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#21
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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 |
#22
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Efficiency of 200-ohm hairpin matching
Hi Wim (and lurkers),
On Apr 8, 1:56 pm, Wimpie wrote: On 8 abr, 18:52, K7ITM wrote: .... I just ran EZNEC on a frequency-scaled version of the 14MHz 5 element Yagi included in the sample files, with the D.E. slightly shortened to allow a decent hairpin match to 200 ohms at the design center frequency. Did you also scale the thickness of the elements? Yes, EZNEC lets you scale everything in the same proportion in one quick operation. I did a frequency sweep, 49 to 51 MHz, in 0.25MHz steps. .... I don't know whether this will give sufficient BW improvement for the Yagi as the Q is also determined by the reflector en directors. I am looking forward to your simulation results. Yes, of course. If the antenna itself is not wide-band, there is rather little you can do about it. It may be possible with a more-or- less complicated network to make the SWR bandwidth somewhat greater (assuming still low loss--if you have a lossy network, you can make the SWR bandwidth very large -- ;-) But making the SWR bandwidth large does not make the antenna's gain bandwidth large. That is, for a given set of directors and reflectors, the pattern will change with frequency without much regard for what you do to get power into the antenna. Thanks for the encouragement about running some simulations. I'll report results as I have a chance, probably this evening (US Pacific coast time). Cheers, Tom |
#23
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Efficiency of 200-ohm hairpin matching
On 9 abr, 01:26, K7ITM wrote:
Hi Wim (and lurkers), On Apr 8, 1:56 pm, Wimpie wrote: On 8 abr, 18:52, K7ITM wrote: ... I just ran EZNEC on a frequency-scaled version of the 14MHz 5 element Yagi included in the sample files, with the D.E. slightly shortened to allow a decent hairpin match to 200 ohms at the design center frequency. Did you also scale the thickness of the elements? Yes, EZNEC lets you scale everything in the same proportion in one quick operation. I did a frequency sweep, 49 to 51 MHz, in 0.25MHz steps. ... I don't know whether this will give sufficient BW improvement for the Yagi as the Q is also determined by the reflector en directors. I am looking forward to your simulation results. Yes, of course. If the antenna itself is not wide-band, there is rather little you can do about it. It may be possible with a more-or- less complicated network to make the SWR bandwidth somewhat greater (assuming still low loss--if you have a lossy network, you can make the SWR bandwidth very large -- ;-) But making the SWR bandwidth large does not make the antenna's gain bandwidth large. That is, for a given set of directors and reflectors, the pattern will change with frequency without much regard for what you do to get power into the antenna. Thanks for the encouragement about running some simulations. I'll report results as I have a chance, probably this evening (US Pacific coast time). Cheers, Tom Hello Tom, I do not have experience with EZNEC, so I didn't know that it also scales element diameter. Fully agree with regards to VSWR BW, gain BW and wide band dummy loads.... Here it is UTC+2, so it is time for me te visit my bed for some hours. Tommorow VAT administration is waiting. Best regards, Wim |
#24
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Efficiency of 200-ohm hairpin matching
On Apr 8, 4:40 pm, Wimpie wrote:
On 9 abr, 01:26, K7ITM wrote: Hi Wim (and lurkers), On Apr 8, 1:56 pm, Wimpie wrote: On 8 abr, 18:52, K7ITM wrote: ... I just ran EZNEC on a frequency-scaled version of the 14MHz 5 element Yagi included in the sample files, with the D.E. slightly shortened to allow a decent hairpin match to 200 ohms at the design center frequency. Did you also scale the thickness of the elements? Yes, EZNEC lets you scale everything in the same proportion in one quick operation. I did a frequency sweep, 49 to 51 MHz, in 0.25MHz steps. ... I don't know whether this will give sufficient BW improvement for the Yagi as the Q is also determined by the reflector en directors. I am looking forward to your simulation results. Yes, of course. If the antenna itself is not wide-band, there is rather little you can do about it. It may be possible with a more-or- less complicated network to make the SWR bandwidth somewhat greater (assuming still low loss--if you have a lossy network, you can make the SWR bandwidth very large -- ;-) But