Suitable Antennas (5c4, 5c5, 5d1)

 

The antenna system must be suitable for the frequency of the transmitted signal. 

 

If the antenna is not correctly designed for the transmitter it will not match the transmitter and will not work effectively.

 

The picture below shows a number of very large Yagi antennas for a variety of hf bands.  Notice that element lengths are longer on bands with the longest wavelengths (and the lower frequencies).

 

 

A dipole antenna is always λ/2.  So a dipole for a wavelength of 40m will be 40m ÷ 2 = 20m long.  A dipole for a wavelength of 4m will be 4m ÷ 2 = 2m long.

Antenna Tuning Units (ATU)

 

On the hf bands, where an antenna has not been designed for the particular frequency, an ATU (Antenna Tuning Unit) makes it possible for the antenna to accept power from the transmitter. 

Balanced and Unbalanced Antennas

 

If an antenna is symetrical (e.g. both halves of a dipole look the same) is called a “balanced” and need symetrical or “balanced” feeder.  Co-axial cable is un-balanced, but can be used with “balun” (balanced to unbalanced transformer).

 

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