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.