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Art Unwin, KB9MZ wrote:
"If you place some circuit close to a normal dipole the circuit will take some of the energy from the dipole via coupling such that the circuit will oscillate and reradiate at its resonant frequency or at a frequency determined by its particular coefficient of coupling." Not exactly.. To obtain continuous oscillation requires a gain equal to loss in a circuit and introduction from the output to the input of the circuit a sufficient fraction in the proper phase to reinforce signals in the circuit. An oscillator is likely to have its frequency affected by any external coupling. That`s a reason to shield an oscillator and to provide a buffer between its output and external circuitry. A characteristic of a radio wave is its amplitude and frequency lock to the generator which produces it. Subsequent mistreatment of the wave by its environment is usually incapable of altering its alternations. Best regards, Richard Harrison, KB5WZI |
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Art Unwin KB9MZ wrote:
If I have a loop circuit unconnected to a transmitter could it not oscillate under ideal conditions? Art, what is the power source? -- 73, Cecil, W5DXP |
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Locate a tuning fork. Mount it any way you please. Then check it from
time to time to see if it spontaneously begins a sustained ringing. As a mechanical engineer, you're well acquainted with the differential equations that describe the motion of a physical object that's been struck, for example a tuning fork. And you'll recall that the form of the solution is a decaying sinusoid. An antenna or other resonant circuit obeys the same equations and behaves the same way. In electrical parlance, this response to excitation is called "ringing", after the obvious physical equivalent. While a tuning fork or an antenna will ring if excited, an antenna won't spontaneously ring, or produce a sustained oscillation without an external source of power -- for exactly the same reasons a tuning fork won't. Roy Lewallen, W7EL Art Unwin KB9MZ wrote: Richard, If I have a loop circuit unconnected to a transmitter could it not oscillate under ideal conditions? If it can then would it not radiate at the frequency that it is resonant at as well as reradiate at the frequency of the energy input Regards Art |
Art Unwin, KB9MZ wrote:
"If I have a loop circuit unconnected to a transmitter could it not oscillate under ideal conditions?" Ideal conditions would require a source of energy to replenish losses in the loop circuit. The source has to be the same frequency as that consumed in operation of the loop. A loop like any conductor or circuit has a self-resonant frequency. At resonance, the conductor`s inductive and capacitive reactances cancel. This zero reactance leaves only resistance to limit current in the conductor. Some of the total resistance may be a coupled load, and some will be radiation resistance, which is the conductor`s loss of r-f energy to radiation. Some energy will be lost in conversion to heat at the surface of the conductor and perhaps other locations. At frequencies not too far from resonance, reactance of the wire rises so high that little current flows and the wire has little effect on anything. Broadcasters are faced with structures which arise near their antenna arrays. At times these are resonant at the broadcast frequency and if so they absorb and re-radiate energy distorting the station`s radiation pattern. The solution is usually simply applying something to the new structure to detune it from resonance at the broadcast frequency. If not very near resonance, the structure won`t pickup enough energy to cause trouble. Too much reactance to allow current flow. The hard fact that a structure must be near resonance to admit significant energy makes broadbanding an antenna by an appurtenance tuned to some frequency other than the fundamental frequency of the antenna challenging. One method that works is a combination of antennas resonant for all the desired frequencies. There are other methods to get a wire to accept current over a wide frequency range. Wave antennas are an example. But, standing wave antennas are the most common and these need resonance or thereabouts. Best regards, Richard Harrison, KB5WZI |
Cecil Moore wrote in message ...
Art Unwin KB9MZ wrote: If I have a loop circuit unconnected to a transmitter could it not oscillate under ideal conditions? Art, what is the power source? Wow, this brought forth a lot of comments to contemplate. Regarding your question. I was thinking of energy radiated by a nearby source such as another radiator with very loose coupling from which it gets energy, and current flows in the loop, I then reason that with energy being applied to the loop it will also create oscillation PLUS a emf feedback to the initiating energy scource which then also reacts giving a 'pulse' to continue oscillation as well as energy for radiation. I see the loop not only reradiating the initial frequency impinged upon it by the outside source but also a radiation at the frequency of oscillation somewhat similar to that seen in a receiver where the object is to bring the two frequencies together. Now I am not electrically based and I am also guilty of using wrong terms in my description but I am using the hobby to experiment and learn. Now I did play with such a set up and was able to see two frequencies on the 141T, Whether I interpreted correctly what I saw is another matter. Fortunately I am not teaching the subject or trying to make a living from it nor do I have a resume to protect which gives me the opportunity to speculate, play around with the hobby without the danger of being beheaded. That is what is great about this hobby, it accepts all, the curious and the appliance operators all of which are not frightened by showing their ignorance of the subject but trying just the same. If I thought I knew a bit about conjugate matches I would have participated in the other postings but that was really beyond my ken and interest so you lucked out there. Cheers Art |
Richard Clark wrote in message . ..
On 10 Oct 2003 08:06:45 -0700, (Art Unwin KB9MZ) wrote: Richard, If I have a loop circuit unconnected to a transmitter could it not oscillate under ideal conditions? If it can then would it not radiate at the frequency that it is resonant at as well as reradiate at the frequency of the energy input Regards Art Hi Art, Distinguish between what resonates and what oscillates. Passive circuits do not oscillate even though they support the flow of alternating current. If what you state above is correct then I am wrong since basically what I am descibing is a passive feedback circuitu Art The ideal conditions for oscillation is the presence of a power source, gain, and feedback. If any of the three is missing, then there is no oscillation. The ideal conditions for resonance is an external source of alternating current coupled to a system that is harmonically related. If that system has no harmonic relation, then there is no oscillation or resonation. ALL systems that support the flow of alternating current radiate. How well this is performed is called efficiency. You cannot change a frequency without a nonlinear interface (like a diode) or without the original excitation source containing spurs (illegal at the antenna). 73's Richard Clark, KB7QHC |
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