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#1
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Owen Duffy wrote:
On Tue, 10 Jan 2006 21:14:40 GMT, Owen Duffy wrote: On Mon, 09 Jan 2006 15:50:20 GMT, Cecil Moore wrote: Ricke wrote: If SETUP right , There the best HF I've ever used. Maybe the only one? :-) The G5RV, with tuner, is a pretty good 80m, 40m, 20m, and 12m antenna. If the series section is varied from 20 feet to 36 feet, it becomes a very good all-HF-band antenna. With the addition of a parallel 1000pf capacitor with the series section at 22 feet, on 75m my "G5RV" has SWR of 1.3:1 and works as well as a 75m 1/2WL dipole. Is this the antenna described at http://www.qsl.net/w5dxp/G5RV.HTM ? I have made a mistake during my analysis, let me try again: Now you tell me after I spent 15 minutes replying to it. :-) The details are there so I won't repeat it here. In that article, on 75m you model a feedpoint impedance of 36-j324, 28' of 300 ohm ladder line, for a Z of 15+j4 (seems to indicate 48.2 deg length of 300 ohm line with 0.007dB loss (optimistic)). At that point, were 50 ohm coax connected directly, the VSWR at the load end of the 50 ohm coax would be 3, however you shunt the 17+j4 with 1000pF to give a new Z of 17.3-j3.0 that results in a VSWR at the load end of the 50 ohm coax of around 2.9, almost identical to the case without the capacitor. Presumably when you say that the capacitor improves the VSWR on 75m, you mean the VSWR on the coax. Did I miss something, how does the capacitor improve the VSWR on 75m? What you missed is that the frequency must be changed to obtain the benefit. The capacitor is *not* installed at the 17+j4 point. It is installed at the 1/50 + j1/X admittance point. You can either increase the length of the feedline past the 17+j4 point to the 1/50 + j1/X admittance point or increase the frequency thus electrically lengthening the feedline to the 1/50 + j1/X admittance point. You cannot keep both of those values constant as you tried to do above. You already know what I am trying to say. I must not be saying it very well. When a parallel cap is used on a 75m screwdriver antenna to achieve 50 ohms, the screwdriver is tuned to 1/50 + j1/X, i.e. slightly inductive. When a parallel coil is used, the screwdriver is tuned to 1/50 - j1/X, i.e. slightly capacitive. -- 73, Cecil http://www.qsl.net/w5dxp |
#2
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On Wed, 11 Jan 2006 00:36:25 GMT, Cecil Moore wrote:
You already know what I am trying to say. I must not be saying it very well. When a parallel cap is used on a 75m screwdriver antenna to achieve 50 ohms, the screwdriver is tuned to 1/50 + j1/X, i.e. slightly inductive. When a parallel coil is used, the screwdriver is tuned to 1/50 - j1/X, i.e. slightly capacitive. Negative susceptances are inductive. An inductive reactance of j5 is a susceptance of 1/j5 or -j1/5. I agree with your words, the sign of the admittances is wrong. Owen -- |
#3
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Owen Duffy wrote:
Cecil Moore wrote: You already know what I am trying to say. I must not be saying it very well. When a parallel cap is used on a 75m screwdriver antenna to achieve 50 ohms, the screwdriver is tuned to 1/50 + j1/X, i.e. ^ should be - slightly inductive. When a parallel coil is used, the screwdriver is tuned to 1/50 - j1/X, i.e. slightly capacitive. ^ should be + Negative susceptances are inductive. An inductive reactance of j5 is a susceptance of 1/j5 or -j1/5. I agree with your words, the sign of the admittances is wrong. Yes, you are correct - sorry. But it now seems that you understand what I was trying to say. If one takes an ordinary G5RV and installs a parallel 1000pf capacitor at the coax/twinlead junction, one will raise the resonant frequency and lower the SWR on the coax for 75m operation. Very close to 50+j0 ohms can be achieved on 75m through that simple act. When I lived in AZ, I switched that cap in automatically using a relay and the frequency output signal on my IC-745. -- 73, Cecil http://www.qsl.net/w5dxp |
#4
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On Wed, 11 Jan 2006 04:25:59 GMT, Cecil Moore wrote:
Yes, you are correct - sorry. But it now seems that you understand what I was trying to say. If one takes an ordinary G5RV and installs a parallel 1000pf capacitor at the coax/twinlead junction, one will raise the resonant frequency and lower the SWR on the coax for 75m operation. Very close to 50+j0 ohms can be achieved on 75m through that simple act. When I lived in AZ, I switched that cap in automatically using a relay and the frequency output signal on my IC-745. OK. I played around a bit using the feedpoint impedances that I modelled for my "Feeding the G5RV" article. With 31' of 554, I needed about 2000pF to "tune" it for low 50 ohm VSWR at 3.6MHz. I plotted the impedance presented to the coax for a range of frequencies from 3.5 to 3.8MHz, they are at http://www.vk1od.net/temp/G5RV-W5DXP.GIF . The Smith chart is normalised to 50 ohms. The solution seems fairly narrow band, the VSWR at 3.55 was 6, at 3.6 it was 1.3, and at 3.65 it was 5. Of course, implementations will have slight differences in actual feedpoint impedances, and the outcome is very sensitive to slight differences in feedpoint Z. This "no-tuner" matching scheme will probably need significant customisation for each implementation. Owen -- |
#5
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![]() "Owen Duffy" wrote Of course, implementations will have slight differences in actual feedpoint impedances, and the outcome is very sensitive to slight differences in feedpoint Z. This "no-tuner" matching scheme will probably need significant customisation for each implementation. ========================================== The World-famous G5RV. --------------------------------- What everyone appears to forget, is that Zo of the balanced twin-line section, on all bands except at 14.15 MHz, has a considerable affect on feedpoint impedances, swr, losses, etc. When describing systems and performance nobody ever mentions what Zo of the feedline actually is. Omission of Zo reduces any following discussion to blythe, innocent nonsense. R.L.Varney himself never gave a value to Zo. He didn't need to. He was concerned mainly with 14.15 MHz. It would be unfair to accuse him of not understanding the serious effects of Zo on other bands. ---- Reg. |
