between adjacent windings. 
The windings may be coupled to a flux-concentrating core which may be of 
soft ferromagnetic material, material of high dielectric constant, a 
non-magnetic conductor or a suitable combination. In the case of a 
cylindrical structure the core may conveniently be longitudinally disposed 
within the structure. 
In another form of the invention the windings comprise a first helical 
winding extending along the length of the structure and a second helical 
winding extending along the length of the structure, the first and second 
windings having approximately the same number of turns as each other but 
in the opposite sense. 
The antenna according to the invention can be connected into a circuit in 
the same way as a conventional monopole antenna. That is to say it may be 
connected by means of a tapping connection or directly at one end. It is 
found in practice that antennae according to the invention can be made 
which present suitable impedances for matching to connections made at the 
end, without any need for tapping connections. 
The arrangement of the conductor is such that a resonant antenna according 
to the invention is physically shorter than a conventional whip antenna 
resonant at the same frequency. The coupling of the conductor with the 
core tends further to reduce the necessary length of the antenna. Antennae 
according to the invention have been found to have a somewhat lower gain 
than conventional whip antenna, but it has been found that a single extra 
radio-frequency amplification stage will adequately compensate for this. 
It is considered that the advantages of compactness and robustness 
possessed by the antennae herein described outweigh the disadvantages of 
the lower gain. 
It appears that the antennae herein described have the additional advantage 
that their performance is less sensitive to the proximity of other objects 
such as walls, furniture, vehicle bodywork or human bodies than 
conventional whip aerials. 
Within the scope of the present invention there is included a radio antenna 
having a resonant frequency in the VHF band, and comprising an extended 
conductor, one end of which is an open-circuit end and the other end of 
which forms an electrical connection to the antenna, forming a series of 
groups of turns around and disposed along the length of a core formed from 
a conducting material or a magnetically soft ferromagnetic material, 
successive groups in the series being wound in left-hand and right-hand 
senses alternately (i.e., having opposite "chirality"). 
The core may consist of beads of ferrite or like material to impart a 
degree of flexibility to the antenna. 
It will be appreciated that the windings in antennae according to the 
invention are so arranged that axial magnetic fields through the windings 
due to electric currents in the windings substantially cancel. 
Antennae according to the invention may be combined to form dipole 
antennae, but where compactness is particularly required a monopole 
antenna is generally more convenient.

In FIG. 1, a ferrite rod 1 has a length of enamelled copper wire wound 
round it in four groups of turns, 2a,2b,2c and 2d. The groups 2 are evenly 
spaced along the length of the rod 1. The group 2a consists of four turns 
in the left-hand sense; the group 2b consists of four turns in the 
right-hand sense; the group 2c consists of four turns in the left-hand 
sense; and the group 2d consists of four turns in the right-hand sense. 
One end of the wire 3, provides an electrical connection for the antenna, 
and the other end 4, is an open circuit end. 
The antenna acts as a quarter wave monopole at a frequency in the VHF band. 
In order to obtain an antenna tuned to a desired frequency, wire is wound 
round the rod 1, forming more groups of turns than is necessary, and the 
resonant frequency is gradually increased by successively clipping off 
turns from the open circuit end 4 of the wire. When the desired resonant 
frequency is nearly reached the turns are stretched out to fill the length 
of the rod 1, whereupon the resonant frequency falls a little. 
One such antenna which has been tested, is wound on a ferrite rod 200 mm (9 
inches) long and comprises four groups each of four turns, as shown in the 
Figure. Another antenna which has been tested is wound on a ferrite rod 
130 mm long (5 inches). A 200 mm antenna has a bandwidth of about 5 MHz, 
and has been tried at an operating frequency of 79 MHz on a portable 
walkie-talkie transceiver. The 130 mm antenna has a similar bandwidth and 
has been tested on a portable VHF radio receiver using BBC transmissions 
(about 94 MHz). When tested in the laboratory over a large ground plane, 
or when mounted on the top of a vehicle, so that the top of the vehicle 
forms a large ground plane, the antennae according to the invention show a 
markedly lower gain than a whip aerial adapted to work at the same 
frequency, and standing over a large ground plane. In subjective tests, 
however, using portable radio apparatus, in which there is no large ground 
plane, the antennae according to the invention gave apparently comparable 
performance with a whip aerial. The sensitivity is definitely lower than 
with a whip aerial, but it is found that a single extra stage of radio 
frequency amplification is enough to make the performance subjectively 
very similar. The ferrite rods used were salvaged from old longwave/medium 
wave radio receivers and were thus not specially adapted for use at VHF 
frequencies. An antenna has been tested using a core of dustiron, though 
to be better adapted for use at VHF frequencies, but the performance was 
not apparently better than with a ferrite core. An antenna has also been 
tried using an aluminium tube, of the type used in building dipole 
antennae of a conventional type, in place of the ferrite rod 1. This 
antenna worked, but was rather less satisfactory than the ferrite antenna. 
