Yagi
antenna theory - the basics
The key element to the Yagi theory is
the phases of the currents flowing in the additional elements of the antenna.
The parasitic elements of the Yagi
antenna operate by re-radiating their signals in a slightly different phase to
that of the driven element. In this way the signal is reinforced in some
directions and cancelled out in others. As a result these additional elements
are referred to as parasitic elements.
In view of the fact that the power in
these additional elements is not directly driven, the amplitude and phase of
the induced current cannot be completely controlled. It is dependent upon their
length and the spacing between them and the dipole or driven element.
As a result, it is not possible to
obtain complete cancellation in one direction. Nevertheless it is still
possible to obtain a high degree of reinforcement in one direction and have a
high level of gain, and also have a high degree of cancellation in another to
provide a good front to back ratio. The Yagi antenna is able to provide very
useful levels of gain and front to back ratios.
Yagi Uda antenna showing element types
To obtain the required phase shift an
element can be made either inductive or capacitive.
Inductive:
If the parasitic element is made inductive it is found that the induced
currents are in such a phase that they reflect the power away from the
parasitic element. This causes the RF antenna to radiate more power away from
it. An element that does this is called a reflector. It can be made inductive
by tuning it below resonance. This can be done by physically adding some
inductance to the element in the form
of a coil, or more commonly by making it longer than the resonant length.
Generally it is made about 5% longer than the driven element.
Capacitive:
If the parasitic element is made capacitive it will be found that the induced
currents are in such a phase that they direct the power radiated by the whole
antenna in the direction of the parasitic element. An element which does this
is called a director. It can be made capacitive tuning it above resonance. This
can be done by physically adding some capacitance to the element in the form of
a capacitor, or more commonly by making it about 5% shorter than the driven
element.
It is found that the addition of further directors increases the directivity of
the antenna, increasing the gain and reducing the beamwidth.
The addition of
further reflectors makes no noticeable difference.
In summary:
Reflectors - longer than driven element
= Inductive
Directors - shorter than driven element
= Capacitive
Yagi Uda antenna showing direction of maximum radiation
Yagi
antenna history
The full name for the antenna is the
Yagi-Uda antenna. The Yagi antenna derives its name from its two Japanese
inventors Hidetsugu Yagi and Shintaro Uda. The RF antenna design concept was
first outlined in a paper that Yagi presented in 1928. Since then its use has
grown rapidly to the stage where today a television antenna is synonymous with
an RF antenna having a central boom with lots of elements attached.
The design for the Yagi antenna appears
to have been initially developed not by Yagi who was a student, but his
colleague Professor Shintaro Uda. However all the original papers were all in
Japanese and accordingly the design was not publicised outside Japan.
It was Hidetsugu Yagi who wrote papers
in English and as a result the design is often incorrectly only attributed only
to Yagi.
Yagi himself did not aim to steal the
publicity, in view of his English papers, and as a result the design now bears
the names of both men and is known as the Yagi-Uda antenna.
Yagi
antenna - the basics
The Yagi antenna design has a dipole as
the main radiating or driven element. Further 'parasitic' elements are added
which are not directly connected to the driven element.
These parasitic elements within the
Yagi antenna pick up power from the dipole and re-radiate it. The phase is in
such a manner that it affects the properties of the RF antenna as a whole,
causing power to be focussed in one particular direction and removed from
others.
basic concept of Yagi Uda antenna
The parasitic elements of the Yagi
antenna operate by re-radiating their signals in a slightly different phase to
that of the driven element. In this way the signal is reinforced in some
directions and cancelled out in others. It is found that the amplitude and
phase of the current that is induced in the parasitic elements is dependent
upon their length and the spacing between them and the dipole or driven
element.
Yagi Uda antenna showing element types
There are three types of element within
a Yagi antenna:
Driven element:
The driven element is the Yagi antenna element to which power is applied. It is
normally a half wave dipole or often a folded dipole.
Reflector :
The Yagi antenna will generally only have one reflector. This is behind the
main driven element, i.e. the side away from the direction of maximum
sensitivity.
Further reflectors behind the first one add little to the performance. However
many designs use reflectors consisting of a reflecting plate, or a series of
parallel rods simulating a reflecting plate. This gives a slight improvement in
performance, reducing the level of radiation or pick-up from behind the
antenna, i.e. in the backwards direction.