making the SWR bandwidth large does not make the antenna's gain bandwidth large. That is, for a given set of directors and reflectors, the pattern will change with frequency without much regard for what you do to get power into the antenna. Thanks for the encouragement about running some simulations. I'll report results as I have a chance, probably this evening (US Pacific coast time). Cheers, Tom Hello Tom, I do not have experience with EZNEC, so I didn't know that it also scales element diameter. Fully agree with regards to VSWR BW, gain BW and wide band dummy loads.... Here it is UTC+2, so it is time for me te visit my bed for some hours. Tommorow VAT administration is waiting. Best regards, Wim Hi Wim and lurkers, Yes, EZNEC has some nice features. I exercised a couple of them tonight, doing frequency sweeps with inductive and capacitive matching. I did cheat: since I do not know the dimensions of the hairpin match, I elected to just use a pure lumped inductance. I suppose the error compared with a transmission line stub (hairpin) won't be great. I also used for the first time ASCII file import for the wire description, making use of Excel to generate the wires. (Why did it take me so long?? ;-) Somewhat to my surprise, I found that the SWR bandwidth of the shortened D.E. (driven element) loaded with a shunt inductance and that of the lengthened D.E. loaded with a shunt capacitance was not so different. The antenna I picked to model was one in the Orr and Cowan book, "The Beam Antenna Book." It's a 6-element 1.20 wave long design, with 0.2 waves D.E. to reflector, and 0.25 waves D.E. to first director and also between each adjacent pair of directors. For 6 meters it is a little shorter than Tony's antenna, but not very much shorter. But the elements (per Orr and Cowan's numbers) are 2 inches (about 50mm) diameter. The large diameter almost certainly contributes to the fairly broad bandwidth. All models below are done in freespace. For the shortened D.E.: D.E. length = 95.4 inches. Feedpoint impedance at 50.1MHz is 16.32-j54.63 ohms. Gain varies from 11.78dBi at 49.5MHz to 11.98dBi at 50.1 to 11.68 at 50.5. f/b varies uniformly from 19.43dB at 49.5 to 8.21dB at 50.5. Matched at 50.1MHz to 200 ohms with 189.4nH shunt at the feedpoint yields a 1.4:1 SWR bandwidth of about 49.89MHz to 50.28MHz. For the lengthened D.E.: length = 115.8 inches; impedance at 50.1 = 29.28+j70.7 ohms. Gain: , 12.05max, . f/b shows same uniform degradation with increased freq; to . 38.35pF shunt across feedpoint yields 1.4:1 SWR BW of about 49.89 to 50.34MHz, just slightly greater than the shortened one. (Neither of these includes frequency effects of the 4:1 balun to get back to 50 ohms...) I also tried a much-lengthened D.E., to get it up to 50+j204.1 ohms. Then a series 15.57pF gives a 50 ohm match. Gain is similar to above, 11.83dBi -- 12.11dBi -- 11.93dBi, and f/b is also similar, 22.82 -- 8.83. I notice both the gain and the f/b improve slightly as the D.E. is lengthened, though it's small enough that you'd never notice it--it would be swamped out by other effects. The 1.4:1 SWR BW for this configuration is about 49.85MHz to 50.32MHz; this of course against a 50 ohm reference which is different from the first two examples above which are against a 200 ohm reference. OK--I think I got all the critical stuff in there... Cheers, Tom |
#25
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Efficiency of 200-ohm hairpin matching
K7ITM wrote in
: .... Yes, EZNEC has some nice features. I exercised a couple of them tonight, doing frequency sweeps with inductive and capacitive matching. I did cheat: since I do not know the dimensions of the hairpin match, I elected to just use a pure lumped inductance. I suppose the error compared with a transmission line stub (hairpin) won't be great. I also used for the first time ASCII file import for It probably isn't. However, you could get an idea from my Two Wire Line Loss Calculator (http://www.vk1od.net/tl/twllc.htm). It looks like a hairpin made of 4mm dia aluminium 50mm spacing and 150mm in length give an impedance of 0.02 +j61... so the Q is probably mainly determined by the end connections. a o/c stub made of 4mm dia aluminium 50mm spacing and 1300mm in length give an impedance of 0.07-j80... so, it is not quite as good electrically, it is unweildly and end connection resistance will probably still be significant. This is no doubt why people use a hairpin in preference to an o/c stub! Owen |
#26
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Efficiency of 200-ohm hairpin matching
On 9 abr, 08:40, K7ITM wrote:
