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[TowerTalk] Mismatch is a Real Loss

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Subject: [TowerTalk] Mismatch is a Real Loss
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Date: Mon, 29 Jun 98 10:25:51

     Another interesting subject.  I am a few days late with this but I 
     would like to add some comments.
     Mismatch loss is real and it does impact system gain and performance 
     by reducing the total power delivered to the antenna for radiation.  
     VSWR (SWR) is a measure of how closely the antenna impedance "matches" 
     the coaxial cable impedance.  A 1.0:1 antenna VSWR indicates no 
     mismatch and hence no mismatch loss.  A 1.5:1 antenna VSWR results in 
     4% reflected energy or 0.18 dB mismatch loss.  A 2.0:1 VSWR results in 
     11% reflected energy or 0.51 dB mismatch loss.  A 3.0:1 VSWR results 
     in 25% reflected energy or 1.25 dB mismatch loss.
     Where does this energy go?  It is reflected back to the tuner and/or 
     transmitter.  If you do not have a tuner, the reflected energy will 
     travel back to the transmitter and will be dissipated as heat. Most, 
     if not all transmitters, will lower their transmit power or shut off 
     completely if too much power is reflected back by the antenna. If the 
     transmitter source impedance does not match that of the coaxial cable, 
     some of the reflected energy will be reflected back towards the 
     antenna resulting in multiple reflections in the coaxial cable.  
     If you have a tuner, remember that it only tunes the impedance at 
     where it is located.  If it is located in your shack at the beginning 
     of your coax, the antenna may still have a high VSWR and may still be 
     reflecting energy back to the tuner/transmitter.
     The other cable issue is attenuation which reduces both the forward 
     and reflected energy as they travel through the coax.  BTW, RG-8 cable 
     has a nominal impedance of 52 ohms and an attenuation of about 0.7 
     dB/100 ft at 10 MHz.
     Steve Best
     Cushcraft Corp.  

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