In the antenna illustrated in FIGS. 2 and 3 the conductor consists of 
thirty-three groups, each of three turns, of enamelled copper wire 5. One 
end of the wire is connected to the central electrode of a coaxial 
connector 6 (FIG. 2 only). The other end of the wire 7 is an open circuit 
end. The core 8 consists of ferrite beads 9 (FIG. 3 only) of the type 
commonly used as parasitic suppressors (Q-killers) threaded on a strip of 
fibreglass 10 (FIG. 3 only). The beads are covered by a heat-shrink 
sleeving to provide protection and added mechanical support. The core 8 
extends for only two thirds of the length of the antenna, its place being 
taken for the third of the length nearest to the open circuit end 7 by a 
length of plastic tubing 12 whose sole purpose is to provide mechanical 
support for the windings. The length of the antenna is about 250 mm (10 
inches) and the diameter is about 6 mm (1/4 inch). The antenna of FIG. 2 
has been tested with a portable VHF receiver using BBC VHF transmissions 
(about 90 MHz). The performance was comparable with, but noticeably less 
good than the telescopic aerial supplied with the set, but with a single 
extra stage of radio frequency amplification, subjectively similar 
performance was obtained. An antenna similar to that of FIG. 2 has been 
built and made to resonate at 450 MHz. This antenna had fourteen groups, 
each of one turn, and the length of the antenna was 65 mm (21/2 inches) of 
which the core only extended for 40 mm (11/2 inches). The antenna of FIG. 
2 has quite marked directional properties, and it is notable that in this 
respect it resembles a rod monopole aerial. It is thus clear that the 
antenna of FIG. 2 is acting as a monopole antenna, rather than as a 
magnetic pick-up which conventional ferrite rod aerials are. 
In the antenna of FIG. 4 the conductor 13 is disposed in ten groups, each 
of one turn, wound alternately in the left-hand and right-hand senses. The 
core is a dielectric core 14 consisting of distilled water contained in a 
cylindrical plastics container. A conducting copper wire 15 runs through 
the axis of the core from one end to the other. The antenna will work 
without the conductor 15, but the insertion of the conductor 15 lowers the 
resonant frequency of the antenna, or, for the same frequency, reduces the 
length of the antenna. The antenna is about 130 mm (5 inches) in length by 
about 30 mm (about 11/4 inches) in diameter. It has been tested with a 
portable VHF receiver using BBC broadcasts. The electrical connection is 
to one end of the conductor 13, the central conductor 15 being left 
floating. 
FIG. 5 shows an antenna with a ferrite core, of a similar type to the 
antenna of FIG. 1, coated with Plasticine (the word Plasticine is a 
trademark of Harbutt's Plasticine Ltd). The Plasticine 16 is pressed into 
the antenna, filling in the air spaces between the turns. The Plasticine 
acts as a dielectric and lowers the resonant frequency of the antenna, or, 
for the same resonant frequency, reduces the necessary size of the 
antenna. 
In the antennae thus far described there are loops of wire formed where the 
conductor turns back on itself between groups of turns. These loops of 
wire are vulnerable to displacement unless secured in place, and give rise 
to difficulties in production, especially where many groups of turns are 
required. In the mode of construction illustrated in FIG. 6 a fibreglass 
strip 17 extends axially along the length of the antenna and the loops 18a 
to 18h are formed round the strip 17. The strip 17 thus serves to hold the 
loops 18 in place and also acts as a guide in making the loops, enabling 
them to be neatly in line. In FIG. 7 is shown a section of an antenna 
constructed in the mode illustrated in FIG. 6 in which the windings are 
closed up together so that there is virtually no air space between 
adjacent windings. With antennae according to the invention which do not 
have any core, it is advantageous that the windings should be as close to 
one another as possible. In these circumstances the strip 17 is of 
particular usefulness because there are many loops and they are close 
together. 
An alternative mode of construction is illustrated in FIGS. 8 and 9. FIG. 8 
shows a rectangular sheet of flexible insulating material 19 on which is 
printed a serpentine conducting strip 20. In FIG. 9 the sheet 19 is shown 
rolled into a cylinder so that the conducting strip 20 is formed into a 
series of windings of alternating sense, thus forming an antenna according 
to the invention. As illustrated in FIG. 9 the windings are each of about 
two turns, but clearly the windings can be made with any desired number of 
turns by rolling the sheet more or less tightly. 
