[Amps] Mismatch loss

[Jim Brown]

On 5/2/2012 6:21 PM, Larry Benko wrote:
> Jim,
>
> You keep changing the topic.

No, it is YOU who changed the topic.  I began this discussion by noting 
that proper UHF connectors are just fine at HF and low to mid-VHF, and 
you proceeded to go to your modeling software to find a worst case 
condition that resulted in significant mismatch at UHF.  I've never 
suggested that N or BNC or DIN doesn't have advantages above mid-VHF.

> 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

What you're calling "mismatch loss" is NOT loss, it is nothing more than 
the impedance transformation that occurs in any mis-matched line.  The 
only signal LOST is that due to excess attenuation.  What you're calling 
"mismatch loss" is entirely the result of where along the line the 
signal is sampled or retrieved, and the "mismatch loss" can be 
eliminated either by changing the line length or transforming the 
impedance.

> Now you are changing the topic again.  Do you dispute that 2 mated UHF
> connections can produce a 2:1 SWR on 70cm?

I don't have enough information to say that -- because I don't work at 
UHF, I haven't seen precise measurements of the characteristics of these 
connectors, but I have no trouble accepting that there is SOME mismatch 
based on their construction.  What I do NOT accept is that

1) roughly 30 electrical degrees (will be dependent on Vp is of the 
connector at the frequency of interest) will do any more than rotate the 
signal around the Smith chart; and

2) that an analysis of ONLY a mating connector pair with ideal 50 ohm 
cable on either end is indicative of real world use of connectors.  Most 
connectors are used AT EQUIPMENT, and both the termination at that 
equipment, as well as the variations of input and output impedance 
within that equipment, are nearly always of greater consequence than 
whatever mismatch may be caused by the connector.

> Many universities such as MIT have the EE courses online.
>
> See: http://ocw.mit.edu/index.htm

Thank you for your insult to my 1964 BSEE from the University of 
Cincinnati, where I studied transmission lines rather thoroughly.  In 
those days, we used slide rules and Smith charts for computations, but 
we also learned when that analysis was appropriate and when it was not.  
And in my 40+ years of professional engineering, I have learned to look 
at the ENTIRE SYSTEM, not pick nits with individual components that 
don't matter IN THAT PARTICULAR SYSTEM, but to consider them when they 
do matter.

> 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.

That's nice. If I needed that capability in my work, I'd do that too.  
But my comments are about the suitability of UHF connectors at HF and 
low to mid-VHF.