Ken Javor wrote: >> I've seen that design in a D-dot sensor. The elements were a couple of spheres. The output impedance of such an "antenna" is quite high, it really is an electric field probe, and works at frequencies where the elements are electrically short. These types of devices really sense the time derivative of the filed, hence the name. They are fitted with ...<<
A dipole of half-spheres on the surface of a single sphere seems popular, perhaps because it is convenient for self-contained devices (inside the sphere) and easy to model. There is at least one - there are probably more -- firm making emissions standards or field sensors using this configuration. The shortening of a dipole can't be attributed solely to the difference between perfect and real materials. Kraus derives the impedance of a half-wave, center fed dipole antenna (Antennas, chapter 10) of negligible thickness and perfect conductivity as 73 + j42.5 ohms. Because this impedance is inductive, resonance requires shortening. However, it has often been noted for real antennas that that the lowest VSWR (50 ohm) does not occur at resonance. Of course we get higher capacitance with a real wire, which adds to the shortening effect, and there's got to be SOME phase shift due to the RC time constant of conductor resistance and capacitance. I don't know that anyone has ever quantified that. Might be an interesting experiment. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