In all the antennae described so far the axes of the windings have been 
parallel to the general direction of extension of the antenna. In FIG. 10 
is illustrated a form of winding in which the axes of the windings are at 
right angles to the general direction of extension. As illustrated in FIG. 
10 the general direction of extension of the conductor is from left to 
right. From a point 21 the conductor spirals inwards anti-clockwise to a 
point 22 and then spirals outwards to a point 23 which is displaced from 
the point 21 in the general direction of extension of the conductor. From 
the point 23 the conductor again spirals inwards to a point 24 and then 
outwards again to a point 25, and so on, repeating the same pattern to 
form a series of double spirals extending in the general direction of 
extension of the conductor. In this form of conductor the anti-clockwise 
spiral from 21 to 22 is one winding and the clockwise spiral from 22 to 23 
is the next. The next anti-clockwise spiral from 23 to 24 is the next 
winding and the clockwise outward spiral from 24 to 25 is the next after 
that. Thus the conductor forms a series of windings, successive windings 
in the series being adjacent and of opposite sense. 
In field tests of a ferrite cored antenna of the type illustrated in FIG. 1 
using walkie-talkie transceivers operating at 79 MHz it was reported that 
the antenna worked particularly well, compared with conventional whip 
aerials, in an indoor location and aboard vehicles. It is thought that 
this may be partly due to the physical convenience of having a compact 
antenna in confined spaces. This is particularly so within vehicles where 
the whip aerial normally used had to be pushed out through a window in 
order to make room for the occupants of the vehicle to ride in any 
comfort. It is also, however, thought to be partly due to the smaller near 
field of the antennae according to the invention, compared with whip 
aerials, which makes the antennae according to the invention less 
vulnerable to so-called proximity effects, by which conventional whip 
aerials are pulled out of tune by the presence of nearby objects. 
FIG. 11 shows schematically a further antenna according to the invention. A 
first helical winding 26 of insulated copper wire is wound around a 
cylindrical former 27. A second helical winding 28 of insulated copper 
wire is wound over the first winding 26. The two windings 26 and 28 are 
joined together at one end, 29, which forms the connection to the antenna, 
so they effectively constitute a single conductor. As illustrated the 
windings 26 and 28 are also joined together at the other end 30. This is 
convenient since it helps to prevent the windings from coming unwound, but 
it makes practically no difference to the operation of the antenna. The 
windings 26 and 28 are coaxial, have the same longitudinal extent and 
number of turns but are wound in the opposite sense, the first winding 26 
being illustrated as left-handed and the second winding 28 as 
right-handed. 
For the sake of clarity the windings are shown loosely wound and the second 
winding 28 is shown standing out well clear of the first winding 26. Also 
only a small number of turns are shown. In an embodiment of the antenna 
which has been built and tested the windings 26 and 28 were closely wound, 
leaving substantially no gaps between turns, and the second winding 28 was 
wound directly over the first winding 26, with substantially no space 
between them. The antenna measured two meters in length and 25 mm in 
diameter and the windings were closely wound in 32 gauge wire. The antenna 
was resonant at 7.4 megahertz (in the HF band) with a bandwidth of about 
2.5 megahertz. The impedance at the point 29 was about 200 ohms, which 
could easily be matched to 50 ohm equipment by means of a small 
autotransformer. For comparison a helical whip antenna resonant at 7.4 
megahertz of similar dimensions was made. The bandwidth of the helical 
whip antenna was only about 250 kilohertz and there were strong multiple 
resonances. The antenna according to the invention had a fairly strong 
half-wave resonance at 15 megahertz, but no strong higher resonances. 
An HF antenna resonant at 7.4 megahertz has also been built using the mode 
of construction illustrated in FIGS. 6 and 7. The antenna was one meter 
long and 65 mm in diameter and comprised a series of windings, each of one 
turn, closely wound in 32 gauge wire. The performance was similar to that 
of the antenna constructed as illustrated in FIG. 11, but the amount of 
labour involved in making the windings was much greater. 
FIG. 12 shows a further antenna 40 which is similar to that of FIG. 11 in 
that the antenna has first and second insulated copper wire helical 
windings, 26 and 28 wound in opposite senses on a cylindrical former 27. 
However, in the FIG. 12 embodiment only the inner winding 26 is connected 
to a receiver circuit via a terminal 29 and the outer winding 28 is not 
electrically connected to winding 26 or any part of a circuit for the 
antenna. It has been found that antennae of FIGS. 11 and 12 of the same 
general construction as regards number of turns, diameter of wire and the 
former 27 dimensions have closely similar performances. Further, it has 
been found that there is little change in the performance of the FIG. 12 
embodiment when the outer winding 25 is connected to the receiver circuit 
and the inner winding 26 is left unconnected. 
