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Bredhurst Receiving and Transmitting Society

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Part 1

Learning outcomes of this part

  • Identify and recall the use of coaxial and twin feeders.

  • Know that in coaxial cable the RF field only exits within the cable and is not affected by objects outside the cable.

  • Know that feeders cause loss of signal strength on both transmit and receive. The longer the cable, the greater the loss.

  • Know that twin feeder usually has lower loss that coaxial cable.

  • Know that loss is measured in dB.

  • Know how to calculate the power delivered to an antenna for a given RF output and given feeder loss (in multiples of 3 dB).

  • 5b Feeder characteristic impedance

  • Know that the term 'impedance' can be thought of as a form of AC resistance.

  • Know that feeders have a characteristic impedance which depends upon the diameter and spacing of the conductors and those commonly available.

  • Know that an antenna has a feed point impedance which is determined by the relative RF potential difference and current at that point.

  • Know that the impedance of an antenna depends upon its dimensions related to the wavelength of the applied signal.

  • Know that an antenna will only present the correct impedance when fed with the frequency for which it is designed, and that a half-wave dipole has a feed point impedance of approximately 50 ohms when used at its designed frequency.

  • Know that if the impedance of the antenna does not match the impedance of the feeder, then some energy will be reflected back down the feeder and that the proportion reflected will depend upon the degree of mismatch.

  • Know that the signal reflected back down the feeder is not lost but will combine with the waves travelling up the feeder from the transmitter.

  • Know about the formation of standing waves.

  • Know that standing waves will affect the impedance presented to the transmitter causing a mismatch between the transmitter and the feeder.5e Antenna tuning unit

  • Know that a transmitter is designed to transfer energy into a specific impedance.

  • An antenna tuner unit (ATU) can change the impedance that a feeder presents to the transmitter, and that when correctly matched, the ATU may reduce harmonic radiation.

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