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How do I develop my intuition regarding the low resistive impedance of such antennas? I understand the high capacitive reactance, but haven't got a grip on why
by wrycoder 7y ago
How do I develop my intuition regarding the low resistive impedance of such antennas? I understand the high capacitive reactance, but haven't got a grip on why the restive part is so low.
What is your general approach to matching these very short radiators?
- madengr 7y agoWhen you bring an antenna close to a conductor, say a dipole next to a plate, the radiation resistance decreases. This is due to the currents induced in the plate, creating an image and reinforcing the currents in the dipole. It’s really the ratio of radiation resistance to conductor resistance. You can shrink an antenna to infinitesimal size, made of perfect conductor, but as the radiation resistance decreases, it’s more difficult to impedance match. An infinitesimal antenna would have zero bandwidth. Sort of like Bode Fano criteria limiting bandwidth versus impedance. There is a Chu theoretical limit which limits antenna efficiency and bandwidth given volume, hence 3D fractals and other stuff. Ain’t no free lunch. A lot of antenna research is who can get closest to the Chu limit. Sort of like coding and the Shannon capacity. https://en.m.wikipedia.org/wiki/Chu–Harrington_limit https://en.m.wikipedia.org/wiki/Chu–Harrington_limit There is still a lot of snake-oil in antennas, some of it fractal related.
- wrycoder 7y agoThanks, I had not heard of that. Also, that link lead me to the WP article Electrically Small Antenna. It's remarkable how much they knew 70 years ago.