FIG. 13 shows a VHF radio receiver 41 which includes the antenna 40 of FIG. 
12. The antenna 40 is pivotally mounted by means of a ball and socket 
joint 43 and may be pivoted from a stowed position where it may be held by 
means of a U-shaped clip 42 to a working position as shown in FIG. 13. 
Typically such an antenna is between about 100 to 200 mm in length and 
thus is a convenient length for mounting on portable VHF radios without 
having to resort to telescopic or folding antennae. 
FIG. 14 shows an antennae 40 which has the same inner and outer windings as 
that of FIG. 12 but has the end 25 of the outer winding formed as a 
terminal. The antenna is connected via a switch 50 to an input terminal of 
a radio receiver so that by operating the switch 50, the inner or outer 
winding can be connected to the receiver to vary the tuning 
characteristics of the antenna. 
FIGS. 15 to 24 relate to experimental work on antennae which are described 
in the following tables: 
TABLE 1 
______________________________________ 
ANTENNA DETAILS 
ANTENNA f.sub.o .DELTA.f 
L.sub.A 
.eta. r.sub.o 
No TYPE (MHz) (MHz) (dB) ( ) N (.OMEGA.) 
______________________________________ 
M1 1/4 wave 150 .about.20 
0 100 1 25 
monopole 
H1 Helical 142 .about.1.5 
1.04 81 4.6 5 
C1 Counterwound 
150 .about.2 
0.4 90 4.15 5 
D1 Doublewound 150 .about.9 
7.8 16 5.6 21 
D2 Doublewound 145 .about.7.5 
7.2 19 4.1 16 
D3 Doublewound 150 .about.12 
10.7 8 5.6 32 
D4 Doublewound 150 .about.10 
.about.12 
-- 5.9 20 
D5 Doublewound 167 .about.13.5 
8.7 14 3.6 20 
DS1 Doublewound 140 3 6 25 5.3 10 
spacing 
DD1 Doublewound 190 &gt;25 6.1 24 2.3 63 
(inner only) 
DD2 Doublewound 175 &gt;30 8.4 14 2.5 48 
(outer only) 
DD3 Doublewound 175 &gt;35 10.4 9.7 2.5 45 
(outer only) 
DD4 Doublewound 174 16 4.7 33 2.5 15 
DU1 Doublewound 166 23 11.3 8 4.1 60 
DU2 Doublewound 165 20 9.8 10.3 4.1 44 
DU3 Doublewound 145 15 13 5 4.9 50 
(inner only) 
DU4 Doublewound 141 10 7.4 18 5 25 
DU5 Doublewound 153 15 8.3 15 4 40 
(inner only) 
DU6 Doublewound 147 10 6 25 4.1 22 
DU7 Doublewound 186 16 6 25 3.8 23 
______________________________________ 
TABLE 2 
______________________________________ 
ANTENNA CONSTRUCTION 
______________________________________ 
M1 1/8" diameter wire. 
H1 28 turns 19 SWG wire, 9 mm internal diameter, 
L = 115 mm. 
C1 35 turns 18 SWG wire 9 mm internal diameter, 
L = 120 mm, 4 turns positive and 4 turns negative 
alternatively for 7 sections. 
D1 136 turns 24 SWG wire, 16 mm internal diameter, 
L = 90 mm, identical turns wound on to form outer 
winding. 
D2 145 turns 22 SWG wire, 16 mm internal diameter, 
L = 125 mm (excluding BNC plug of 20 mm), identical 
turns wound on to form outer winding. 
D3 103 turns 22 SWG wire, 22 mm internal diameter, 
L = 90 mm, outer winding 98 turns only to adjust 
tuning. 
D4 89 turns 20 SWG wire, 28 mm internal diameter, 
L = 85 mm (not including plug), outer winding 86 turns 
only to adjust tuning. 
D5 318 turns 29 SWG wire, 7.5 mm internal diameter, 
L = 128 mm, 1 identical outer winding. 
DS1 83 turns 22 SWG wire, 6.5 mm internal diameter, 
L = 100 mm, outer winding of 58 turns 16 SWG on 12.5 
mm former to create annular cylindrical air gap. 
DD1 400 turns 29 SWG wire, 6.3 mm internal diameter, 
L = 172 mm, outer winding 200 turns 22 SWG on top 
of inner. Inner coil excited only (outer not 
joined at feed). 
