An antenna transforms radiofrequency energy into an electromagnetic wave field. Shortwave antennas are comparable in size to the wavelength and can achieve conversion efficiencies close to 100 percent, whereas medium and particularly long wave antennas generally are electrically short and have much lower efficiencies. The ratio of radiation (effective) resistance to loss resistance in the overall system and ground determines efficiency.
>> Electromagnetic fields
Radiated power from dipole as function of wavelength.
Height is the single most expensive parameter in antenna design, and budget restraints often limits it to less than the electrical optimum. Time-proven techniques such as top loading capacitance, multiple tuning and electrically fat antennas using wire cages or multiple electrically short masts, can help maintain high radiation resistance and true resonance without unnecessary sacrifice of bandwidth. Effective ground systems reduce loss resistance.
>> Radio antenna engineering
Very low frequency antennas, Nova Scotia 1943.
Computers can accurately predict the performance of a proposed antenna by modelling it with electrical segments. The simulation is performed using the "method of moments" solution of the electric field integral equation for thin wires and the magnetic field integral equation for closed, conducting surfaces. The simulation can include wires buried in homogeneous ground, insulated wires and impedance loads. Free software based on professional cores is available.
>> 4NEC2 design software
3D plot of a short wave antenna using 4NEC2 software.
Electrically short antennas exhibit not only a low useful radiation resistance, but also a large reactive component that does not contribute to radiation. The reactive component only stores energy and thus decreases bandwidth by increasing the circuit Q, also increasing peak voltage levels and the risk of arcing. A matching network can mask the antenna's reactive component and transform the resistive component to the impedance of the transmitter.
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DRM extension tuning network at Oranienburg 177 kHz.