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How much can the impedance of coax vary from its characteristic impedance?
Hello,
I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the coax a little longer because I intended to tune it with my MFJ259B or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into ON4UN's program for calculating impedance along a coax line: Given the coax is 75 Ohms and the load is 54 Ohms, the program reported that if the line was a 1/4WL long the impedance at the source end would be about 105+j0 Ohms. However my actual measurement with both the MFJ259B and LP100 showed an impedance of 74 + j0 Ohms. http://remote.wu2x.com:8888/lee/quar...-75-meters.jpg I plugged in a few numbers into ON4UN program and calculated it would take coax that had a characteristic impedance of 64 Ohms to see the transformation that I am seeing. Is there any error in my logic here? If this coax really is 64 Ohms, then I'd like to find something that really is closer to 75 Ohms so I can achieve the 2:1 ratio that I intended.I still have another 100 feet of it and can do more tests with the tools I have on hand. 73, Scott, WU2X |
How much can the impedance of coax vary from its characteristic impedance?
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How much can the impedance of coax vary from its characteristic impedance?
Richard,
You asked if the load is shielded. I was using a MFJ dummy load and the resistor is inside a metal enclosure. Not sure if this mets your shielding criteria. I did have twelve #31 ferrite beads (1.125"X.0.5") on both the input and output end of the cable, so I can say for sure that aspect was taken care of. I didn't notice any changing values whenever I would move the coax around or grab onto either. -Scott, WU2X On Aug 19, 1:20 pm, Richard Clark wrote: On Sun, 19 Aug 2007 16:28:32 -0000, wrote: However my actual measurement with both the MFJ259B and LP100 showed an impedance of 74 + j0 Ohms. Hi Scott, Unless both ends are completely shielded (which is to ask: Is the load a shielded load?), then you need to choke things. A quick test would be to take the load end of the line and coil it 8 - 12 turns around a 6 inch cylinder. Does this change your impedance reading? You will need this choke for your antenna anyway, so plan on something like it. 73's Richard Clark, KB7QHC |
How much can the impedance of coax vary from its characteristic impedance?
I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I
went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the coax a little longer because I intended to tune it with my MFJ259B or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into ON4UN's program for calculating impedance along a coax line: Given the coax is 75 Ohms and the load is 54 Ohms, the program reported that if the line was a 1/4WL long the impedance at the source end would be about 105+j0 Ohms. How long is the coax (in feet)? Are you sure you didn't actually cut a half wavelength? Tor N4OGW |
How much can the impedance of coax vary from its characteristic impedance?
How long is the coax (in feet)? Are you sure you didn't actually cut a half wavelength? Which wouldn't make sense either, then you should get 54 ohms. Tor N4OGW |
How much can the impedance of coax vary from its characteristic impedance?
|
How much can the impedance of coax vary from its characteristic impedance?
On Aug 19, 5:35 pm, Owen Duffy wrote:
I am not sure whether you are solving a coax problem, or validating the LP100. Well, the LP100 and MFJ259B readings are virtually identical. That gives me some confidence in both instruments. The goal was to tune a matching section for my loop. Which BTW, I did and it is working fine. However, the SWR is only reaching a minimum of 1.6:1 because I didn't get the full 2:1 step down ratio. Now, I am not going to lose sleep over that, but it seemed like a good opportunity to understand what happens outside the textbook. The impedance plot certainly clears away questions about whether you have properly accounted for velocity factor. The phase plot has a strangely flattened minimum. Yeah, I was surprised by that too - but I was pleased to see that - if it is true. I know the loop won't be bandwidth limited by my matching section. I wonder about your confidence in the 54 ohm load, and your answers to Richard about choking the cable suggests something non-ideal. Confidence in that it really is 54 Ohms? Well, I wouldn't bet my life on it, but all my instruments seem to agree that its 54 Ohms. I choked the cable because it was driving me crazy. No seriously, I was building the matching section to use on my loop, so you have to slide those ferrite beads on before you solder on the PL259s. I didn't put them on because I thought they were necessary when I measured the matching section, they just happened to be there at the time. You could try a test with s/c or o/c stub (or both), so eliminating the 54 ohm load. Of course, in such a test, the line loss becomes more important. It also tests your instrument at extreme mismatch, so you might want to validate it on o/c, s/c, and some reactive 100:1 loads using known coax sections. I measured it with an open end with the MFJ and it had zero reactance right in the range the LP100 showed it not have any reactance. I have had good luck with Belden, so maybe I'll order some of that in 75 Ohms and see if its any different. -Scott, WU2X |
How much can the impedance of coax vary from its characteristicimpedance?
