The Design of Dipole and Monopole Antennas with Low Uncertainties

Close agreement between the measured and predicted insertion loss between dipole antennas above a conducting ground plane has produced a calculable standard dipole antenna. Antenna factors of dipole antennas in the frequency range 30 MHz to 500 MHz have been determined to an uncertainty of 0.1 dB. Though the design of the dipole is not new, the key achievements are: the use of a large standard ground plane, the use of the program NEC to obtain broadband results, careful design of antennas and supports and precision measurements. Insertion loss and antenna factor are calculated by two independent methods, numerical and analytical, and the results agree to better than 0.05 dB. The NPL calculable dipole antenna was originally designed for electromagnetic compatibility (EMC) applications in the frequency range 30 MHz to 1 GHz, but it can be used wherever electric field strength of free-field signals needs to be precisely measured. The technique was applied to monopole antennas for which antenna factor was determined to an uncertainty of 0.15 dB over the frequency range 10 MHz to 100 MHz. In this paper, traceability of antenna factor to the predicted value rather than a measured value is justified.