DD2 As DD1 but outer coil excited only (inner not joined at 
feed). 
DD3 As DD2 with outer excited and inner connected to 
outer at the other end to feed point. 
DD4 As DD1 but with both inner and outer excited at 
feed point. 
DU1 282 turns 29 SWG wire, 6.3 mm internal diameter, 
L = 110 mm, outer winding 340 turns 32 SWG wound 
directly on top of inner winding. 
DU2 340 turns 32 SWG wire, 6.3 mm internal diameter, 
L = 110 mm, outer winding 285 turns 29 SWG wound 
directly on top of inner winding. 
DU3 252 turns 29 SWG wire, 12.5 mm internal diameter, 
L = 105 mm, outer winding 162 turns 26 SWG wound 
directly on inner. Inner coil excited only (outer 
not joined at feed). 
DU4 As DU3 but both windings excited at feed point. 
DU5 200 turns 26 SWG wire, 12.5 mm internal diameter, 
L = 125 mm, outer winding 150 turns 22 SWG wound 
directly on inner. Inner coil excited only (outer 
not joined at feed). 
DU6 As DU5 but both windings excited at feed point. 
DU7 250 turns 29 SWG wire, 6.3 mm internal diameter, 
L = 105 mm, outer winding 104 turns 20 SWG wound 
directly on inner. 
______________________________________ 
TABLE 3 
______________________________________ 
Antenna excitation 
both coils 
inner only 
f.sub.o Q L.sub.A 
r.sub.o 
______________________________________ 
DD4 DD1 +9% -30% +30% +310% 
DU4 DU3 +2.5% -33% +75% +100% 
DU6 DU5 +4% -31% +54% +20% 
______________________________________ 
Percentage change in f.sub.o, Q, L.sub.A and r.sub.o when only the inner 
coil is excited. 
The following symbols are used in the above tables and in the following 
text: 
______________________________________ 
N 
##STR1## 
L.sub.A = antenna system loss factor 
.eta. = antenna efficiency 
.DELTA.f = antenna bandwidth 
G + j b = antenna admittance 
j 
##STR2## 
r.sub.N + j X.sub.N 
= normalised (50.OMEGA.) antenna impedance 
r.sub.o = r.sub.r + r.sub.L = resistance at resonance 
r.sub.r = equivalent antenna radiation resistance 
r.sub.L = equivalent antenna internal loss resistance 
f.sub.o = frequency of antenna resonance 
L = antenna overall length 
Q = f.sub.o /.DELTA.f 
______________________________________ 
The antennae listed in Tables 1 and 2 had enamelled copper wire windings on 
lossless cylindrical formers. In some antennae a BNC plug was mounted on 
the antenna base and in other antennae their feed points were excited by 
inserting their wire ends into a test rig socket. All the antennae were 
tested in the vertical position. The inner and outer windings were 
separated by a layer of `Sellotape.` 
While all the experimental work has been carried out on monopole-like 
devices on a ground plane the designs could, with suitable excitation, be 
changed to obtain dipole action with the feed applied centrally. The 
radiation patterns of all the antennae tested are essentially those of a 
conventional wire monopole or dipole and thus no patterns are presented 
although checks have been made to verify the pattern shape. 
The types of antennae that have been investigated or used are: 
(a) Conventional quarter wave monopole (M) 
(b) Conventional helical antenna (H) 
(c) Counterwound (C) 
(d) Doublewound (D) 
(e) Doublewound with spaced coils (DS) 
(f) Doublewound with unequal turns and only inner or outer excited (DD and 
DU) 
The term `counterwound` refers to windings of the configuration shown in 
FIG. 1 and `doublewound` refers to windings as in FIG. 11 or FIG. 12. The 
spacing refers to an annular cylindrical airspace between windings. The 
helical antenna is included because it is the most common form of small 
antenna and forms a useful comparison. 
At lower frequencies it is not practical to measure the properties of 
antennae within the laboratory and reliance is put on actual field trials 
or perhaps system measurements outside. This introduces many unknown 
propagation effects, matching problems and difficulties of assessing the 
quality of the received signal. At VHF however it is practical to measure 
the antenna properties indoors. The parameters of interest are antenna 
bandwidth .DELTA.f, and the inherent power loss in the antenna when 
working under matched conditions. The latter is expressed as a system loss 
L.sub.A or alternatively as an efficiency .eta.%. The ratio of the height 
of the small antenna to that of a conventional quarter-wave monopole is 
defined as N which is taken as &gt;1. 