Typically a reflector will add around 4 or 5 dB of gain in the forward
direction.
Director:
There may be none, one of more reflectors in the Yagi antenna. The director or
directors are placed in front of the driven element, i.e. in the direction of
maximum sensitivity. Typically each director will add around 1 dB of gain in
the forward direction, although this level reduces as the number of directors
increases.
The antenna exhibits a directional
pattern consisting of a main forward lobe and a number of spurious side lobes.
The main one of these is the reverse lobe caused by radiation in the direction
of the reflector. The antenna can be optimised to either reduce this or produce
the maximum level of forward gain. Unfortunately the two do not coincide exactly
and a compromise on the performance has to be made depending upon the
application.
Yagi antenna radiation pattern
Yagi
antenna advantages
The Yagi antenna offers many advantages
for its use. The antenna provides many advantages in a number of applications:
Antenna has gain allowing lower
strength signals to be received.
Yagi antenna has directivity enabling
interference levels to be minimised.
Straightforward construction. - the
Yagi antenna allows all constructional elements to be made from rods
simplifying construction.
The construction enables the antenna to
be mounted easily on vertical and other poles with standard mechanical fixings
The Yagi antenna also has a number of
disadvantages that need to be considered.
For high gain levels the antenna
becomes very long
Gain limited to around 20dB or so for a
single antenna
Typical Yagi Uda antenna used for television reception
The Yagi antenna is a particularly
useful form of RF antenna design. It is widely used in applications where an RF
antenna design is required to provide gain and directivity. In this way the
optimum transmission and reception conditions can be obtained.
Yagi
gain / beamwidth considerations
It is found that as the Yagi gain
increases, so the beam-width decreases. Antennas with a very high level of gain
are very directive. Therefore high gain and narrow beam-width sometimes have to
be balanced to provide the optimum performance for a given application
Yagi-Uda antenna gain vs beam-width
Yagi-Uda
antenna gain considerations
A number of features of the Yagi design
affect the overall gain:
Number of elements in the Yagi:
One of the main factors affecting the Yagi antenna gain, is the number of
elements in the design. Typically a reflector is the first element added in any
yagi design as this gives the most additional gain. Directors are then added.
Element spacing:
The spacing can have an impact on the Yagi gain, although not as much as the
number of elements. Typically a wide-spaced beam, i.e. one with a wide spacing
between the elements gives more gain than one that is more compact. The most
critical element positions are the reflector and first director, as their
spacing governs that of any other elements that may be added.
Antenna length:
When computing the optimal positions for the various elements it has been shown
that in a multi-element Yagi array, the gain is generally proportional to the
length of the array. There is certain amount of latitude in the element
positions.
The gain of a Yagi antenna is governed
mainly by the number of elements in the particular RF antenna. However the
spacing between the elements also has an effect. As the overall performance of
the RF antenna has so many inter-related variables, many early designs were not
able to realise their full performance. Today computer programmes are used to
optimise RF antenna designs before they are even manufactured and as a result
the performance of antennas has been improved.
Yagi
gain vs number of elements
Although there is variation between
different designs and the way Yagi-Uda antennas are constructed, it is possible
to place some very approximate figures for anticipated gain against the number
of elements in the design.
APPROXIMATE
YAGI-UDA ANTENNA GAIN LEVELS
|
|
|
APPROX
ANTICIPATED GAIN
DB OVER DIPOLE
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
It should be noted that these figures
are only very approximate.
As an additional rule of thumb, once
there are around four or five directors, each additional director adds around
an extra 1dB of gain for directors up to about 15 or so directors. The figure
falls with the increasing number of directors.
Yagi
Front to Back ratio
One of the figures associated with the
Yagi antenna gain is what is termed the front to back ratio, F/B. This is
simply a ratio of the signal level in the forward direction to the reverse
direction. This is normally expressed in dB.
Yagi front to back ratio
The front to back ratio is important in
circumstances where interference or coverage in the reverse direction needs to
be minimised. Unfortunately the conditions within the antenna mean that
optimisation has to be undertaken for either front to back ratio, or maximum
forward gain. Conditions for both features do not coincide, but the front to
back ratio can normally be maximised for a small degradation of the forward
gain.
Feed
impedance of Yagi driven element
It
is possible to vary the feed impedance of a Yagi antenna over a wide range.
