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Efficiency of 200-ohm hairpin matching
On Apr 8, 12:31 pm, "Antonio Vernucci" wrote:
Over that range, the equivalent series capacitance changes from 59pF at the low end to 138pF at the high end, and at least by NEC2's prediction, the impedance changes especially quickly around 51MHz-- both reactive and resistive parts. 50.75MHz: 10.3-j31.76; 51MHz: 3.91- j22.56, quite a large percentage change in 250kHz. Having the effective series capacitance change that quickly will cause the matching network to behave very differently than it would with a capacitance element that is fixed. That is exactly the point! It would not be correct to calculate bandwidth on the basis of the Q factor at resonance and assuming that the capacitive antenna reactance is equivalent to that of a fixed capacitor. Today I have discovered another shortcoming of that antenna. After raining cats and dogs, the antenna resonant frequency gets lowered by about 130 kHz due to the influence of the wet terrain. That is really a lot if you consider that, after making very accurate measurements with a Bird wattmeter, the antenna bandwidth is only 100 kHz at 1.4 SWR! I am considering to re-build the driven element for 50-ohm match, by using a longer driven element and a 1:1 balun. However it will not be easy to find the optimum situation because there are two variables to be adjusted, that is the driven element length and the hairpin length. Also, I am not too sure on to which extent using a longer driven element would influence the antenna radiation pattern. Any comment? 73 Tony I0JX Though the Q calculation doesn't give the right SWR bandwidth for the antenna/matching system, it does tell you that (with such a low loaded Q), it should not be difficult to make a hairpin or even standard helical coil inductor that has low enough loss that you can ignore the effect. I believe that the physical length of the driven element in a Yagi is much less important than the tuning and spacing of the parasitic elements. The question becomes something like this: what is the relative amplitude and phase of the current in each parasitic element, for some excitation of the driven element? A Yagi is a system of coupled resonators, like a system of coupled pendulums. If one of the pendulums is driven at a particular amplitude and frequency, even if it's not that pendulum's natural frequency, the rest of the pendulums will follow along pretty much the same as if the driven pendulum was tuned to have that natural frequency. In the antenna, the difference will only be in the coupling from the driven element to the others, and I believe that changes only slightly as the length of the driven element changes. But I may be wrong about that, and await my re-education. ;-) But I just ran EZNec on the example "NBS" 3-element 50.1MHz Yagi, varying the nominal 110 inch long D.E. by +/- 10 inches, and saw the expected fairly large variation in impedance, but only 0.02dB change in gain over that whole range, with similarly small variation in F/B ratio and beam width. The longest D.E. I ran was also the highest gain (by that tiny amount), and provided enough inductive reactance that the feedpoint could be tuned to resonance and present 200 ohms by shunting with about 55pF capacitance. Next to try: compare the SWR bandwidths of the hairpin (inductive) shunt of a shortened D.E. and the capacitive shunt of a lengthened D.E.. Unless someone offers a better test case, I'll use the NBS 3 element Yagi... Cheers, Tom |
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