FIG. 15 is a diagram of a test rig for antenna admittance and impedance 
measurements for an antenna 3. A Wayne-Kerr admittance bridge 2 is fed 
from a signal generator 4 coupled to a frequency counter 5 and 
out-of-balance signals detected by a radio receiver 8 which has its output 
connected to a loudspeaker 7 or headphones 6 via a switch, and to an audio 
output meter 9. The bridge 2 is balanced to yield antenna admittance G+j 
b. To assess the antenna efficiency by the `Wheeler Can` method, the 
admittance is re-measured at each frequency but with a large screening can 
10 placed over the antenna. Leakage of radiation between instruments and 
cables around edges of a ground plane 1 below the antenna 3 was reduced by 
careful choice of earthing points and screening under the ground plane. 
The ground plane is continuous and of large enough extent so that at 
frequencies of interest these conditions were satisfied. The `Wheeler Can` 
method can produce erroneous results if the can is either too large or too 
small. A 0.7 meter diameter cylindrical can of height 0.4 meters which is 
about the smallest can which gives acceptable detuning effects was used. 
The value of L.sub.A (dB) had a .+-.10% error tolerance. 
FIG. 16 shows a test rig for checking RF power handling characteristics of 
antennae and enables incident and reflected powers in the antenna feed to 
be monitored and may be used to check the operation of a transmitter 
system which incorporates the antenna. A 150 MHz 0-50 watt RF source is 
connectable to an antenna 5 on a ground plane 4 and to a 50 ohm dummy load 
3, via a wattmeter 2. 
FIG. 17 shows a further test rig which enables input impedance properties 
of an antenna to be obtained as a function of frequency, and gives the 
resonant frequency and tuning effects with good accuracy but the impedance 
level is only a qualitative assessment indicating for example well above, 
well below or near 50 ohms. A Rhode and Schwarz Polyskop meter 4 has its 
RF output connected to an antenna 6 or a ground plane 5 and the antenna 
output measured using a grid dip oscillator 7. A frequency meter 1 is 
connected to an input of a signal generator 2 and the output fed to an 
external frequency marker input of the meter 4 via an amplifier 3. 
The admittance G+j b read from test rig shown in FIG. 15 was converted to a 
normalised impedance r.sub.N +jX.sub.N with respect to 50.OMEGA.. G was 
read directly in mhos from the Wayne Kerr Bridge but B had to be converted 
from the capacity reading of Cpf which could be +ve or -ve. 
##EQU1## 
where f=frequency. The results were computer processed and then plotted on 
a Smith chart of which a typical example is shown in FIG. 18. Antenna 
efficiency was calculated by noting the value of the antenna resistance at 
resonance, both with and without the Wheeler can. Let r.sub.r and r.sub.L 
denote the equivalent radiation and loss resistance of the antenna at 
resonance then it follows that 
##EQU2## 
For the example in FIG. 18 we have 
EQU r.sub.r +r.sub.L =0.32.times.50.OMEGA. 
EQU r.sub.L =0.26.times.50.OMEGA. 
giving .eta.=18.75% and L.sub.A =7.2 dB. 
The bandwidth of an antenna depends on its input impedance characteristic 
and also the impedance of the transmitter or receiver. The former can be 
read directly from the Smith chart and if the frequency at which r.sub.N 
=X.sub.N sets the limits of the bandwidth this is readily given by the 
locus of r.sub.N =X.sub.N on the chart. In practice however the antenna 
impedance would be transformed in some way to the desired impedance level 
which was taken as 50.OMEGA.. A circle delineates a region of operation 
whereby a resulting input VSWR is less than a given value. The radius of 
the circle for various VSWR values is indicated on the chart. As 
electrical transformation networks for the antennae are not precisely 
known, the VSWR circles are centred about the antenna resonance point 
which presupposes that the curves will not change too much under 
transformation. The bandwidth assessments are therefore taken as an 
approximate average of both the `r.sub.N =X.sub.N and `VSWR circle` 
methods as appropriate, to obtain a realistic appraisal. 
Referring to FIG. 18 and FIG. 15 the conventional quarterwave wire monopole 
M1 exhibited a lower input resistance r.sub.o than was expected which 
indicates the effect of wire connections which were used between a 
measurement socket in the ground plane 1 and the measurement bridge 2. 
Proximity effects due to distant walls etc also made some small 
contribution to this. The helical (H1) and counterwound (C1) antennae were 
similar in characteristics exhibiting very low input impedances; curves 
for H1 are given in FIG. 19 and the usefulness of the Polyskop trace is 
evident since it provides, by comparison with the bridge a very quick 
measurement. 
Antenna characteristics of doublewound antennae with different diameters 
were investigated. This was done by using thicker wire for larger 
diameters but it was not possible to keep the precise N value throughout. 
