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#1
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Efficiency of 200-ohm hairpin matching
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 |
#2
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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 |
#3
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Efficiency of 200-ohm hairpin matching
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 Hi Tom, the results you got on EZNEC are encouraging. Nevertheless I would not like to try using a lengthened element in conjunction with a capacitor, as the difference between that configuration and the original configuration would be the maximum (although it would be much easier to adjust a capacitor than the inductance of an hairpin). What puzzles me is that the antenna manufacturer reported me having sold several hundreds of those antennas, and no one has reported him the bandwidth being too narrow or the exagerated wet terrain influence. I am not sure on what I am going to do, also because I am not 100% sure on whether the bandwidth problem is only due to the matching system, or it is also due to the particular antenna design. My original intention was to compare this 50-MHz long Yagi antenna (32-foot boom) against a smaller antenna (11-foot boom) I have on another tower, so as to determine how much a bigger antenna really helps during multiple-hop sporadic openings to US and Japan. Probably for the forecoming sporadic-E season (May-August) I will leave things as they are, and just try to assess the practical advantages of the bigger antenna. After that I will see what I shall do. Thanks very much for the useful discussion. 73 Tony I0JX |
#4
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Efficiency of 200-ohm hairpin matching
"Antonio Vernucci" wrote in
: .... the results you got on EZNEC are encouraging. Nevertheless I would not like to try using a lengthened element in conjunction with a capacitor, as the difference between that configuration and the original configuration would be the maximum (although it would be much easier to adjust a capacitor than the inductance of an hairpin). Tony, Some thoughts. You are suggesting that it is easier to make a low loss capacitor that is located at the feedpoint in a hostile environment, than it is to make a low loss inductor (the hairpin). Just as the hairpin is a s/c stub for inductive reactance, you could use an o/c stub... but remember that transmission line elements are a path to low Q reactors, use thick conductors for the transmision line (which for an o/c stub will need to be much longer than for the s/c stub). What puzzles me is that the antenna manufacturer reported me having sold several hundreds of those antennas, and no one has reported him the bandwidth being too narrow or the exagerated wet terrain influence. Only hundreds? Hy-Gain have used this feed system on 2m antennas for a very long time. Yes, their gain figures seem a bit generous, but the hairpin is a viable commercial option. I am not sure on what I am going to do, also because I am not 100% sure on whether the bandwidth problem is only due to the matching system, or it is also due to the particular antenna design. My gut feeling is that optimised long Yagis have narrow bandwidth because of the large number of elements with role that is frequency critical. A short Yagi has wider bandwidth with the same feed system. It is a long time since I read your first post, but narrow bandwidth can be an advantage. It reduces out of band signal reach your first amplifier where it will mix and produce IMD products that may be in-band. Narrow band antennas help to provide the selectivity that is lacking in many / most modern radios. Owen |
#5
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Efficiency of 200-ohm hairpin matching
You are suggesting that it is easier to make a low loss capacitor that is
located at the feedpoint in a hostile environment, than it is to make a low loss inductor (the hairpin). What I was maning to say that is easier to determine the correct capacitance (just using a variable capacitor) rather than the correct inductance (using a sliding short on the hairpin). Once one determined the correct value, the variable element should anyway be replaced with an equiavalent fixed element Only hundreds? Hy-Gain have used this feed system on 2m antennas for a very long time. Yes, their gain figures seem a bit generous, but the hairpin is a viable commercial option. Well, this is an antenna just for 6-meter enthusiasts fabricated in Italy, and there are not too many of them around here. It is a long time since I read your first post, but narrow bandwidth can be an advantage. It reduces out of band signal reach your first amplifier where it will mix and produce IMD products that may be in-band. Narrow band antennas help to provide the selectivity that is lacking in many / most modern radios. yes, but the bandwidth is so tight that it barely fits my needs. Also the SWR response shift when it rains is a problem to which I have no solution 73 Tony I0JX |
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