Yagi Antenna Gain & Directivity

- Yagi antenna gain is dependent upon many factors including the number of elements, spacing and several other minor factors..

One of the chief reasons for using a Yagi antenna is the gain it provides.

The Yagi or Yagi-Uda antenna gain is of great importance, because it enables all the transmitted power to be directed into the area where it is required, or when used for reception, it enables the maximum signal to be received from the same area.

Gain for reception and transmission are equal when a passive antenna is used - i.e. one without any active elements.


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

A comparison of the radiation pattern of higher and lower gain Yagi-Uda antennas showing the impact on beamwidth
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
Number of elements Approx anticipated gain
dB over dipole
2 5
3 7.5
4 8.5
5 9.5
6 10.5
7 11.5

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.

Diagram showing the front to back ratio of a Yagi antenna on the raditiaion pattern diagram
Yagi front to back ratio

Yagi front to back ratio formula

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.

By Ian Poole


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