Although the impedance of the dipole itself would be 73 ohms in free space,
this is altered considerably by the proximity of the parasitic elements.
The
spacing, their length and a variety of other factors all affect the feed
impedance presented by the dipole to the feeder. In fact altering the element
spacing has a greater effect on the impedance than it does the gain, and
accordingly setting the required spacing can be used as one design technique to
fine tune the required feed impedance.
Nevertheless
the proximity of the parasitic elements usually reduces the impedance below the
50 ohm level normally required. It is found that for element spacing distances
less than 0.2 wavelengths the impedance falls rapidly away.
Yagi
matching techniques
To
overcome this, a variety of techniques can be used. Each one has its own
advantages and disadvantages, both in terms of performance and mechanical
suitability. No one solution is suitable for all applications.
The
solutions below are some of the main solutions used and applicable to many
types of antenna. There also not the only ones:
Balun:
A balun is an impedance
matching transformer and can be used to match a great variety of impedance
ratios, provided the impedance is known when the balun is designed.
Folded dipole:
One method which can
effectively be implemented to increase the feed impedance is to use a folder
dipole. In its basic form it raises the impedance four fold, although by
changing various parameters it is possible t raise the impedance by different
factors.
Delta match:
This method of Yagi impedance
matching involves "fanning out" the feed connection to the driven
element.
Gamma match:
The gamma match solution to
Yagi matching involves connecting the out of the coax braid to the centre of
the driven element, and the centre via a capacitor to a point away from the
centre, dependent upon the impedance increase required.
Balun
for Yagi matching
The
balun is a very straightforward method of providing impedance matching. 4:1
baluns are widely available for applications including matching folded dipoles
to 75Ω coax.
Baluns
like these are just RF transformers. They should have as wide a frequency range
as possible, but like any wound components they have a limited bandwidth.
However if designed for use with a specific Yagi antenna, this should not be a
problem.
One
of the problems with a balun is the cost - they tend to be more costly than
some other forms of Yagi impedance matching. They may also be power limited for
a given size.
Folded
dipole
The
folded dipole is a standard approach to increasing the Yagi impedance. It is
widely used on Yagi antennas including the television and broadcast FM
antennas.
The
simple folded dipole provides an increase in impedance by a factor of four.
Under free space conditions, the dipole impedance on its own is raised from 75Ω
for a standard dipole to 300Ω for the folded dipole.
Note
on folded dipole:
The
folded dipole is a from of dipole that has a higher impedance than the standard
half wave dipole - in the standard version it has four times the impedance.
However different ratios can be obtained by changing the mechanical attributes.
Another
advantage of using a folded dipole for Yagi impedance matching is that the
folded dipole has a flatter impedance versus frequency characteristic than the
simple dipole. This enables it and hence the Yagi to operate over a wider
frequency range.
While
a standard folded dipole using the same thickness conductor for the top and
bottom conductors within the folded dipole will give a fourfold increase in
impedance, by varying the thickness of both, it is possible to change the
impedance multiplication factor to considerably different values.
Delta
match
The
delta match for of Yagi matching is one of the more straightforward solutions.
It involves fanning out the ends of the balanced feeder to join the continuous
radiating antenna driven element at a point to provide the required match.
Delta match for dipole - often
used for Yagi impedance matching
Both
the side length and point of connection need to be adjusted to optimise the
match.
One
of the drawbacks for using the Delta match for providing Yagi impedance
matching is that it is unable to provide any removal of reactive impedance
elements. As a result a stub may be used.
Gamma
match
The
gamma match is often used for providing Yagi impedance matching. It is
relatively simple to implement.
Gamma match for dipole - often
used for Yagi impedance matching
As
seen in the diagram, the outer of the coax feeder is connected to the centre of
the driven element of the Yagi antenna where the voltage is zero. As a result
of the fact that the voltage is zero, the driven element may also be connected
directly to a metal boom at this point without any loss of performance.
The
inner conductor of the coax is then taken to a point further out on the driven
element - it is taken to a tap point to provide the correct match. Any
inductance is tuned out using the series capacitor.
When
adjusting the RF antenna design, both the variable capacitor and the point at
which the arm contacts the driven element are adjusted. Once a value has been
ascertained for the variable capacitor, its value can be measured and a fixed
component inserted if required.