Most of the important points have been well covered, but let me add a
general observation: Loss will skew any measurement toward the characteristic impedance of the cable. This is intuitively evident, since if the cable is lossy enough, you'll see its characteristic impedance at the input regardless of what's connected to the other end. So when you say you're seeing 74 ohms when you should be seeing 105, I'd bet without further evidence that the cable has more loss than your model thinks it has. Roy Lewallen, W7EL |
How much can the impedance of coax vary from its characteristic impedance?
On Aug 19, 9:28 am, wrote:
Hello, I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the coax a little longer because I intended to tune it with my MFJ259B or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into ON4UN's program for calculating impedance along a coax line: Given the coax is 75 Ohms and the load is 54 Ohms, the program reported that if the line was a 1/4WL long the impedance at the source end would be about 105+j0 Ohms. However my actual measurement with both the MFJ259B and LP100 showed an impedance of 74 + j0 Ohms. http://remote.wu2x.com:8888/lee/quar...-75-meters.jpg I plugged in a few numbers into ON4UN program and calculated it would take coax that had a characteristic impedance of 64 Ohms to see the transformation that I am seeing. Is there any error in my logic here? If this coax really is 64 Ohms, then I'd like to find something that really is closer to 75 Ohms so I can achieve the 2:1 ratio that I intended.I still have another 100 feet of it and can do more tests with the tools I have on hand. 73, Scott, WU2X My rule-of-thumb is that I shouldn't be surprised if the actual impedance of coax is anywhere within ten percent of the nominal value. A while back I went looking for precision 50 ohm coax to be used in a test system that among other things does a calibration of test instruments, and the impedance of the coax really does matter. Even cables that cost several hundred dollars for a one or two meter length (that's admittedly with connectors attached...) didn't guarantee impedance closer than a couple ohms out of 50. Add to that that I just went looking for typical loss specs for RG-59- type cable on the web and the first one I looked at lists it as 70 ohm line, not 75. Although the expected impedance you calculated is for 75 ohm line with no loss, adding a dB loss doesn't change things all that much, certainly not enough by itself to account for your reading. Seems like with your impedance analyzer, you could pretty easily find the impedance and the loss of your line, and use those in your formulas to see if the performance is what you expect. Have you measured the line with the far end shorted and with it open? Assuming accurate measurements, the line impedance will be the square root of the product of those two measured values. Then there's also the possibility that your impedance measuring device isn't all that accurate at some impedances.... Cheers, Tom |
How much can the impedance of coax vary from its characteristic impedance?
Roy - Your comment made me double check my work. I was using ON4UN's
software and I was entering the estimated loss at my design frequency. Then I realized when I cranked up the loss (to see the effect clearly) that it wasn't doing anything with impedance along the line. And then I visited Owen's link for the second time and plugged in the details from my cable. I must have done something wrong the first time - because let me tell you it was dead nuts what I saw (Zo 75.01-j1.45 ). Case closed! -Scott, WU2X Parameters Transmission Line Belden 9204 (RG-59/U) Code B9204 Data source Belden Frequency 3.800 MHz Length 42.700 feet Zload 54.00+j0.00 Yload 0.018519+j0.000000 Results Zo 75.01-j1.45 Velocity Factor 0.660 Length 89.94 °, 0.250 Line Loss (matched) 0.269 dB Line Loss 0.279 dB Efficiency 93.77% Zin 102.11-j3.74 Yin 0.009780+j0.000359 , , RL, VSWR (source end) 1.53e-1-j8.49e-3, 0.153 -3.2°, 16.3dB, 1.36 , , RL, VSWR (load end) -1.63e-1+j9.38e-3, 0.163 176.7°, 15.7dB, 1.39 7.24e-4+j3.68e-2 k1, k2 3.19e-6, 1.79e-11 Loss model source data frequency range 10.000 MHz - 1000.000 MHz Correlation coefficient (r) 0.999661 |
How much can the impedance of coax vary from its characteristic impedance?
