Don: Regarding the complex impedance of an antenna at the end point, you raise some interesting questions.
I was always of the impression that the definition of resonance of a half wave radiator is the condition in which the current at the center is a maximum and the current at the ends is at zero. The current distribution in a half wave antenna is analogous to the displacement of a violin string, which when vibrating at resonance has zero displacement at the ends and maximum displacement at the center. (Such a resonance is easily detected with a grid dip meter, even if no feedline at all is connected to the radiator. Admittedly, the feedpoint would need to be shorted. Sweeping the grid dip meter through a range of frequencies is analogous to the broadband energy in the "pluck" on the violin string. In either case (assuming very high Q), only the energy at the resonant frequency actually gets coupled into the device.) In the case of center fed half wave element, a zero value of imaginary component of impedance at a center feed point is a coincidental indication of antenna resonance rather than the definition of antenna resonance; it is used by amateurs because it is easy to measure, whereas the current distribution is almost impossible to measure directly. If you run the "BY dipole" simulation on EZNEC at a frequency and radiator length for which the center feedpoint impedance is pure resistance, and then move the feedpoint around, the radiation pattern comes out just the same (within the limits of computational error) regardless of the feedpoint location. Also, the magnitude of the current distribution remains about the same, big in the center and approaching zero at the ends. Thus, my sense is that by the analogy to the violin string, the antenna is resonant at that length for that frequency irrespective of the feedpoint location, or the fact that the feedpoint impedance is complex. Admittedly, that much of the discussion is literally academic, depending on how we define resonance. However, you raise another question that is more practical than academic. You make the perfectly reasonable point that if I play around with the radiator length, I should find a length that has an end feedpoint impedance of some big value of R plus J0. I am sure you can do that, but my question to you is, what is the advantage to doing so? (Note: This is not intended as a smart aleck comment. If there is some advantage easily obtained by tweaking the radiator length, I'd really like to know what it is.) I do not expect that minimizing radiation from the transmission line is one of those advantages. Changing the radiator length such that you move away from the length that gives "violin string resonance" would make the current distribution on the radiator more asymmetrical and would increase the probability of feed line radiation. This line of reasoning got me curious about something else. Elecraft rigs are usually rated as being able to operate normally for any load that has an SWR of 2 or less compared to a characteristic impedance of 50+J0. Am I correct in assuming that that means that the rig expected to be able to operate normally into any complex load on or inside the SWR = 2 circle on the Smith Chart? Why that matters is the following. Using a high impedance quarter wave (approximately) transformer to Zepp feed a half wave radiator, it is relatively straightforward to tweak the transformer length such that the subsequent 50 Ohm coax has an SWR well inside the SWR = 2 circle. If so, the Elecraft rig should be perfectly happy, even if I seldom if ever actually find an impedance with a zero imaginary component. Have I missed something in my thinking? (It would not be the first time.) TNX & 73, Steve Kercel AA4AK ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
