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[TowerTalk] Reflections, Conjugate Matching and Jim Reid's comments

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Subject: [TowerTalk] Reflections, Conjugate Matching and Jim Reid's comments
From: sbest@cushcraft.com (sbest@cushcraft.com)
Date: Thu, 23 Jul 98 08:32:38

     To All:
     
     I enjoyed all of the comments, although I will say some more than 
     others.
     
     To Jim Reid:  
     
     The history of mankind is quite interesting.  Have you ever noticed 
     that anytime someone challenges another person's (or group of 
     people's) beliefs that they respond immediately with persecution.  Is 
     the world still flat and does it still revolve around the sun?
     
     I was very disappointed to see your personal attacks towards me 
     regarding my discussion, my background and the DOD $$.   Given my 
     educational and professional background, I could respond much more 
     severely to you, but I won't.
        
     First, I could not respond to questions last night because my e-mail 
     at home does not access the reflector.  I enjoy the technical exchange 
     of opinions on the reflector.  When I prepare my submittals, I realize 
     that I am challenging many opinions and that I will be subject to 
     great disagreement.  I do not, however, expect personal ridicule. 
     
     Given my extensive technical background on antenna and transmission 
     theory and design, I attempt not to state technical opinions that are 
     invalid.  Although, I will admit that I am not familiar with the 
     operation of ALL amateur tuners.  Maybe I should have said some 
     dissipate the power rather than most. 
     
     My only reason for posting the discussions about transmission line 
     theory (reflections, VSWR, etc.) was to educate those people that 
     might be interested in broadening their base of knowledge.  I am not 
     an expert class ham but I am somewhat of an expert at the topics I 
     address.  (On the 200 Watts on the bird wattmeter - you don't provide 
     enough information for me to explain it properly too you.  In the 
     meantime, you might want to read up on the theory of "conservation of 
     energy"). 
     
     As far as reading materials - I have not read Maxwell's reflections 
     and cannot comment on it until I have done so.  K4SB has been kind 
     enough to send me a copy to review.  The only Maxwell I have read is 
     James Clerk Maxwell - you might find this interesting Jim.  Anyway, I 
     have numerous electrical engineering books completely devoted to 
     transmission line theory and practice.  I would highly recommend you 
     read "Schaum's Outline Series - Transmission Lines" by Robert Chipman. 
      A quote from page 203 "At the signal source end of the line the 
     reflection coefficient, ps = 0 because Zs = Zo, so that none of the 
     power reflected by the terminal load impedance is re-reflected on 
     returning to the input end of the line."  Another one is "Transmission 
     Lines and Networks by Walter Johnson".  A quote from page 16 "Thus, a 
     terminating impedance different from Zo will give rise to a reflected 
     wave which travels away from the termination.  The reflection, upon 
     reaching the other end, will itself be reflected IF the impedance at 
     that end is DIFFERENT from Zo".
     
     I don't believe I have said anything technically incorrect.  My 
     assumptions regarding source impedance being equal to Zo, etc. have 
     always been clearly stated.  A great many physical systems operate 
     this way.  I challenge you to open your mind to new concepts.  Read 
     the short article I wrote.  
     
     Oh, one last thing, I too have designed many, many antennas that are 
     ALL still working fine.  Some even for satellites.
     
     To Anyone Else Interested:
     
     I guess that we are beyond the semantics of "antenna gain" versus 
     "system gain".
     
     I also think that all of us understand that an antenna has a complex 
     impedance and that VSWR cannot be defined until the antenna is 
     connected to a transmission with a characteristic impedance Zo.
     
     In any event, the real issue is not VSWR but rather what happens to 
     the reflected energy?  Most on the reflector appear to believe that it 
     is never really lost and that it does not impact system performance - 
     like operating into an antenna with a 20:1 VSWR and not experiencing 
     even a fraction of dB loss.  Why do ALL the antenna engineers in the 
     world waste their time minimizing VSWR?
     
     I made several controversial statements yesterday.  One was that the 
     reflected energy is dissipated in the transmitter if the source 
     impedance is 50 ohms.  If Zo is 50 ohms and Zs is 50 ohms, how can 
     energy be reflected from the source?  I also stated that if the source 
     impedance is not equal to Zo, then SOME, BUT NOT ALL, energy would be 
     reflected back towards the antenna.  The only way ALL of the energy 
     could be reflected back towards the antenna is if the source impedance 
     had an infinite VSWR, i.e. it was either an open circuit or a short 
     circuit.  This also applies to tuners and amplifiers as well.  A tuner 
     (even Jim's) will never reflect all energy unless the VSWR seen by the 
     antenna reflected wave is infinite.   Again, some energy will be 
     reflected if the VSWR seen by the antenna reflected wave is not 1:1 
     and some will be absorbed and dissipated as heat OR some will be 
     canceled (maybe a new concept for some).
     
     My discussions and examples assumed a source with an impedance of 50 
     ohms.  I clearly stated this and many real transmitters and amplifiers 
     operate in this manner.  The concepts I discussed in the article I 
     wrote (which some of you asked for) are valid for all situations (real 
     and theory) and should never "be left in the lab".  The formulas 
     presented in the article are the basis for all transmission line work. 
      
     
     Ask yourself why transceiver manufacturers, even ones with built in 
     tuners, specify a maximum VSWR load (like 3.0:1) and why the output 
     power is lowered when operating into a high VSWR?  If all of the 
     antenna reflected power was reflected back to the antenna, why would 
     it make a difference?  And yes, some transceivers and tuners do burn 
     up because of reflections.
     
     One other concept that has to be considered relates to "lost energy".  
     This is important.  Electromagnetic energy in a transmission line 
     system can be dissipated as heat, it can be reflected by an impedance 
     mismatch, or it can be canceled.  Cancellation is the issue that is 
     sometimes forgotten and hard to understand in practice.  On a 
     transmission line, two voltages that are of equal amplitude and 180 
     degrees out of phase will cancel completely.  Many matching devices 
     use this method to eliminate or minimize reflections on the line.
     
     Conjugate Matching:  Actually, I think Dick Green gave a pretty good 
     summary but was challenged by someone (was it Jim?).  Conjugate match 
     theory was derived for a transmission line with a complex 
     characteristic impedance, complex source impedance and complex load.  
     Anyway, conjugate theory states that more power will be delivered to a 
     terminal load whose impedance is the conjugate of characteristic 
     impedance.  There are still reflections on the line but what happens 
     is the forward and reflected waves combine in such a manner that the 
     total line loss is not increased as with a transmission line whose 
     characteristic impedance is real.  With tuners, the conjugate match 
     theory is a bit different.  Once the complex input impedance to the 
     line is determined (after reflections reach the tuner) the tuner 
     impedance is adjusted to be the conjugate of the input impedance. 
     (Assumption on my part as to how they work)  In this case, the 
     reflections will still exist on the line between the tuner and 
     antenna.  When the reflections arrive back at the tuner, they will be 
     prevented from going to the transmitter (so it sees a 1:1 match and 
     thinks there are no reflections).  Three things can happen to the 
     energy at the tuner - some will be reflected back to the antenna 
     because the reflected wave no longer see a matched condition, some 
     will be dissipated as heat in the tuner and some can be canceled as 
     described in the previous paragraph. 
     
     Enough said.
     
     73 
     Steve Best    
     

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