The main reason antennas are hard to understand is that their dimensions are comparable to the wavelength they are tuned for. Unless you are dealing with microwaves, the systems used to generate and detect RF are modeled using "lumped circuit elements". For example, resistors, capacitors, inductors, and transistors. And, at first, big sparks. These elements are small compared to the wavelength.
The design of those elements and the circuits using them was historically pretty independent from formal electromagnetic theory, as developed by Maxwell. The intersection was Oliver Heaviside, at the end of the 19th century.
Before Heaviside, RF electronic design was a largely a matter of groping though the practicalities of employing those circuit elements to create oscillations and couple them to resonant wires supported as high as possible.
As the peer posts explained, antennas are resonant structures in which electrons are caused to slosh back and forth. As they slosh, they accelerate, and as they accelerate, they radiate electromagnetic energy. But, it is best to stick to the rules of thumb at the level of the ARRL handbooks. To understand the EM theory related to radiation and antennas, you really need to work through to the final chapters of Griffiths, "Introduction to Electrodynamics".
But that is not necessary to get an intuitive understanding of antennas, to construct them, or to run the modeling software.
Resonant antennas are only one class of antennas, with the other being traveling wave antennas. A log periodic looks similar to a Yagi, but it’s not. Same with a biconical and dipole.
It’s an easy day when I have dimensions on the order of a wavelength. Usually it’s 1/10 or less, and shoved up against metal.
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?
When 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.
Thanks, 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.
The design of those elements and the circuits using them was historically pretty independent from formal electromagnetic theory, as developed by Maxwell. The intersection was Oliver Heaviside, at the end of the 19th century.
Before Heaviside, RF electronic design was a largely a matter of groping though the practicalities of employing those circuit elements to create oscillations and couple them to resonant wires supported as high as possible.
As the peer posts explained, antennas are resonant structures in which electrons are caused to slosh back and forth. As they slosh, they accelerate, and as they accelerate, they radiate electromagnetic energy. But, it is best to stick to the rules of thumb at the level of the ARRL handbooks. To understand the EM theory related to radiation and antennas, you really need to work through to the final chapters of Griffiths, "Introduction to Electrodynamics".
But that is not necessary to get an intuitive understanding of antennas, to construct them, or to run the modeling software.