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Antenna tuner/Smith chart question
Still having fun playing with my 8.5 foot whip on the metal patio roof, with
coax feedline to an antenna tuner in the shack. Operating on 15,12, and 10. The antenna tuner is a typical t network with series capacitors and an inductor to ground. I was unable to get a match on 15 meters with the antenna tuner. So, I thought I would add more coax to the feedline and move around the SWR circle on the Smith chart. Apparently this provides a set of matching parameters that the antenna tuner can now cope with. I don't have instruments to measure many parameters, but a rough guess is that the 15 meter SWR is at least 20:1. So the question is: are there parts of the Smith chart that are going to be more difficult to match, and should be avoided? |
#2
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Antenna tuner/Smith chart question
On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:
Still having fun playing with my 8.5 foot whip on the metal patio roof, with coax feedline to an antenna tuner in the shack. Operating on 15,12, and 10. The antenna tuner is a typical t network with series capacitors and an inductor to ground. I was unable to get a match on 15 meters with the antenna tuner. So, I thought I would add more coax to the feedline and move around the SWR circle on the Smith chart. Apparently this provides a set of matching parameters that the antenna tuner can now cope with. I don't have instruments to measure many parameters, but a rough guess is that the 15 meter SWR is at least 20:1. So the question is: are there parts of the Smith chart that are going to be more difficult to match, and should be avoided? Try rearranging your network, try pi series C, pi shunt C, etc. Also make friends with someone with a vswr analyzer ;-) |
#3
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Antenna tuner/Smith chart question
"dave" wrote in message ... On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote: Still having fun playing with my 8.5 foot whip on the metal patio roof, with coax feedline to an antenna tuner in the shack. Operating on 15,12, and 10. The antenna tuner is a typical t network with series capacitors and an inductor to ground. I was unable to get a match on 15 meters with the antenna tuner. So, I thought I would add more coax to the feedline and move around the SWR circle on the Smith chart. Apparently this provides a set of matching parameters that the antenna tuner can now cope with. I don't have instruments to measure many parameters, but a rough guess is that the 15 meter SWR is at least 20:1. So the question is: are there parts of the Smith chart that are going to be more difficult to match, and should be avoided? Try rearranging your network, try pi series C, pi shunt C, etc. Also make friends with someone with a vswr analyzer ;-) - Or, I could leave in the additional feedline ;-) But a good point is that I have an old homebrew 2 element tuner with alligator clip configuration. Might give that a try just for fun. I have one of the older MFJ analyzers, but I wouldn't trust the accuracy at high VSWRs. |
#4
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Antenna tuner/Smith chart question
On Jan 2, 3:39*am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio roof, with coax feedline to an antenna tuner in the shack. I was unable to get a match on 15 meters with the antenna tuner. EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms, depending on feedline length. Not many tuners will match 1.25 ohms with reasonable efficiency. In this case, a feedline length of an integer number of wavelengths (N*30') plus 13 feet would probably allow for a reasonable tuner match for VF=0.66 coax if you can tolerate the considerable feedline losses. -- 73, Cecil, w5dxp.com |
#5
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Antenna tuner/Smith chart question
"W5DXP" wrote in message ... On Jan 2, 3:39 am, "Wayne" wrote: Still having fun playing with my 8.5 foot whip on the metal patio roof, with coax feedline to an antenna tuner in the shack. I was unable to get a match on 15 meters with the antenna tuner. EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms, depending on feedline length. Not many tuners will match 1.25 ohms with reasonable efficiency. In this case, a feedline length of an integer number of wavelengths (N*30') plus 13 feet would probably allow for a reasonable tuner match for VF=0.66 coax if you can tolerate the considerable feedline losses. -- 73, Cecil, w5dxp.com - Thanks for the comments. When I used EZNEC I didn't know what to use for ground, so I used Real/Minimec. I'm showing about 18-j142 as the feedpoint impedance at 21.1 Mhz, which is not totally out of the ballpark with your number. At any rate, I have bought enough telescoping tubing sections to build about a 20 foot vertical. Final height is subject to approval by "the Admiral". Fabrication of a more substantial base support will be necessary. In the meantime, this AM I replaced the 8.6 ft whip with about 13 feet of telescoping tubing, using the same mobile whip mount clamped to conduit. Resonance is around 18.7 MHz and it loads up nicely on 18 through 10, with 10 meters having the highest calculated swr of 17. With the antenna removed, the feedline shows a 1/4 wave null at 4.33MHz. This gives an estimated 1/2 wave electrical length of 57 feet at 8.66 MHz, or with 0.66 VF a physical length of 37.6 ft. That seems about right. I'll get a Smith chart and see where the existing feedline moves the parameters. Per your comments I'll try to avoid low r values. Wish the bands would open back up like they were in Oct and Nov. - Wayne W5GIE |
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Antenna tuner/Smith chart question
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#7
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Antenna tuner/Smith chart question
On Jan 8, 10:21*pm, Owen Duffy wrote:
Did the Smith chart reveal any of that? There are loss scales at the bottom of the Smith chart that allow losses to be included. That's why I mentioned "tuner efficiency" and "considerable feedline losses" in my posting. -- 73, Cecil, w5dxp.com |
#8
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Antenna tuner/Smith chart question
Owen Duffy wrote in
: .... So, if the feed point impedance was 18-j142, and you used 38' of RG58 (for example, I could not find that info), then you would expect to lose 70% of the power out of the ATU as heat. Of course, it is likely that the power out of the ATU is somewhat less than the transmitter output as the ATU load is some 7+j10, relatively low R (but typical ATUs tend to be more efficient on the higher bands). Did the Smith chart reveal any of that? Well, subject to how carefully you plotted the data, you might get a reasonable answer for the line input impedance (as seen by the ATU), but the Smith chart will not directly read the line loss with a mismatched losd. With a lot of care in plotting, you might scale off the values of G at line input and output, and find the ratio Vload/Vin from the chart to calculate the ratio of Pload/Pin and so find the loss under mismatched load. The line loss in this case is just over 5dB, more than 10 times the 0.5dB loss you would observe with a matched load. An exercise for the reader is to use a Smith chart to find the loss under the mismatched load with RG213 where the matched line loss is about 0.31dB. Owen PS: if you had in mind using the handy dandy "Additional loss due to VSWR" graph in the ARRL, the line input VSWR is 7.7, load VSWR is 23.7. If you get the same loss figure are mentioned above, it is by accident of the scenario rather than soundness of the method. |
#9
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Antenna tuner/Smith chart question
On Jan 9, 3:12*pm, Owen Duffy wrote:
Well, subject to how carefully you plotted the data, you might get a reasonable answer for the line input impedance (as seen by the ATU), but the Smith chart will not directly read the line loss with a mismatched losd. Owen, I respect your obvious technical ability and the only time I ever disagree with you is when you go off the deep end and accuse me of saying things that I never said and/or try to pick a fight like you are trying to do on this thread. What in the world do you expect to gain from such antisocial behavior? -- 73, Cecil, w5dxp.com |
#10
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Antenna tuner/Smith chart question
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