[TowerTalk] mismatch loss and SWR

[w8ji.tom]

Hi Steve,

>      I hope this does not come off as arrogant.

Naw. Tell it like it is (or like it isn't, hihi)

>      The above statements are perhaps the biggest misconception in the 
>      amateur hobby regarding antennas.  Antenna VSWR does reduce antenna 
>      gain.  No question.  If you feed an antenna with a 20:1 VSWR you
will 
>      give up 7.41 dB of radiated power.  VSWR results in a mismatch loss 
>      regardless of cable attenuation.   

That is conditionally true IF you are considering the reduction in
generator power output and ONLY if the generator meets certain well defined
criteria! 

Let me give an example where your statement is true. Suppose we drive a 20
dB 50 ohm pad with a transmitter and feed a 50 ohm cable, and the cable
feeds a 50 ohm load. All the power on the antenna side of the attenuator is
available for the load, less the transmission loss. If we misterminate this
system, then what you said is correct. We have mismatch loss of the value
you indicate.

The same would be true if the cable had near-infinite or very high
attenuation. The higher the attenuation, the more correct your statement
becomes.
  
If we measure our antenna efficiency or feedline efficiency, mismatch loss
does not apply. The mismatch loss is a problem at the source reducing
available power, NOT at the junction of the 50 cable and the load. If the
cable has high attenuation, it becomes the "source" because the non-linear
generators we all use are effectively disconnected from the load. 

If we remove the 50 ohm pad, all bets are off. The source (unless from a
class A PA or generator with no feedback that is carefully matched to the
load) does not look like a linear source impedance that is the complex
conjugate of the load impedance (50 ohms j0 in this example) so efficiency
and power transfer might very well INCREASE with a mismatch between the
line and load.

 Any non-class A PA looks like a non-linear source (if conjugately matched
at one power level) and will also produce a mismatch loss when the line is
misterminated, but the attenuation by the mismatch will vary with mismatch
level and NOT follow the formulas you used.   
      
What I say is pretty well documented and accepted in engineering circles
Steve. Mismatch loss ONLY applies under very specific circumstances, and
those are conditions where the generator is a linear source (meaning the
source resistance does not change with load impedance or power level
changes) that is conjugately matched to the load.

If what you said was true, a large number of antenna systems in the world
would suddenly stop working efficiently, and open wire lines and large
curtain arrays would be blown away by TA-33's. VOA used curtains with over
18 dBd gain, and these curtain arrays operated with feedline mismatches of
over 10 to one and even larger mismatches in the arrays internal power
distribution system.

All of this is the very reason I fought to have match efficiency, %
transmitter power, and other non-sense that misleads people removed from
software in the antenna analyzer. Mismatch loss belongs in the lab where
class A generators feed 50 ohm pads before being used, not in the hamshack
where it does not apply in virtually 100 cases out of 100.

73 Tom

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