Dipole Antenna Feeders & Feed Impedance

- knowing how to properly feed a dipole antenna can improve its performance as well as reducing unwanted effects. Calculating the feed impedance is also a key part of this process.

The feed impedance is of particular importance along with knowing the best way of feeding a dipole antenna.

To ensure the optimum transfer of energy from the feeder, or source / load, the dipole antenna feed impedance should be the same as that of the source or load.

By matching the feed impedance of the dipole to the source or load, the antenna is able to operate to its maximum efficiency.

Dipole feed impedance basics

The feed impedance of a dipole is determined by the ratio of the voltage and the current at the feed point. A simple Ohms Law calculation will enable the impedance to be determined.

Although a dipole can be fed at any point, it is typically fed at the current maximum and voltage minimum point. This gives a low impedance which is normally more manageable.

Most dipoles tend to be multiples of half wavelengths long. It is therefore possible to feed the dipole at any one of these voltage minimum or current maximum points which occur at a point that is a quarter wavelength from the end, and then at half wavelength intervals.

 A dipole antenna using three half wavelengths showing the distribution of voltage and current.
Three half wavelength wave dipole antenna showing feed point
points λ/4 from either end could also be used

The vast majority of dipole antennas are half wavelengths long. Therefore they are centre fed - the point of the voltage minimum and current maximum.

Basic half wave dipole antenna
The basic half wave dipole antenna with centre feed point

The dipole feed impedance is made up from two constituents:

  • Loss resistance:   The loss resistance results from the resistive or Ohmic losses within the radiating element, i.e. the dipole. In many cases the dipole loss resistance is ignored as it may be low. To ensure that it is low, sufficiently thick cable or piping should be used, and the metal should have a low resistance. Skin effects may also need to be considered.
  • Radiation resistance:   The radiation resistance is the element of the dipole antenna impedance that results from the power being "dissipated" as an electromagnetic wave. The aim of any antenna is to "dissipate" as much power in this way as possible.

As with any RF antenna, the feed impedance of a dipole antenna is dependent upon a variety of factors including the length, the feed position, the environment and the like. A half wave centre fed dipole antenna in free space has an impedance 73.13 ohms making it ideal to feed with 75 ohm feeder.

Factors that alter the dipole feed impedance

The feed impedance of a dipole can be changed by a variety of factors, the proximity of other objects having a marked effect. The ground has a major effect. If the dipole antenna forms the radiating element for a more complicated form of RF antenna, then elements of the RF antenna will have an effect. Often the effect is to lower the impedance, and when used in some antennas the feed impedance of the dipole element may fall to ten ohms or less, and methods need to be used to ensure a good match is maintained with the feeder.

Dipole height above ground

For larger dipole antennas like those used for frequencies below about 30 to 50 MHz, the height above ground can be a major influence on the feed impedance. At these frequencies the distance between the antenna and the ground may be only a wavelength or two in many instances. At these sorts of heights, the ground can have a major influence on the impedance, especially when the antenna is mounted horizontally as is often the case.

 A plot of the impedance variations of a half wave dipole antenna for different heights above ground.
Variation of a half wave dipole at different heights above ground

As can be seen from the impedance variation plot, the largest swings of impedance are seen when the dipole antenna is closest to the ground. It then closes in on the free space value.

By Ian Poole

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