On Apr 8, 4:40 pm, Wimpie wrote: On 9 abr, 01:26, K7ITM wrote: Hi Wim (and lurkers), On Apr 8, 1:56 pm, Wimpie wrote: On 8 abr, 18:52, K7ITM wrote: ... I just ran EZNEC on a frequency-scaled version of the 14MHz 5 element Yagi included in the sample files, with the D.E. slightly shortened to allow a decent hairpin match to 200 ohms at the design center frequency. Did you also scale the thickness of the elements? Yes, EZNEC lets you scale everything in the same proportion in one quick operation. I did a frequency sweep, 49 to 51 MHz, in 0.25MHz steps. ... I don't know whether this will give sufficient BW improvement for the Yagi as the Q is also determined by the reflector en directors. I am looking forward to your simulation results. Yes, of course. If the antenna itself is not wide-band, there is rather little you can do about it. It may be possible with a more-or- less complicated network to make the SWR bandwidth somewhat greater (assuming still low loss--if you have a lossy network, you can make the SWR bandwidth very large -- ;-) But making the SWR bandwidth large does not make the antenna's gain bandwidth large. That is, for a given set of directors and reflectors, the pattern will change with frequency without much regard for what you do to get power into the antenna. Thanks for the encouragement about running some simulations. I'll report results as I have a chance, probably this evening (US Pacific coast time). Cheers, Tom Hello Tom, I do not have experience with EZNEC, so I didn't know that it also scales element diameter. Fully agree with regards to VSWR BW, gain BW and wide band dummy loads.... Here it is UTC+2, so it is time for me te visit my bed for some hours. Tommorow VAT administration is waiting. Best regards, Wim Hi Wim and lurkers, Yes, EZNEC has some nice features. I exercised a couple of them tonight, doing frequency sweeps with inductive and capacitive matching. I did cheat: since I do not know the dimensions of the hairpin match, I elected to just use a pure lumped inductance. I suppose the error compared with a transmission line stub (hairpin) won't be great. I also used for the first time ASCII file import for the wire description, making use of Excel to generate the wires. (Why did it take me so long?? ;-) Somewhat to my surprise, I found that the SWR bandwidth of the shortened D.E. (driven element) loaded with a shunt inductance and that of the lengthened D.E. loaded with a shunt capacitance was not so different. The antenna I picked to model was one in the Orr and Cowan book, "The Beam Antenna Book." It's a 6-element 1.20 wave long design, with 0.2 waves D.E. to reflector, and 0.25 waves D.E. to first director and also between each adjacent pair of directors. For 6 meters it is a little shorter than Tony's antenna, but not very much shorter. But the elements (per Orr and Cowan's numbers) are 2 inches (about 50mm) diameter. The large diameter almost certainly contributes to the fairly broad bandwidth. All models below are done in freespace. For the shortened D.E.: D.E. length = 95.4 inches. Feedpoint impedance at 50.1MHz is 16.32-j54.63 ohms. Gain varies from 11.78dBi at 49.5MHz to 11.98dBi at 50.1 to 11.68 at 50.5. f/b varies uniformly from 19.43dB at 49.5 to 8.21dB at 50.5. Matched at 50.1MHz to 200 ohms with 189.4nH shunt at the feedpoint yields a 1.4:1 SWR bandwidth of about 49.89MHz to 50.28MHz. For the lengthened D.E.: length = 115.8 inches; impedance at 50.1 = 29.28+j70.7 ohms. Gain: , 12.05max, . f/b shows same uniform degradation with increased freq; to . 38.35pF shunt across feedpoint yields 1.4:1 SWR BW of about 49.89 to 50.34MHz, just slightly greater than the shortened one. (Neither of these includes frequency effects of the 4:1 balun to get back to 50 ohms...) I also tried a much-lengthened D.E., to get it up to 50+j204.1 ohms. Then a series 15.57pF gives a 50 ohm match. Gain is similar to above, 11.83dBi -- 12.11dBi -- 11.93dBi, and f/b is also similar, 22.82 -- 8.83. I notice both the gain and the f/b improve slightly as the D.E. is lengthened, though it's small enough that you'd never notice it--it would be swamped out by other effects. The 1.4:1 SWR BW for this configuration is about 49.85MHz to 50.32MHz; this of course against a 50 ohm reference which is different from the first two examples above which are against a 200 ohm reference. OK--I think I got all the critical stuff in there... Cheers, Tom Hello Tom, Regarding the "cheating", no problem with that. When length of hairpin is far below 1/8 lambda, difference with lumped component is negligible (for this situation). Owen mentioned that a 150mm hairpin has the required reactance. The 0.02 Ohms loss resistance in the hairpin does not dissipate significant power (not measurable in practice within the antenna setup). This also shows that Tony does not have to worry about hairpin loss, even in his 200 Ohm intermediate impedance. Your simulation made fully clear that Tony's problem is in the antenna and not in the matching. Thanks for sharing your results, Wim PA3DJS www.tetech.nl please remove abc from the address. |