Well ***Opps****. I took out my contacts already. And I read the cable
Zo as the impedance at the input end. As you can see it says 102.11- j3.74. I am still going to search out some new RG-59/U... -Scott, WU2X |
How much can the impedance of coax vary from its characteristicimpedance?
Sorry I don't have the time to dig into this more deeply right now. I'm
sure Owen has done a great job in estimating loss, but here are some things to think about: 1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated steel center conductor for strength. At 3.8 MHz, depending on the copper thickness, current might be entering the steel. If it is, the loss will be a lot more than a simple model for solid copper would predict. I notice that the statement at the bottom of the data you posted says "Loss model source data frequency range 10.000 - 1000.000 MHz". You're well below that. A good reason for a lower limit on the model would be not accounting for current penetrating into the steel. 2. Some common RG-59 type cables have stranded center conductors and tin plating. Both increase the loss. More importantly, stranding results in much thinner copper for a given percentage of wire diameter of copper cladding. 3. A logical way for a cable manufacturer to cut costs is to put a thinner copper cladding on the center conductor. This would have no effect on the performance at VHF and above, where the cable is most likely to be used. So thin copper wouldn't surprise me. The only way to really know the loss is to measure it. And this might not be the reason for any apparent error. But it might be. As Tom said, though, 10, or even 15% deviation from nominal isn't unusual. Let me relate a story. Years ago, I came across a very large surplus quantity of approximately 0.1" diameter 75 ohm cable. It was just before Field Day, and because it looked to be in good physical condition, I measured off 100 feet, put a couple of BNC connectors on it, and tossed it into the pack as feedline for the 40 meter antenna. (I backpack my gear on Field Day, so weight is a major consideration.) Field Day went ok, but it was one of those years when we were just at the other stations' noise level, requiring a lot of repeats, QRZs, etc. Afterward, my FD partner was saying that all we needed was another 2 or 3 dB gain on 40, and we'd do a lot better. I agreed. Not too long afterward, I was measuring the impedance of a folded dipole through a half or full wave of that 75 ohm coax (since I had a lot of it), and was getting bizarre results. And that's when I first learned of the importance of cable loss on impedance transformation. I had been assuming lossless cable for my calculations of load Z given input Z, but got suspicious that loss might play a role. When I modified by equations to account for loss, I was surprised at how much difference even a little loss made. (As it turns out, loss makes more difference when the load Z is far from the cable Z0, as it was in this case, than when they're about the same.) I put more and more loss into the formula until I got about what I expected for load Z, given the input Z I was measuring. 4 dB at 7 MHz! A quick measurement with the wattmeter confirmed that the cable did indeed have that much loss. The problem was the thinness of the copper cladding on the very small steel center conductor strands. Even though the cladding was a substantial portion of the wire diameter, it was still very thin because of the tiny wire diameter. At our next sked, I told my FD partner that I'd figured out a way to get a couple more dB out of our 40 meter antenna. . . Roy Lewallen, W7EL |
How much can the impedance of coax vary from its characteristic impedance?
Roy Lewallen wrote in news:13cid39fvvdsk28
@corp.supernews.com: .... 1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated steel center conductor for strength. At 3.8 MHz, depending on the copper thickness, current might be entering the steel. If it is, the loss will be a lot more than a simple model for solid copper would predict. I notice that the statement at the bottom of the data you posted says "Loss model source data frequency range 10.000 - 1000.000 MHz". You're well below that. A good reason for a lower limit on the model would be not accounting for current penetrating into the steel. Roy, the reason I show the freq range on which the model is built is for exactly the case you are discussing. It makes it clearer when the model is an extrapolation, and confidence limits should be wider. One of the things that I have done when doing the regressions on the source data is to discard low frequency points that have high error wrt the regression. This effect occurs almost intirely with CCS centre conductor type cables. Not all RG59 and RG6 type cables have CCS, and the worry with low cost CCS is whether the coating is even thinner than the 9204. I use RG6 quite a bit for ham work, and the cable I buy uses a HDC centre conductor. I would avoid CCS for lower HF. Back to the original problem, it would take a huge loss to deliver an input impedance of just under 75 ohms from a 54 ohm load and a quarter wave of nominal 75 ohm line. The Zo looks low. Owen PS: The quoted output from TLLC is somewhat hard to read due to the Greek characters and some other symbols not copying to plain text. |
How much can the impedance of coax vary from its characteristic impedance?
