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:Original poster: gary350@xxxxxxxxxxxxxI have an idea. A transmitter antenna radiates output power just like the topload of the tesla coil radiates sparks.Does the physical size of an antenna increase or decrease the output power?Neither. There's a tradeoff between size, gain, bandwidth, and efficiency. A small antenna, if efficient, will tend to to be narrow band.I know an antenna needs to be tuned to the circuit. A multi element beam antenna for example, 2 element beam compaired to a 4 element beam. Next a 4 element beam antenna compaired to an 8 element beam antenna. The 8 element is physically larger than a 4 element and it is larger than a 2 element. Reception and power output are both better with the physically larger antenna.The "gain" of the anntenna is higher with the bigger antennas, but the actual power radiated is still the same. It's just squirted in a smaller range of directions. On receive, there is a physical size impact, particularly for antennas very much larger than a wavelength. For small antennas, the effective aperture is more driven by the size of the reactive near field, rather than the physical size of the antenna.My next thought is, an amateur radio antenna like 80 meters, 40 meters, 20 meters, 2 meters do not physically radiate visible discharge sparks like a Tesla coil. WHY?Sparks are mostly a matter of what the electric field is. In a typical amateur radio antenna, the electric field at the ends of the dipole aren't high enough to spark. However, run a lot of power through a short antenna, and you'll get breakdown at the ends. I've seen corona discharge off the end of a short 40meter vertical with a kilowatt going to it.If the amateur radio antenna were to be retuned would it radiate sparks that could be seen just like a tesla coil? If so then would it still transmit a signal several 100 miles to a receiver? If a tesla coil topload were to be retuned would it transmit just like an amature radio antenna?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. Higher Q means narrower bandwidth of course. In commercial "long wave" broadcast antennas operating around 180 kHz (still quite a few around the world) the antenna consists of several different towers with bases in parallel to get the reactance and the Q down enough to pass speech and music.
The whole subject of electrically short antennas is a complicated one and has been understood since the days of Tesla and Marconi. "Long wave" transmitting antennas tend to be huge (up to miles in length for horizontal wires and up to a thousand feet or more for vertical towers), the cost of the low-loss loading coil becomes a major expense in setting up a station, and very extensive ground systems are used to reduce the ground resistance. One thing which has always made me suspicious of Tesla's claims of wireless transmission (including for sure the ones in Leland Anderson's book) is the extremely casual way he treats frequency and doesn't seem to remember what he used. If he really had an efficient antenna he should have had a vivid memory all of the gory design details, even years later. For this reason I'm very suspicious of his claims. Wardenclyffe was an extreme example of this. The tower (even the big one he dreamed about but couldn't afford) was so short that it would have been a lousy radiator (of course he said he wanted to reduce radiation). I'll leave it to the free power transmission cult to rebut this.
Ed