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.