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Re[2]: [TowerTalk] Mismatch Loss and Tuners

To: <towertalk@contesting.com>
Subject: Re[2]: [TowerTalk] Mismatch Loss and Tuners
From: sbest@cushcraft.com (sbest@cushcraft.com)
Date: Tue, 25 Aug 98 08:52:22

     I think Hank (KN6DI) has made a few good points.
     
     With the exception of responding to the question posted by Pete 
     (N4ZR), this will be my last post on this subject for awhile.  After 
     this post, I think it only fair that Walt Maxwell be given a chance to 
     provide his rebuttal to my comments.  In the meantime, I have sent 
     Walt a copy of my "proof" of the concepts I have outlined.  I will 
     value his review, comments and opinions.
     
     In plain English:
     
     One of the objectives in the transmit system is to eliminate 
     reflections arriving at the front end of the transmit circuit.  This 
     can be accomplished in many ways.  The most common are to use an 
     antenna tuner or to use an antenna with a feedpoint impedance equal to 
     the characteristic impedance of the transmission line.
     
     With a matched antenna, all power arriving at the antenna will be 
     delivered to the antenna with no reflections.  The significant source 
     of loss in the system will be that which occurs as a result of cable 
     attenuation between the transmitter and the antenna.
     
     To properly analyze a tuner we must first describe the initial state 
     of the tuner and then develop into the steady state of the tuner.  To 
     do a full analysis, voltages must be used in all calculations.  Steady 
     state power levels can be determined once the steady state voltages 
     are known.
     
     In the initial state (the state prior to reflections arriving at the 
     tuner output from the antenna), the tuner will not present a 1.0:1 
     VSWR at its input.  Because of this, there will be an initial 
     reflected voltage present at the tuner input.  Therefore, the initial 
     power delivered through the tuner to the antenna will be less than the 
     total power delivered to the tuner from the transmitter.  The voltage 
     (power) delivered through the tuner will then arrive at the antenna 
     where some will be reflected (based upon antenna VSWR).  This 
     reflected voltage will arrive back at the tuner output.  At the tuner 
     output, another reflected voltage will be created based upon the tuner 
     VSWR.  Note that the tuner output VSWR will not be infinite therefore, 
     ALL of the voltage will not be re-reflected.  This process will 
     continue until the steady state condition is reached.  In the steady 
     state condition, the multiple reflections will create a steady state 
     voltage at the tuner output.  This voltage will be delivered back to 
     the tuner input through the tuner components.  The voltage delivered 
     back to the tuner input will be the negative of the initial reflected 
     voltage at the tuner input causing a complete cancellation of the 
     initial reflection. Hence, the steady state 1.0:1 VSWR.
     
     The difference between this discussion and that of Walt Maxwell's is 
     that Walt does not start by considering the initial state of the 
     tuner. He immediately "assumes" that all of the transmitter power is 
     delivered through the tuner, some is reflected by the antenna and then 
     ALL is re-reflected at the tuner output.  His initial assumption may 
     support his conclusion but it does not accurately describe the 
     operation of the tuner, nor all of the voltage reflections and 
     cancellations that occur.  With a lossless transmission line between 
     the tuner and antenna his results are numerically correct, however, I 
     am uncertain if this would be true when considering a case with 
     transmission line attenuation.
     
     In any event, even considering the transmission line losses, the 
     matched antenna will radiate more steady state power than the 
     mismatched antenna and tuner.  Remember that with the tuner, the 
     reflections from the antenna must travel the transmission line twice 
     before arriving back at the antenna.
     
     Which "works" better - matched antenna or mismatched antenna/tuner?  
     The answer to this question requires a much more involved analysis.  
     In order to "work", the transmit system only needs to provide a 
     useable signal level at the receive site.  In many cases, both will 
     "work" quite well.
     
     73, Steve, VE9SRB
     
     


______________________________ Reply Separator _________________________________
Subject: Re: [TowerTalk] Mismatch Loss and Tuners
Author:  kn6di@groupone.net
Organization: Global Pack & Mail at InterNet
To: <towertalk@contesting.com>
Date:    8/24/98 10:22 PM


Wouldn't be great if people could talk in plain english and discuss this thread 
in a straight forward manner instead of every one trying to get the last piece 
of fly S--- out of the black pepper.
It would be nice for a plain statement of facts of what works and does not work.
As we know no 2 antennas work the same because the conditions are not/never the 
same. This could go on until cycle 30 at least.
Hank KN6DI
     
     
J
     
     
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