Log Periodic Antenna Feed & Matching

- notes and details on the feed or feeder arrangements and system used feeding a log periodic dipole array.

One key issue with any antenna is that of providing the correct feed arrangements.

Most antennas have a single feed point, but the log periodic dipole array applies power to all the elements, and the ones in the active region accept the power. This means that the overall feed system for the log periodic antenna is more integrated intot he antenna than for many other types.

Log periodic feed arrangements

The log periodic dipole antenna presents a number of difficulties if it is to be fed properly.

The feed impedance is dependent upon a number of factors.

However it is possible to control this by altering the spacing, and hence the impedance for the feeder that connects each of the dipole elements together.

Despite this the impedance varies with frequency, but this can be overcome to a large extent by making the longer elements out of a larger diameter rod. Even so the final feed impedance does not normally match to 50 ohms on its own. It is normal for a further form of impedance matching to be required. This may be in the form of a stub or even a transformer.

The actual method employed will depend to a large degree on the application of the antenna and its frequency range.

By Ian Poole

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