[Amps] Mismatch loss

[Larry Benko]

Jim,

You keep changing the topic.

My original comment was that 2 UHF connector pairs separated by 155 
degrees (worst case) of 50 ohm cable on 70cm _can_ produce slightly over 
a 2:1 SWR with low loss transmission line with a perfect 50 ohm load.  
Now the mismatch loss is ~.55dB.  To most people a 2:1 SWR is 
significant.  This is measurable easily done and is very real.   Mated 
UHF connectors are ~0.9" of 30 ohm transmission line.  Then you changed 
the topic AGAIN with your comment about the ARRL Handbook which calls 
additional loss in transmission line mismatch loss.  Unfortunately the 
rest of the technical world uses the definition that is on the Wikipedia 
page.  Additional loss in a transmission line due to SWR (which produces 
heat) is NOT the same as mismatch loss (which does NOT produce heat).

See http://en.wikipedia.org/wiki/Mismatch_loss

Now you are changing the topic again.  Do you dispute that 2 mated UHF 
connections can produce a 2:1 SWR on 70cm?  It also happens that if the 
separation of the 2 UHF connections is 67 degrees the SWR returns to 1:1 
at the transmitter with the 67 degree section of transmission line 
having as SWR of 1.43:1.   I just made the measurement with an HP8753 
network analyzer getting the same answer as WinSmith or SuperSmith show.

Mismatch loss is as super easy to see at HF.  Take a tuner and adjust 
the impedance seen by the transmitter to various values "near" 50 ohms 
and measure the forward power.  You are virtually guaranteed to see a 
larger forward power at some value other than 50 ohms.  The maximum 
power (assuming no transmitter protection issues) occurs when this 
impedance is the complex conjugate of the transmitters output impedance.

At HF, the minimum loss on transmission lines occurs when the line has 
constant current on it (matched) since standing waves cause repeated 
higher and lower currents in the cable.  Loss is proportional to current 
squared and the higher currents more than offset the loss contribution 
of the lower currents.  I have said this before but the free program 
LTSpice can show this behavior perfectly.

Many universities such as MIT have the EE courses online.

See: http://ocw.mit.edu/index.htm

If you wish to continue to argue about these topics I would suggest you 
learn/use LTSpice, take an online RF EE class, or get a current RF class 
textbook.  I know how to do the math of the transmission line impedances 
almost in my sleep and have the test equipment and the understanding how 
to make the measurements properly.

73,
Larry, W0QE

On 5/2/2012 6:13 PM, Jim Brown wrote:
> On 5/2/2012 11:42 AM, Larry Benko wrote:
>> here are two different topics going on with this question:
>> 1.) Mismatch loss
>> 2.) Additional attenuation in a transmission which is operated with an
>> SWR different than 1.0:1.
> 1) None of this has any relevance without considering the actual complex
> source impedances and load impedances at each connector. Those
> impedances are likely to have far greater effect than the small
> discontinuity of a proper UHF connector.
>
> 2) At HF, and at low VHF, mating UHF connectors are nothing more than a
> discontinuity in the impedance of a line that is too small to matter.
> They are too short as a fraction of a wavelength for transmission line
> analysis to matter.
>
> 73, Jim K9YC
>
>
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