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Re: Antenna physical size & shape



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:15 AM 4/9/2006, Tesla list wrote:
Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Tesla list wrote:

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 12:29 PM 4/8/2006, Tesla list wrote:

Here, you're back to the tradeoff between physical size, efficiency, etc. A physically small antenna (in terms of wavelength) will tend to be less efficient *as a radiator* than a physically large.

I'd qualify what Jim says just a bit. His remarks about the same radiated power independent of size only apply for the same power actually flowing in the antenna. As the length of a grounded vertical (or half of a dipole) gets shorter the radiation resistance goes down and it's much harder to match the input impedance of the antenna to the output of the transmitter. For very short antennas (<< 1 wavelength) the radiation resistance varies as the square of the ratio of the length to the wavelength while at the same time the antenna capacitance goes down, requiring a larger series coil for resonance. Of course, the losses in the series (loading) coil go up as the inductance goes up so the overall efficiency goes to pot in a hurry unless extraordinary steps are taken to keep losses down. For the grounded vertical the ground circuit loss usually dominates and there's no way to get much efficiency.

The remark about the bandwidth is important too. The effective Q of the antenna circuit is the ratio of the reactance of its capacitance (all electrically short antennas have capacitve reactance) to the radiation resistance so it also goes up as the antenna gets shorter.


Actually, that's just an approximation. The "real" Q when talking about antennas is the ratio of the energy stored in the antenna system (including the near field) and the energy radiated away (or lost in thermal dissipation). For a low loss antenna, the ratio of radiation resistance and inductive (or capacitive) X is pretty close. If the antenna is lossy (and most low frequency antennas are), then you also have to add in the loss resistance.



Higher Q means narrower bandwidth of course.

Narrower bandwith *if driven from a constant impedance source*. Drive it from a current source, and it can be pretty wideband, albeit inefficient.



The whole subject of electrically short antennas is a complicated one and has been understood since the days of Tesla and Marconi.

With some significant work in the last half century or so by Chu, Harrington, Thiele and Hansen, among others, at least as far as theoretical limits of performance go.