#27
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Efficiency of 200-ohm hairpin matching
Tom and Wim,
I agree with your concluion that the bandwidth narrowness is due to the antenna design and not to the matching system. I have simulated the antenna on EZNEC and the narrow band property of the antenna is very evident. I would like to also put the hairpin in the EZNEC model, but I am not very good at EZNEC and I would not now know to do it. If you are interested I can send you the EZNEC file via e-mail. 73 Tony I0JX |
#28
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Efficiency of 200-ohm hairpin matching
On Apr 9, 2:08 am, Owen Duffy wrote:
K7ITM wrote : ... Yes, EZNEC has some nice features. I exercised a couple of them tonight, doing frequency sweeps with inductive and capacitive matching. I did cheat: since I do not know the dimensions of the hairpin match, I elected to just use a pure lumped inductance. I suppose the error compared with a transmission line stub (hairpin) won't be great. I also used for the first time ASCII file import for It probably isn't. However, you could get an idea from my Two Wire Line Loss Calculator (http://www.vk1od.net/tl/twllc.htm). It looks like a hairpin made of 4mm dia aluminium 50mm spacing and 150mm in length give an impedance of 0.02 +j61... so the Q is probably mainly determined by the end connections. a o/c stub made of 4mm dia aluminium 50mm spacing and 1300mm in length give an impedance of 0.07-j80... so, it is not quite as good electrically, it is unweildly and end connection resistance will probably still be significant. This is no doubt why people use a hairpin in preference to an o/c stub! Owen Thanks, Owen. Actually, the error I was thinking of was not the Q, since both a good hairpin and a good helical coil will have Qu's very much higher than the loaded Q of the matching network -- but rather of the slope of reactance versus frequency, since the hairpin is a transmission line and will presumably show a sharp resonance at a different frequency from the coil's self resonance. But that error should also be small--negligible as Wim notes. Cheers, Tom |
#29
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Efficiency of 200-ohm hairpin matching
On Apr 9, 9:02 am, "Antonio Vernucci" wrote:
Tom and Wim, I agree with your concluion that the bandwidth narrowness is due to the antenna design and not to the matching system. I have simulated the antenna on EZNEC and the narrow band property of the antenna is very evident. I would like to also put the hairpin in the EZNEC model, but I am not very good at EZNEC and I would not now know to do it. If you are interested I can send you the EZNEC file via e-mail. 73 Tony I0JX Hi Tony, I would be happy to have a look at the file and add the hairpin if you wish. But I can also guide you through doing it yourself. It should be quite easy. Simply click on the " Trans Lines" button, the second one below Sources in the main window. Specify the first end the same as the source--perhaps 50% along wire 2, assuming the D.E. is wire 2. That end will then be in parallel with the source. You can type "S" in for the other end's wire #, or if you click in the End 2 Wire # box, you should see a list of "open" and "short" and you can select "short" there. Then enter the length, Z0, velocity factor (1) and loss. Reverse or normal doesn't matter since only one end is connected. In as much as the hairpin actually IS in the field of the antenna, and couples to it to some degree, there may be a small error in using the EZNEC transmission line, instead of entering it as wires, but for a short stub I would not expect that to be a significant issue. I do know that the 1/4 wave stub used to separate elements of a collinear does couple to the antenna fields and that coupling is a significant factor in the performance of the antenna--or at least that is what R.W.P. King tells us. And...thanks much for the interesting discussion topic. I learned a few things from it. Cheers, Tom |
#30
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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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