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.
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