Impedance curves for D2 with and without the Wheeler can are given in FIG. 
18 and are fairly typical of the antennae in this test. It was found with 
this series of antennae that one of the windings in complete isolation 
from the other resonates at a wavelength which is between 8 to 10 times 
greater than that of the doublewound configuration. Thus a 150 MHz 
doublewound antenna resonates around 20 MHz when the outer winding is 
removed and is a useful guide to the number of turns required. 
Clearly the additional wire used above that for a conventional helical 
antenna increases antenna losses and consequently the bandwidth and input 
impedance. The question then arises as to whether the counterwound antenna 
bandwidth can be obtained by simply damping down a helical antenna. It was 
evident however from the previous experiments that double humped bandpass 
input characteristic can be obtained by varying the turns ratio and coil 
spacing. Additional bandwidth can be acquired by coupling turns in 
opposition. Increased bandwidth is not just due to additional losses as 
maybe judged by comparing actual loss values. For example D2 has about 
five times the bandwidth of H1 yet the r.sub.o value has increased from 
only 5.OMEGA. to 16.OMEGA. which is approximately 3:1. 
Antenna DD was measured with four configurations of winding connections and 
the results are shown in FIGS. 20 and 21. The bandwidth and losses can be 
changed in a ratio of about 2:1 by exciting either one or the other coils 
or both. Antenna DD3 is very wideband but at the expense of high loss but 
DD1 on the other hand has improved characteristics over DD4 with little 
extra loss. 
The series of antennae DU1-7 were used to investigate the effect of unequal 
turns for various diameters. The response curves are similar in shape to 
those of DD1-4 and the results are summarised in Table 1. Antenna DU1 has 
more turns on the inner coil while DU2 has more turns on the outer. The 
latter gives a lower input resistance r.sub.o and L.sub.A value. Antennae 
DU3-6 are for larger diameters and different lengths, and include tests on 
exciting the inner coil only. Finally DU7 is a compromise between the 
turns and diameters previously tried. It appears that the tuning shifts 
upward if only the inner winding is connected while tuning is not affected 
much by exciting only the outer winding whereas the other parameters 
change. 
FIG. 22a illustrates how the changes in antenna parameters relates to the 
diameter of the plastic former for antennae D2 to D5. As somewhat 
different N values were used in each case the curves have been adjusted by 
scaling the parameters in the ratio of the N value to an average figure of 
4.5 at about f.sub.o =150 MHz. The resistance r.sub.o is a sensitive 
parameter but lies scattered between 15 and 25.OMEGA. with no discernible 
trend. The Q value on the other hand increases with diameter inferring 
that a small diameter is to be preferred but then the loss factor L.sub.A 
appears to be minimum for moderate diameters; D2 is on the whole an 
optimum choice. The trend curves of FIG. 22b reveal the large change in 
characteristics that are brought about by unequal turns and these curves 
are again scaled to approximately adjust for the different N values used 
by using N=4.5 at 150 MHz. Q and L.sub.A vary much less than r.sub.o about 
the unity ratio condition so this could prove a useful technique for 
adjusting the input resistance. It could however make the antenna 
sensitive to production tolerances if the design was critical. It appears 
that the Q is consistently reduced by about 30% and the frequency of 
resonance raised by between 2.5 and 9%; L.sub.A and r.sub.o also increase 
but no other trend is apparent. This technique appears to provide a means 
of obtaining more bandwidth from a doublewound antenna at the expense of 
increased loss; the increase in r.sub.o may offset the increased loss 
factor in some systems however if a better match is obtained. 
An N value of between 4 and 5 is suitable and at 150 MHz a diameter of 
between 10 and 20 mm appears optimal for a doublewound coil with equal 
turns and both coils excited, with a length to diameter ratio of between 
12:1 and 6:1. 
A design procedure which was adopted was to choose N and a diameter within 
the above recommended range and select a wire gauge that permits enough 
turns to be wound on to obtain resonance. After adding the identical outer 
coil the characteristics are measured and if increases in r.sub.o are 
required the turns ratio are made slightly different from unity. The ratio 
can be greater or less than unity to give an increase in r.sub.o. Finally 
if more bandwidth is required then only the inner need be excited and the 
outer coil is then left in position but disconnected at the base. A 
comparison of D2 and DU6/DU5 indicates that where unequal turns are used 
and only the inner is excited then both higher bandwidth and r.sub.o are 
obtained for a 54% increase in L.sub.A. 
A commercial VHF radio (Fidelity Radio LTD) was fitted with a doublewound 
antenna of length 130 mm instead of the conventional pull-up whip antenna 
which is typically about 6 to 7 times longer. The above design procedure 
was followed and 295 turns of 26 SWG were wound on a 16 mm former and the 
same number of turns were wound directly on top to make the outer winding. 