#6
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Reg Edwards wrote:
R.L.Varney himself never gave a value to Zo. He didn't need to. He was concerned mainly with 14.15 MHz. It would be unfair to accuse him of not understanding the serious effects of Zo on other bands. However, in The ARRL Antenna Compendium #1, he did describe the matching section well enough to calculate his Z0. It is #14 copper open-wire separated by 1.75 inches. I'll bet that's an improvement over 300 ohm twinlead. -- 73, Cecil http://www.qsl.net/w5dxp |
#7
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Reg Edwards wrote:
R.L.Varney himself never gave a value to Zo. He didn't need to. He was concerned mainly with 14.15 MHz. It would be unfair to accuse him of not understanding the serious effects of Zo on other bands. However, in The ARRL Antenna Compendium #1, he did describe the matching section well enough to calculate his Z0. It is #14 copper open-wire separated by 1.75 inches. I'll bet that's an improvement over 300 ohm twinlead. -- 73, Cecil ========================================== Cec, the facts are, G5RV never mentioned Zo, either because he never attached any importance to it, or he didn't understand its relevance to other bands. When considering other than 14.15 MHz perhaps he should have done. In any event, everybody else has followed suit. Now we have the situation where dozens of people are busily seriously comparing all-band versions, one with another, and unknown to each other they are all using different value Zo transmission lines. Many of those who purchased the antennas, or just copied G5RV's construction, havn't the foggiest idea what their particular Zo is. Yet, from an analysis point of view, which you are involved with, it is a crucial matter. Your own Zo is a non-standard 375 ohms because you measured it. Yet you describe its performance to others as if everybody else's Zo is the same, whereas there is no hope of anybody else reproducing your particular results. Its all a load of nonsense! If anybody, at this very late stage in the art, should still wish to acquaint themselves with the less than mediocre performance of a G5RV, then download program DIPOLE3 from website below. DIPOLE3 is a general purpose program which deals with a dipole of any length, at any frequency, plus balanced-twin feedline of any length and any Zo, plus balun, plus coax line of any length and Zo, plus L-tuner. So it happens to include a G5RV. All the data for the original G5RV can be inserted in the program by depressing one key. Modifications to the system, such as changes in Zo, can easily be done and changes in performance immediately seen. The final important output figure is overall loss between transmitter and radiated power. Individual losses in the antenna, in the two transmission lines, and in the tuner, are reported seperately. Even the tuner L and C settings and their circuit locations are predicted. It is very easy to sweep over the HF frequency range to check when low swr happens to fall into amateur bands. (Unfortunately it doesn't do this very often.) It is also easy to change the length of transmission line (as Cecil recommends and advertises) to try to obtain an swr of less than 2:1. But you can use any dipole length other than 102-feet to play with this useful aspect. Accuracy is better than needed for the intended purposes and is generally as good as the accuracy of program input data. There is only one known trivial bug which occurs when dipole length is extremely short compared with wavelength. But clearly this is not of consequence. What more could you want from a G5RV? There's nothing to do but erect it and then compare it with a dipole of any other length with an open wire feedline of thick wire, of no particular length, all the way to the shack. Plus a choke balun. I'm still on Red, South African, Western Cape. Download DIPOLE3. Its free to USA citizens. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
#8
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Owen Duffy wrote:
I played around a bit using the feedpoint impedances that I modelled for my "Feeding the G5RV" article. With 31' of 554, I needed about 2000pF to "tune" it for low 50 ohm VSWR at 3.6MHz. I'm just reporting what it took for my actual antenna under the existing conditions at my QTH. The cap is actually 950 pf for a minimum SWR of 1.3:1 on 3.9 MHz. The optimum value of the cap would no doubt change at lower frequencies. With 22.5' of Wireman #554 and a 950 pf cap, the 3:1 bandwidth is 145 kHz. Adding sections of ladder-line lowers the resonant frequency. Incidentally, this is a method for modifying the G5RV to work, not only without a tuner, but with built-in tuners. When using a built-in tuner, the antenna configuration doesn't have to be changed as often. My IC756PRO will tune my present configuration from 3.72-4.0 MHz. or 280 kHz. I plotted the impedance presented to the coax for a range of frequencies from 3.5 to 3.8MHz, they are at http://www.vk1od.net/temp/G5RV-W5DXP.GIF . The Smith chart is normalised to 50 ohms. The solution seems fairly narrow band, the VSWR at 3.55 was 6, at 3.6 it was 1.3, and at 3.65 it was 5. Changing the length of the series section will shift the resonant frequency. I can vary mine from 22.5 ft. to 38.5 ft for a near- perfect SWR on all HF ham frequencies. Of course, implementations will have slight differences in actual feedpoint impedances, and the outcome is very sensitive to slight differences in feedpoint Z. This "no-tuner" matching scheme will probably need significant customisation for each implementation. IMO, that is what ham radio is all about - warm up the old MFJ-259B and get with the program. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
#9
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Owen Duffy wrote:
http://www.vk1od.net/temp/G5RV-W5DXP.GIF Owen, what software did you use to generate that graphic? -- TNX & 73, Cecil, http://www.qsl.net/w5dxp |
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