Roy Lewallen wrote:
Sorry I don't have the time to dig into this more deeply right now. I'm sure Owen has done a great job in estimating loss, but here are some things to think about: 1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated steel center conductor for strength. At 3.8 MHz, depending on the copper thickness, current might be entering the steel. If it is, the loss will be a lot more than a simple model for solid copper would predict. I notice that the statement at the bottom of the data you posted says "Loss model source data frequency range 10.000 - 1000.000 MHz". You're well below that. A good reason for a lower limit on the model would be not accounting for current penetrating into the steel. 2. Some common RG-59 type cables have stranded center conductors and tin plating. Both increase the loss. More importantly, stranding results in much thinner copper for a given percentage of wire diameter of copper cladding. 3. A logical way for a cable manufacturer to cut costs is to put a thinner copper cladding on the center conductor. This would have no effect on the performance at VHF and above, where the cable is most likely to be used. So thin copper wouldn't surprise me. The only way to really know the loss is to measure it. And this might not be the reason for any apparent error. But it might be. As Tom said, though, 10, or even 15% deviation from nominal isn't unusual. Let me relate a story. Years ago, I came across a very large surplus quantity of approximately 0.1" diameter 75 ohm cable. It was just before Field Day, and because it looked to be in good physical condition, I measured off 100 feet, put a couple of BNC connectors on it, and tossed it into the pack as feedline for the 40 meter antenna. (I backpack my gear on Field Day, so weight is a major consideration.) Field Day went ok, but it was one of those years when we were just at the other stations' noise level, requiring a lot of repeats, QRZs, etc. Afterward, my FD partner was saying that all we needed was another 2 or 3 dB gain on 40, and we'd do a lot better. I agreed. Not too long afterward, I was measuring the impedance of a folded dipole through a half or full wave of that 75 ohm coax (since I had a lot of it), and was getting bizarre results. And that's when I first learned of the importance of cable loss on impedance transformation. I had been assuming lossless cable for my calculations of load Z given input Z, but got suspicious that loss might play a role. When I modified by equations to account for loss, I was surprised at how much difference even a little loss made. (As it turns out, loss makes more difference when the load Z is far from the cable Z0, as it was in this case, than when they're about the same.) I put more and more loss into the formula until I got about what I expected for load Z, given the input Z I was measuring. 4 dB at 7 MHz! A quick measurement with the wattmeter confirmed that the cable did indeed have that much loss. The problem was the thinness of the copper cladding on the very small steel center conductor strands. Even though the cladding was a substantial portion of the wire diameter, it was still very thin because of the tiny wire diameter. At our next sked, I told my FD partner that I'd figured out a way to get a couple more dB out of our 40 meter antenna. . . Roy Lewallen, W7EL Interesting topic! I've never encountered an RG59 that had a stranded center conductor but I'll take your word for it. I *have* seen an 80 ohm RG59, though (Belden 8221). Bryan WA7PRC |
How much can the impedance of coax vary from its characteristic impedance?
wrote in message ups.com... Well ***Opps****. I took out my contacts already. And I read the cable Zo as the impedance at the input end. As you can see it says 102.11- j3.74. I am still going to search out some new RG-59/U... -Scott, WU2X If you are going to buy new coax, might as well get something decent. I used RG11 FOAM for a 1/4 wave transformer at 3.8 MHz, and it worked per theory. Tam/WB2TT |
How much can the impedance of coax vary from its characteristic impedance?
