[TowerTalk] Mismatch Loss and Tuners

sbest@cushcraft.com sbest@cushcraft.com
Tue, 08 Sep 98 14:07:56

     Hello All:
     > Think about the receive signal. When it hits a mismatch, some 
     >portion will be reflected back to the antenna, to be re-radiated and 
     >forever lost.
     >Not really. What really happens with a tuner in line is the system is 
     >conjugately matched for the receive signal. Efficiency is no 
     >different on receive as it is on transmit.
     Actually, the first statement above is more correct.  When an antenna 
     intercepts a signal, it develops a voltage across its terminals which 
     then "drives" the feeder transmission line.  The level of "driving" 
     voltage is a function of the antenna impedance and the transmission 
     line impedance (antenna VSWR). 
     This "driving" voltage will then travel towards the receiver.  Arriving 
     at a conjugate match in the system, some portion of the voltage will be 
     reflected back towards the antenna.  The portion not reflected will be 
     delivered through the conjugate match towards the receive circuit. A 
     conjugate match is not a "perfect termination" as stated in one of the 
     previous posts.  Like any impedance at the end of a transmission line, 
     a conjugate match will reflect voltage (hence power) based upon the 
     reflection coefficient (due to VSWR) it presents to the transmission 
     The reflected signal from the conjugate match will arrive back at the 
     antenna where some signal will be radiated and some will be 
     re-reflected back towards the receiver.  The level of signal radiated 
     and reflected at the antenna is a function of the antenna VSWR.  
     This process of multiple reflections between the antenna and the 
     conjugate match will continue until the total voltage decays as a 
     result of the following:  Some is delivered to (or through) the 
     conjugate match, some is lost in the transmission line and some 
     re-radiated by the antenna.
     I don't want to get into the "time/speed of light" discussion again, 
     however, if you want to determine the TOTAL power delivered to (or 
     through) the conjugate match and the TOTAL power re-radiated by the 
     antenna, you would have to add up the power contributions from all of 
     the individual voltage reflections.
     In order to fully and accurately describe the operation of a conjugate 
     match "tuner", a complete reflection analysis must be done with 
     voltages NOT power.  Once the system voltages are determined, power  
     delivery in the system can be determined.  It must also be understood 
     that for any incident voltage arriving at the conjugate match output 
     from the antenna (transmit or receive), only a portion will be 
     reflected back towards the antenna.  The amount of voltage/power 
     reflected will be a function of the conjugate match VSWR.  Only if it 
     has an infinite VSWR will ALL voltage, hence power, be reflected.  
     In describing the transmit case, some posts have stated that a 
     conjugate match adds reflection gain by re-reflecting ALL power 
     reflected by the antenna.  If this is true, then why in the receive 
     case does it not re-reflect ALL power delivered by the antenna thus 
     preventing it from reaching the receiver?  Obviously, it does not, in 
     either case. It functions as described above.  The conjugate match is 
     simply seen as an impedance at the end of a transmission line, 
     reflecting voltage or power simply as a function of the impedance 
     mismatch (VSWR) it presents.   

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