[TowerTalk] Antenna Gain vs. VSWR

[sbest@cushcraft.com]

     I will use this one message to respond to many.
     
     *****
     
     Ward's (N0AX) statement: 
     
     "I think there is some confusion on nomenclature.  I disagree that 
     VSWR affects antenna gain, but agree that VSWR affects *system 
     efficiency*. Antenna gain is only a function of antenna geometry.  
     System efficiency includes antenna gain and line loss, but the two are 
     independent."
     
     I will agree with this statement.  The IEEE standard definition for 
     antenna gain does not include the effect of VSWR.  VSWR does impact 
     system gain.  However, as an antenna manufacturer, we directly compare 
     antenna performance to a reference antenna with a good match.  Since 
     the difference in the two antenna's VSWR's results in a "gain" 
     difference between the two, we include it in our antenna gain figures. 
     The real issue is working with system engineers - they always need to 
     know how we define "antenna gain".  Most end users (maybe commercial 
     NOT amateur) are not aware of these subtle definitions.
     
     
     *****
     
     Ron's (KK1L) statement:  
     
     "Now wouldn't this also mean that you are creating a nice reflection 
     back up the line to the antenna? This should provide more power at the 
     antenna for it to radiate."
     
     Most tuners will totally dissipate the reflection such that it will 
     not be delivered to either the transmitter or the antenna.
     
     
     *****
     
     Guy's (K2AV?) statement: My understanding of "loss" is power 
     dissipation. If the 7.41 db is dissipated, and not in the feedline, 
     where is it dissipated? Or do you intend "loss" as some ratio concept 
     that does not actually dissipate power?
     
     Absent the presence of a tuner, the transmitter will internally 
     dissipate the power as heat.  If the transmitter source impedance is 
     not the same as the characteristic impedance of the cable then energy 
     will be reflected back towards the antenna.
     
     ****
     
     Gerald's (K5GW) statements:
     
     "You may or may not be aware that virtually all amateur transmitters 
     and amplifiers contain tuning networks that are either manually or 
     automatically adjusted to minimize the vswr seen by the internal 
     amplifying device. This same proccess also provides (through conjugal 
     matching theory) the needed shift or distortion of the output z of the 
     matching device to  re-reflect the wave back toward the antenna.
     
     In other words, after matching adjustments are completed, the 
     transmitter or amplifier no longer "looks" like 50+j0 ohms.
     
     For this reason, the vswr of the line does cause extra losses, but 
     only the by the amount caused by the multiple trips up and down the 
     line.
     
     This proccess is of course not valid for broadband systems with fixed 
     50 ohm output impedance as there is no mechanism to get the reflected 
     wave turned around at the output of the device."
     
     Gerald, thank you.  My background has virtually all been with 50 ohm 
     broadband systems, although, I do understand the concept and theory of 
     conjugal matching.  I am going to think about this one for a bit.  
     Might have to reconsider a few statements I made. I am confident that 
     I am correct, given my assumptions.  Now I need to rethink my 
     assumptions.
     
     
     *****
     
     More from Gerald: 
     
     
     "A more accurate statement about loss of antenna gain by vswr would be 
     "loss of antenna SYSTEM gain. A 20 to vswr created by a 1000 ohm feed 
     impedance can easily be matched to 50 ohms via an LC network with no 
     apparent loss of gain. With no matching network, there would be 
     considerable power loss in most 50 ohm cables used by amateurs. With a 
     matching network, the the antenna gain is still the same, but the 
     SYSTEM gain will be much greater."
     
     This is correct.  From an antenna designer's perspective, the LC or 
     other type of matching network becomes part of the antenna, reducing 
     "antenna" VSWR such that the antenna is now matched.  However, not all 
     of the power will be radiated since some will be lost into the 
     matching components.
     
     73
     Steve Best 
     

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