A layer of `Sellotape` separated the windings. The antenna tuned to 85 MHz 
and the input resistance r.sub.o was around 40.OMEGA. according to the 
Polyskop response. No admittance measurements were made and it is assumed 
that the results at 150 MHz gave a good guide to behaviour at 85 MHz. The 
antenna was placed horizontally and at right angles to the radio case. 
User tests showed little difference between the reception with the small 
antenna, as compared to that with a 600 mm pull-up whip. This assumed that 
the antenna was suitably oriented but this presents a problem if the 
antenna is fixed to the set. Attempts were made to place the antenna 
completely inside the set in close proximity to the radio circuitry but 
the reception was impaired. This was examined by placing circuitry against 
the antenna when connected to the Polyskop and it was found that both 
detuning and damping effects occurred. In one experiment the detuning was 
allowed for by rewinding the antenna to suit but the damping effects 
continued to impair the reception. 
A small antenna in accordance with the invention is less prone to detuning 
effects produced by objects within a housing because a conventional pull 
out whip antenna has a radius of reach five or six times greater than the 
small antenna. 
A small antenna in accordance with the invention may be used on a domestic 
VHF set with a loss of signal strength as little as 2-4 dB which can be 
made up for by increasing radio gain. In particular tests confirm that the 
antenna needs to be freely mounted both to avoid damping by the set and to 
allow orientation in weak signal conditions. 
Helical antennae are extensively employed on portable transreceivers but 
their narrow bandwidth and very low impedance characteristics as 
illustrated by antenna H1 create both design and operational problems. By 
comparison the doublewound technique offers stable wideband operation with 
an opportunity of matching more directly to the transmitter which in turn 
can make up for the additional system loss L.sub.A. System loss on an 
infinite ground plane and also the relative sensitivity of the doublewound 
and helical antennae on small ground planes were investigated. Antenna D2 
was connected to a receiver and its VSWR using the test rig of FIG. 16 was 
about 2:1 at 145 MHz. On allowing for this mismatch, the antenna system 
loss above that of a matched quarter-wave monopole was between 3 and 7 dB 
thus agreeing in order with the measurements of L.sub.A in Table 1. In 
these tests a 2 ft diameter ground plane was used. 
For the small ground plane tests several metal boxes with open ends were 
constructed and their rectangular shape was proportioned to resemble that 
of typical portable transreceivers. The dimensions of the transreceivers 
were as follows: 
TABLE 4 
______________________________________ 
DIMENSIONS OF METAL CASES USED AND MEANS 
OF CONNECTION TO POLYSKOP 
ANTENNA 
SURFACE 
CASE HEIGHT WIDTH DEPTH AREA 
No h (mm) w (mm) d (mm) cm.sup.2 
______________________________________ 
1 242 190 62 12200 
2 160 130 62 690 
3 140 110 50 500 
4 152 90 42 440 
5 120 83 32 300 
______________________________________ 
A coaxial lead was connected to an antenna socket 3 on one end of each case 
as shown in FIG. 23 so that the impedance characteristic as a function of 
frequency could be displayed on the Polyskop. Antennae D2, M1 and H1 were 
used in the tests whereby the antennae were measured on each of the boxes 
in turn. The shift of frequency brought about by the effect of a small 
ground plane is shown in FIG. 24 for each box. 
Monopole M1 is very sensitive to the ground plane size and presumably the 
shape. Large frequency changes can be obtained by varying the position of 
the hands. The test in isolation was done by supporting the case on 
polystyrene foam. By comparison the small antennae were very stable with 
the helical antenna showing less frequency shift. However the Q of the 
helical antenna is some five times greater than the Q of the doublewound 
and the latter is therefore the most stable in operation. This useful 
experiment shows the benefits of small antennae when used in conjunction 
with a small ground plane. 
The illustrated embodiments of the invention are not intended to provide an 
exhaustive catalogue of possible embodiments, and a person skilled in the 
relevant art will be able to produce others. For example the antennae can 
be made shorter and wider, and other core materials can be used. For 
example, distilled water was used as a dielectric because of its easy 
availability and convenience in a laboratory context, but for practical 
production purposes, other known dielectrics may well be more convenient 
and more effective. For example, rutile type dielectrics are known which 
have higher dielectric constants than water at VHF frequencies and have 
very low losses. Also titanate ferro-electric dielectric materials have 
very high dielectric constants, but they have comparatively high losses, 
and their properties are temperature dependent to an undesirable degree.