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Roy Lewallen wrote:
Cecil Moore wrote: Roy Lewallen wrote: A transmission line can be very lossy, yet have a completely real characteristic impedance. That's what I thought. Is RG-174 one of those transmission lines? No. Distortionless lines are specially made, or periodically loaded with fixed components to achieve distortionless characteristics. Incidentally, I recently carefully measured the Z0 of nine pieces of RG-58 type cables at 10 MHz. R varied from 48.1 to 57.2 ohms, and X from -0.67 to -2.32 ohms. Assuming 57.2 - j2.32 ohms Z0, our 50 ohm SWR meters may be off by 15%? Could this be the answer to Richard C's SWR readings? I suggested that as a possibility early on but he dismissed it. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Tue, 07 Oct 2003 10:49:50 -0500, Cecil Moore
wrote: Richard Clark wrote: If you choose to put forward a variant employing reactance, you could at least step up to the bench to offer confirmatory or rejecting evidence as I did. OK, here's an interesting data point. I adjusted my IC-756PRO for 5W output on 7.2 MHz using the following circuit. 7.2MHz 5W source---(+j442)---(-j442)---50 ohm dummy load SWR meter at the dummy load read 5W forward with an SWR of 1:1 Then I installed the SWR meter between the coil and the cap. With 5W supplied by the source, the forward power read 150 watts. Indicated SWR was 3:1 Hi Cecil, And so? 73's Richard Clark, KB7QHC |
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Cecil Moore wrote:
Roy Lewallen wrote: A transmission line can be very lossy, yet have a completely real characteristic impedance. That's what I thought. Is RG-174 one of those transmission lines? No. Distortionless lines are specially made, or periodically loaded with fixed components to achieve distortionless characteristics. Incidentally, I recently carefully measured the Z0 of nine pieces of RG-58 type cables at 10 MHz. R varied from 48.1 to 57.2 ohms, and X from -0.67 to -2.32 ohms. I made one measurement at 1 MHz, on a cable whose Z0 at 10 MHz was 49.0 - j0.69 at 10 MHz. That cable's Z0 at 1 MHz was 50.7 - j2.05 ohms. I wasn't able to make good measurements below 1 MHz with my setup. . . . Roy Lewallen, W7EL |
Richard Clark wrote:
wrote: OK, here's an interesting data point. I adjusted my IC-756PRO for 5W output on 7.2 MHz using the following circuit. 7.2MHz 5W source---(+j442)---(-j442)---50 ohm dummy load SWR meter at the dummy load read 5W forward with an SWR of 1:1 Then I installed the SWR meter between the coil and the cap. With 5W supplied by the source, the forward power read 150 watts. Indicated SWR was 3:1 Hi Cecil, And so? And so it seems to support your variable SWR observations. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Cecil,
Very interesting. Can you take one more reading. Leave the meter between the coil and cap, and then short out the coil. If shorting out the coil makes any difference, you are seeing the imperfection due to the meter. This is what I was alluding to in my response to Walter. Tam/WB2TT "Cecil Moore" wrote in message ... Richard Clark wrote: If you choose to put forward a variant employing reactance, you could at least step up to the bench to offer confirmatory or rejecting evidence as I did. OK, here's an interesting data point. I adjusted my IC-756PRO for 5W output on 7.2 MHz using the following circuit. 7.2MHz 5W source---(+j442)---(-j442)---50 ohm dummy load SWR meter at the dummy load read 5W forward with an SWR of 1:1 Then I installed the SWR meter between the coil and the cap. With 5W supplied by the source, the forward power read 150 watts. Indicated SWR was 3:1 -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Tarmo Tammaru wrote:
Very interesting. Can you take one more reading. Leave the meter between the coil and cap, and then short out the coil. If shorting out the coil makes any difference, you are seeing the imperfection due to the meter. This is what I was alluding to in my response to Walter. Shorting out the coil will leave the load at 50-j442 ohms, a very high SWR. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
There could be quite a number of reasons Richard's readings aren't
indicating what he thinks, and this is certainly one of them. Of one thing I'm certain -- the reason is something other than actual SWR being modified by source impedance. And yes, our SWR meters can easily be that far off when attempting to measure the real SWR on real cables. Good thing it doesn't matter, huh? Roy Lewallen, W7EL Cecil Moore wrote: Assuming 57.2 - j2.32 ohms Z0, our 50 ohm SWR meters may be off by 15%? Could this be the answer to Richard C's SWR readings? I suggested that as a possibility early on but he dismissed it. |
"Cecil Moore" wrote in message ... David Robbins wrote: "Cecil Moore" wrote: Does (R+jXL)/(G+jXC) really equal 2500 for RG-174 on 12m? The specs say the Z0 of RG-174 is a nominal 50 ohms. of course its not exactly 2500, otherwise there would be no loss. but its close, maybe 2500+j10 or something like that. and even the resistive part may not be exact, the nominal 50 ohms could be 45 to 55 depending on the tolerances of the manufacturer. Comparing the 6dB loss of RG-174 to the 0.14 dB loss for hardline - is all that extra loss accounted for in the +j10 term? no, its more complicated than that. the attenuation constant (usually alpha) = Re(gamma) where gamma is sqrt((R+jwL)(G+jwC)) Zo is sqrt((R+jwL)/(G+jwC)) so there is not a simple way to relate the characterisitic impedance to loss. for a low loss line the approximation for alpha is (R/2Zo)+(GZo/2) which can probalby be applied for most normal cases, but again, you have to get the R and G values of the line which can not be directly calculated from Zo. |
On Tue, 07 Oct 2003 13:54:34 -0500, Cecil Moore
wrote: Richard Clark wrote: wrote: OK, here's an interesting data point. I adjusted my IC-756PRO for 5W output on 7.2 MHz using the following circuit. 7.2MHz 5W source---(+j442)---(-j442)---50 ohm dummy load SWR meter at the dummy load read 5W forward with an SWR of 1:1 Then I installed the SWR meter between the coil and the cap. With 5W supplied by the source, the forward power read 150 watts. Indicated SWR was 3:1 Hi Cecil, And so? And so it seems to support your variable SWR observations. Hi Cecil, With 5W supplied you read 150W forward? Well that aside, it is not very remarkable to see 1:1 into a dummy load. It is also not very remarkable to see 3:1 into a complex load. You do not state you have any transmission line between the two reactances until you dropped in the SWR meter, that isn't particularly meaningful either. So, in the end, you demonstrate nothing of my examples that have always been premised with a transmission line being integral to the concept. The short of it: I have always described this as a problem involving two resistors and a hank of line. The long of it: You have merely demonstrated your own invention of two conjugated reactances and one resistor - not the same thing at all, not even conceptually. Now, if you added a 1foot length between the two reactances, and then replaced that with a two foot length, and then replaced that with a three foot length, and then replaced that with a four foot length.... out to at least half a wave of electrical length. And all the while taking forward and reverse power readings (or SWR, take your choice) and specified the frequency THEN and ONLY THEN would you be able to make a first pass comparison. To take an observation from Metrology: one measurement tells you nothing of any accuracy, two measurements only confuse, three begins to reveal a true measure, more improves matters. Your two readings say nothing to the matter (Mismatch Uncertainty) and actually confirm expectations that lie outside of my examples. 73's Richard Clark, KB7QHC |
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