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#1
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Mismatched Zo Connectors
At risk of being called a Troll, and having little else to do at
present, I copy from another newsgroup the following text. "Reg Edwards" wrote - "The effect of an impedance mis-match at a coaxial connector of ordinary dimensions is practically zero at frequencies up to UHF. Regardless of its other dimensions, the primary requirement of a mismatched connector is that its length should be appreciably less than 1/4-wavelength at the highest working frequency. People are unnecessarily worried at VHF and below. Mix up 50 and 75-ohm connectors, and indeed connectors of unknown Zo, and carry on regardless. For an analysis of performance, download in a few seconds and run immediately program CONNECT from website below." Program CONNECT will calculate the effect on performance of inserting any relatively short length of line, of Zo different from system Zo, into the system. It's less than the inexperienced might imagine from reading frightening magazine and handbook articles about impedance and conjugate mis-matches. If you have a short length of coax lying around, of unknown Zo, just use it! ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
#2
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Mismatched Zo Connectors
Reg Edwards wrote:
Regardless of its other dimensions, the primary requirement of a mismatched connector is that its length should be appreciably less than 1/4-wavelength at the highest working frequency. Yes, at HF the reflections from one impedance discontinuity and the reflections from a complimentary impedance discontinuity 3 inches away almost entirely cancel each other. -- 73, Cecil http://www.qsl.net/w5dxp |
#3
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Mismatched Zo Connectors
Some of us are so cheeky as to use CATV, RG-6, Rg-8, R8-11, etc., with
no regard as to the supposed 'surge impedence'.. What is the load impedence of a 50 ohm antenna at any frequency removed from its resonance point anyway? If the antenna is 30 ohms or 80 ohms, with some reactance, does the SWR meter notice the inexpensive run of supposed 70 ohm coax that you cadged from the cable tv company? Once in a while I get an antenna that is fed with bits and pieces of this and that, which swears differently back at the shack than it did at the antenna feed point... Inserting another 5 or 10 feet of coax jumper usually cures it... denny / k8do - antenna monger from way back... Another fine myth you've gotten us into, Ollie! |
#4
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Mismatched Zo Connectors
Reg Edwards wrote:
Regardless of its other dimensions, the primary requirement of a mismatched connector is that its length should be appreciably less than 1/4-wavelength at the highest working frequency. ====================================== Yes, at HF the reflections from one impedance discontinuity and the reflections from a complimentary impedance discontinuity 3 inches away almost entirely cancel each other. -- 73, Cecil ======================================== Reflections are functions of TIME, not frequency. Oliver Heaviside had the right idea long before the invention of the SWR meter. ----- Reg, G4FGQ. |
#5
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Mismatched Zo Connectors
"Denny" wrote If the antenna is 30 ohms or 80 ohms, with some reactance, does the SWR meter notice the inexpensive run of supposed 70 ohm coax that you cadged from the cable tv company? ===================================== Of course it doesn't. But at least it does try. The meter thinks it indicates SWR on the transmission line, whereas it actually indicates SWR on a long, imaginary, non-existent, 50-ohm line between tuner and transmitter. What sort of mess would Laurel & Hardy have found themselves in when trying to disentangle forward from reflected power? =================================== |
#6
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Mismatched Zo Connectors
On Tue, 22 Aug 2006 17:28:01 GMT, Cecil Moore
wrote: and the reflections from a complimentary impedance discontinuity What is a "complimentary impedance discontinuity", or even a "complementary impedance discontinuity" if you meant that? Owen -- |
#7
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Mismatched Zo Connectors
Cecil Moore wrote:
Reg Edwards wrote: Regardless of its other dimensions, the primary requirement of a mismatched connector is that its length should be appreciably less than 1/4-wavelength at the highest working frequency. Yes, at HF the reflections from one impedance discontinuity and the reflections from a complimentary impedance discontinuity 3 inches away almost entirely cancel each other. What's a "complimentary impedance discontinuity," Cecil? 73, Tom Donaly, KA6RUH |
#8
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Mismatched Zo Connectors
"Owen Duffy" wrote in message ... On Tue, 22 Aug 2006 17:28:01 GMT, Cecil Moore wrote: and the reflections from a complimentary impedance discontinuity What is a "complimentary impedance discontinuity", or even a "complementary impedance discontinuity" if you meant that? ======================================= Owen, For the benifit of innocent bystanders, he means the same magnitude discontinuity with the opposite sign. But you knew that of course, didn't you? On the other hand, I'm not sure *I* have described it correctly. It can best be described in terms of the reflection coefficient. There are two reflections, of opposite signs, which do not occur at exactly the same instant in TIME. Whatever it is, Cecil is (im)perfectly correct. ---- Reg. |
#9
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Mismatched Zo Connectors
"Reg Edwards" wrote Reflections are functions of TIME, not frequency. Oliver Heaviside had the right idea long before the invention of the SWR meter. ==================================== For "reflections" also read "Echos". ----- Reg, G4FGQ. ==================================== |
#10
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Mismatched Zo Connectors
On Wed, 23 Aug 2006 00:54:51 +0100, "Reg Edwards"
wrote: "Reg Edwards" wrote Reflections are functions of TIME, not frequency. Oliver Heaviside had the right idea long before the invention of the SWR meter. ==================================== For "reflections" also read "Echos". Hi Reggie, You are in fact wrong in all accounts. Reflections are functions of distance - as are echoes. This is a phase issue. Time and frequency are always inseparable as Kelvin would instruct you in first principles before another hunk of chalk was winged off your noggin. You may choose to render phase into time, but shift the frequency and the phase shifts, not the distance. Mismatched Zo connectors do not shrink or enlarge with frequency - the effects may, but physical components rarely follow such perturbations.... until an arc-over that is the classic failure mechanism for such mismatches (obviously, and deliberately ignored in this thread). 73's Richard Clark, KB7QHC |
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