Radiation resistance, antenna current and ERP

Radiation Resistance

The radiation resistance of a short, straight antenna can be calculated using the formula:

                                            2
                              (   length   )
                   Rr = 789 x ( ---------- )  ohms
                              ( wavelength )

Thus doubling the antenna length will increase the radiation resistance by a factor of four.

From this we can calculate the radiation resistance of antennas at 73 kHz:

Length (m)   Length (feet)    Rr (ohms)

   6           20             0.0017 
  12           40             0.0067 
  18           60             0.015
  24           80             0.027
  30          100             0.042 
  36          120             0.061
  42          140             0.082

This formula is a simplification: it assumes an electrically short and straight antenna in free space. In the real world, where ground effects exist, or if the antenna does not run in a straight line, the radiation resistance will probably be even lower.

Antenna Current

Having built an electrically short antenna, the next step is to get as much current into it as you can. This can be measured using a thermocouple ammeter connected in series with the `base' of the antenna.

Estimating ERP

The Effective Radiated Power can be estimated using the formula:

           2 
      P = I  x Rr


Where P  = ERP is in Watts 
      I  = Antenna current (amps)
      Rr = Radiation resistance (ohms)

Last updated 23rd June 1996.