On Sun, 19 Aug 2007 16:28:32 -0000, wrote:
Hello, I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the coax a little longer because I intended to tune it with my MFJ259B or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into ON4UN's program for calculating impedance along a coax line: Given the coax is 75 Ohms and the load is 54 Ohms, the program reported that if the line was a 1/4WL long the impedance at the source end would be about 105+j0 Ohms. However my actual measurement with both the MFJ259B and LP100 showed an impedance of 74 + j0 Ohms. http://remote.wu2x.com:8888/lee/quar...-75-meters.jpg I plugged in a few numbers into ON4UN program and calculated it would take coax that had a characteristic impedance of 64 Ohms to see the transformation that I am seeing. Is there any error in my logic here? If this coax really is 64 Ohms, then I'd like to find something that really is closer to 75 Ohms so I can achieve the 2:1 ratio that I intended.I still have another 100 feet of it and can do more tests with the tools I have on hand. After looking over some of the followups to your post, I had a thought. Addressing the errors' introduced by loss, and the possibility that you're seeing worst case loss at a lower frequency, why don't you go back and re-measure your matching section at 3X, 5X, 7X 3.8 mcs.? It's the reverse of a trick I've used to cut n*1/2WL 1:1 coax runs for VHF -- by measuring at *sub* multiple freqs. Anyway -- it's a thought. Jonesy -- Marvin L Jones | jonz | W3DHJ | linux 38.24N 104.55W | @ config.com | Jonesy | OS/2 *** Killfiling google posts: http://jonz.net/ng.htm |
How much can the impedance of coax vary from its characteristic impedance?
Owen Duffy wrote in
: PS: The quoted output from TLLC is somewhat hard to read due to the Greek characters and some other symbols not copying to plain text. I have added a 'No symbols in output' checkbox to the TLLC input form to allow selection of a more ASCII friendly output format. Using that switch on Scotts problem yields: RF Transmission Line Loss Calculator / Enhanced Parameters Transmission Line Belden 9204 (RG-59/U) Code B9204 Data source Belden Frequency 3.800 MHz Length 0.250 wavelengths Zload 54.00+j0.00 ohms Yload 0.018519+j0.000000 ohms Results Zo 75.01-j1.45 ohms Velocity Factor 0.660 Length 90.00 deg, 0.250 wl, 13.024 m Line Loss (matched) 0.269 dB Line Loss 0.279 dB Efficiency 93.77% Zin 102.10-j3.81 ohms Yin 0.009780+j0.000365 ohms Gamma, rhotheta, RL, VSWR (source end) 1.53e-1-j8.82e-3, 0.153-3.3 deg, 16.3dB, 1.36 Gamma, rhotheta, RL, VSWR (load end) -1.63e-1+j9.38e-3, 0.163176.7 deg, 15.7dB, 1.39 gamma 1.24e-1+j6.28e+0 k1, k2 5.46e-4, 3.06e-9 Loss model source data frequency range 10.000 MHz - 1000.000 MHz Correlation coefficient (r) 0.999661 |
How much can the impedance of coax vary from its characteristicimpedance?
Tam/WB2TT wrote:
If you are going to buy new coax, might as well get something decent. I used RG11 FOAM for a 1/4 wave transformer at 3.8 MHz, and it worked per theory. I think you were lucky. I've found that the velocity factor and characteristic impedance of foam dielectric coax, even quality coax, to vary a lot more than solid dielectric cable. Apparently they don't have very good control over the foam density. Roy Lewallen, W7EL |
How much can the impedance of coax vary from its characteristic impedance?
I use RG6 quite a bit for ham work, and the cable I buy uses a HDC centre
conductor. I would avoid CCS for lower HF. For what it's worth, I looked up the specs on the Carol C5785 that is locally available at Home Depot here in the States. It's quad-shield RG-6 and they list the losses down to 1MHz 1MHz .26dB/100ft 10MHz .81dB/100ft 50MHz 1.46dB/100ft According to your calculator for RG-6/U it should be ..19 ..6 1.37 As a percentage difference in dB (boy that's a bad unit) it's actually a good bit more loss at 1 and 10MHz, but in a practical sense it's probably pretty negligible. So unless you're going 1000 feet to transmitting antennas on mid to low HF, I doubt it's a worry. I don't know what the price differential is between CCS and hard drawn but i do know that cable with about the same loss as RG-213 that costs 12 cents a foot is pretty attractive. As far as the original post, I picked my stuff up and made a bunch of twelfth-wave transformers for it and that seemed to work out fine, but I guess they're less sensitive if you put one at each end because they would tend to match to whatever cable you're making the transformers out of and they're very broadband too. As far as velocity factor goes, I measured mine before I started cutting (I built XFMRS for 2m and 70cm so I had to be sort of accurate. It worked). 73, Dan |
How much can the impedance of coax vary from its characteristic impedance?
