Antenna Gamma Impedance Match

The gamma impedance match is often used with antennas where the main element needs to remain as one physical or mechanical structure

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Impedance matching techniques: Antenna delta match Antenna gamma match λ/4 transmission line matching

As with all antenna systems it is very important that there is a good impedance match between the antenna and the feeder. This is aprticaulrly true where transmitters are used: broadcast transmission, radio communications, HF radio systems, etc . . .

There are many ways of achieving this, but one that has gained popularity in a number of scenarios is the gamma impedance match or as it is often called, the gamma match.

Concept of the gamma match for impedance matching antennas — Concept of an antenna gamma impedance matching system

One of the main uses for the gamma match is for use with large Yagi antennas where the drive element needs to remain as one rod, and not be broken in the centre for mechanical reasons. Here the gamma match is an ideal solution.

The gamma match provides a convenient option for many antenna systems for a variety of different applications.

Gamma match basics

The antenna gamma match is a form of match where the outer from the coaxial cable is connected to the centre of the main driven element of a half wave element.

This can be done because the RF voltage at the centre of the half wave antenna is zero - the voltage rises to a maximum at the ends.

This point may also join with the boom of the antenna if the the overall antenna is of the form of a Yagi or similar type of antenna. This can considerably aid the mechanical aspects and make the structure more robust.

The centre conductor from the coax is taken via a variable capacitor to an off-centre feed point where the impedance will be higher than if the half wave antenna had been broken in the centre in the traditional way to form a dipole.

The capacitor acts to tune out the inductance of the arm connecting the centre of the coax to the point along the antenna element.

Once its value has adjusted, it is then possible to replace the variable capacitor with a fixed component of the same value. This will enable the gamma match system to be more resilient to the weather, assuming that it is outside.

One of the advantages of the gamma match is that it is inherently unbalanced and therefore it can be driven directly by coaxial feeder without the need for a balun for the balanced to unbalanced transition.

In fact the gamma match is half of what os sometimes referred to as a "T" match where the offset feed is implemented for both sides of the element, and the overall system is fed with a balanced feeder.

Implementation of the gamma match

The implementation of the gamma match can be achieved in many ways, but it lends itself to the type of construction used for many Yagi beam antennas where the antenna is constructed from tubing.

It is possible to construct the gamma rod or line out of metallic tubing and the and connecting clamp out of a strip of bent metal plate. Normally aluminium is the preferred metal of choice because it is light and rigid.

It is also necessary to try to use like metals to reduce galvanic corrosion caused by dissimilar metals touching in a moist environment - the weather will always throw water at an antenna at some time or another.

Typical implementation approach for an antenna gamma impedance matching system

One of the disadvantages of the gamma match system is that there are a lot of variables to contend with when designing or building an antenna that uses one. It can take a considerable amount to time to adjust the system for a good level of VSWR across the band required.

One rule of thumb that has been adopted in many quarters for Yagi style beam antennas fed with 50Ω unbalanced feeder is for the matching or gamma rod length to be around 0.05λ and its diameter to be around 0.5 that of the main element diameter.

The spacing from between the main element and the gamma rod is also important and this is often around 0.007λ. The capacitor value is then calculated to be around 7pF for every metre of the wavelength of the antenna for its operating frequency.

As an example, this would mean that for operation at 30MHz with a wavelength of 10 metres the maximum value of the variable capacitor will need to be 70 pF - a little higher would be advisable to ensure that the adjustment can be properly made for every eventuality.

It is also possible to counteract the reactance of the matching rod by reducing the length of the antenna element itself. This can simply the construction of the matching rod system as there is no need to add the capacitor and also adjust it in the antenna system.

The gamma impedance match is often used for HF Yagi beam antennas and sometimes for VHF ones as well. As the impedance of the driven element of a Yagi falls significantly when the parasitic elements are added, some means of matching is needed along with a method of transitioning from the normal balanced feed of a dipole to the unbalanced coaxial feeder normally used.

The gamma match is an ideal solution for this, although it can be difficult at times to adjust for a perfect impedance match.

The gamma match may also be used for other antennas as well - it has been used on halo antennas where the impedance falls as a result of the shape of the antenna. There are also other instances where it may be used. It can be used for radio communications systems, broadcasting, and several other applications.