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Small Antenna Characteristics

Radiation Resistance:

  • The antenna radiation resistance is probably the most critical factor in determining the performance of an ESA.


  • The antenna radiating efficiency is determined by the ratio of the radiation resistance to all of the combined resistances present in the antenna system.




  • Only the current consumed in the antenna by the radiation resistance produces radio waves. The rest of the power is wasted producing heat in the other resistive antenna components.


  • The radiation resistance gets lower by the inverse square law as the antenna gets shorter.




  • Low radiation resistance means less damping of the current oscillating in the antenna, resulting in high currents and voltages.


  • The low radiation resistance will result in a high antenna Q value if resistive losses are kept low.





Antenna Q:

  • The Q value is generally inversly proportional to the size of the unmodified antenna.


  • As with other tuned systems and circuits, an antenna with a high Q value will also have a correspondingly narrow bandwidth.


  • Since the received signal noise level is generally directly proportional to the bandwidth, the result from the high Q of an ESA can be great listening asset. This is especially true for small magnetic loop antennas.


  • On the other hand, the bandwidth can become so narrow that tuning becomes problematic and the ESA may even become unusable for the wider modulation schemes.


  • An ESA with a low Q value and a wide bandwidth is an indication of a poorly functioning antenna due to a low radiation resistance and high resistive losses, regardless of the antenna SWR reading.


  • The high Q and the correspondingly high currents and voltages results in strong local antenna fields.





Antenna Efficiency and Effectiveness:

  • High efficiencies are difficult to attain in ESAs because generally, as radiation resistance goes down, so does the radiation efficiency.

    Examples below assume a ground and coil loss resistance of 15 ohms:

    • 1/4 Wavelength Vertical Antenna: (Radiation Resistance will be about 36 ohms)

      - Radiation Efficiency = 36/36+15 = 71%


    • 1/16 Wavelength Vertical Antenna: (Radiation Resistance will be about 2 ohms)

      - Radiation Efficiency = 2/2+15 = 12%


  • Because of the low radiation resistance in an ESA, a lot of current will be necessary to radiate adequate power.



  • The low radiation resistance and resulting higher currents means a lot of power gets wasted in resistive components.


  • Since current is closer to the ground in an ESA, ground losses will be greater than in a full size antenna.


  • Because radiated power is directly related to the in-line amper-feet of an antenna, ESAs, being short, will not be able to exactly match the radiated power of a full size antenna.


  • Building an ESA with high performance, considering the above characteristics, can be challenging. However, when carefully constructed, ESAs can and do perform surprisingly well for their size.