" wrote in
oups.com: I use RG6 quite a bit for ham work, and the cable I buy uses a HDC centre conductor. I would avoid CCS for lower HF. For what it's worth, I looked up the specs on the Carol C5785 that is locally available at Home Depot here in the States. It's quad-shield RG-6 and they list the losses down to 1MHz 1MHz .26dB/100ft 10MHz .81dB/100ft 50MHz 1.46dB/100ft According to your calculator for RG-6/U it should be .19 .6 1.37 Dan, You didn't say which of the RG6 cables you used. The figures you quote are very similar to Belden 1189A. In respect of 1189A, note that the regression model is based on data points from 55MHz to 1000MHz. That is either because that is what Belden supplied, or it could be that I excised some low frequency data points that were a bad fit to the model. Beware of results where the estimate is an extrapolation. (The frequency range is red when the estimate is an extraplation.) Owen |
How much can the impedance of coax vary from its characteristic impedance?
"Roy Lewallen" wrote in message ... Tam/WB2TT wrote: If you are going to buy new coax, might as well get something decent. I used RG11 FOAM for a 1/4 wave transformer at 3.8 MHz, and it worked per theory. I think you were lucky. I've found that the velocity factor and characteristic impedance of foam dielectric coax, even quality coax, to vary a lot more than solid dielectric cable. Apparently they don't have very good control over the foam density. Roy Lewallen, W7EL I didn't have the exact Vp number for the SIW cable; so, I trimmed it. Length came out reasonable, though. If I measure a 50 Ohm load through the cable, my MFJ269 reads 102/J0 at resonance. A little on the low side, but probably within tolerance. If it was not raining outside, I would measure the cable to see what exact length I ended up with. Bottom line is that my (late) 75 m loop did have an SWR of 1.1:1 at resonance. Tam/WB2TT |
How much can the impedance of coax vary from its characteristic impedance?
The RG-59/U I am using does in fact have a stranded center conductor.
I am using RG-59/U because I am feeding a vertical (quad) loop on the vertical side and I am running the coax away at a 90 degree angle. The lighter/smaller coax is more suited for this arrangement. I didn't use RG6 (which I actually have) because the reducers for the PL259 and ferrite beads I have don't fit this size coax. I figure for 3.8Mhz that loss wasn't such a big issue...in addition to the 1/4WL section of 75 ohm coax is only another 25 feet of 50 Ohm coax to the shack. -Scott, WU2X On Aug 21, 1:15 am, Owen Duffy wrote: " wrote groups.com: I use RG6 quite a bit for ham work, and the cable I buy uses a HDC centre conductor. I would avoid CCS for lower HF. For what it's worth, I looked up the specs on the Carol C5785 that is locally available at Home Depot here in the States. It's quad-shield RG-6 and they list the losses down to 1MHz 1MHz .26dB/100ft 10MHz .81dB/100ft 50MHz 1.46dB/100ft According to your calculator for RG-6/U it should be .19 .6 1.37 Dan, You didn't say which of the RG6 cables you used. The figures you quote are very similar to Belden 1189A. In respect of 1189A, note that the regression model is based on data points from 55MHz to 1000MHz. That is either because that is what Belden supplied, or it could be that I excised some low frequency data points that were a bad fit to the model. Beware of results where the estimate is an extrapolation. (The frequency range is red when the estimate is an extraplation.) Owen |
How much can the impedance of coax vary from its characteristic impedance?
wrote in message oups.com... The RG-59/U I am using does in fact have a stranded center conductor. I am using RG-59/U because I am feeding a vertical (quad) loop on the vertical side and I am running the coax away at a 90 degree angle. The lighter/smaller coax is more suited for this arrangement. I didn't use RG6 (which I actually have) because the reducers for the PL259 and ferrite beads I have don't fit this size coax. I figure for 3.8Mhz that loss wasn't such a big issue...in addition to the 1/4WL section of 75 ohm coax is only another 25 feet of 50 Ohm coax to the shack. -Scott, WU2X You might also want to take a look at feeding one of the bottom corners. This will give both vertical and horizontal polarization. I found it worked great for DX; a dog for working locals, but probably no worse than vertical p. I subsequently added a 1:1 current balun, but don't think it did anything relative to the original setup, which had the center conductor connected to the vertical wire. Tam/WB2TT |
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