On Apr 26, 1:59*am, Richard Clark wrote:
On Mon, 25 Apr 2011 17:35:29 -0700 (PDT), "Sal M. Onella"

wrote:
Something I haven't seen is a discussion of the source impedance of
the transmitter.

sigh....

My point: *Using 75-ohm cable to improve the match at the antenna
won't help me *... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio.

Hi OM,

Look at the prospective SWR and how much is
lost/reflected/absorbed/what-have-you? *More heat comes from a less
than optimal system efficiency than what your computation will reveal.
So much that it will swamp it.

But the trick here is that the reflected "power" (arguments turn on
this word) doesn't always mean heat and it could actually cool -
however hot or cold it may alter the situation, that same "power"
never got out into the air. *

Now, as for source impedance, that is a subject fraught with denial in
the face of the obvious: *Those fins in the back of your rig are to
help bookend your QSL cards into groups (the heat bears no relation to
efficiency nor match loss).

A standard definition (courtesy of Wikipedia) for Return Loss is:
* * * * where Zs is the impedance toward the source and
* * * * Zl is the impedance toward the load.
and we find from the values you supply that it is
* * * * 0.20

Of course, such a definition is utterly useless when the concept of Zs
is replaced with (in most cases) "it ain't 50 Ohms, thet's fur
shure").

If, perchance, some brave soul steps into the breach of NOT 50 Ohms to
suggest what Zs is, then we can give it the acid test of engineering
(an act that I am usually reminded is beyond the understanding of
readers and the province of discussion here). *Let's be gentle and go
only by an order of two (which is reasonably available and can be
coaxed out of my TS-440). *Return loss for a rig exhibiting an Zs of
25 Ohms into the 75 Ohm line (presuming it is infinite in length)
would give us:
* * * * 0.50
that doesn't look good, so let's try Zs of 100 Ohms:
* * * * 0.14
that looks better all 'round. *Even intuition agrees.

Let's press intuition to the proximal limit and say that Zs is 74 Ohms
(yes, my thumb is on the scale):
* * * * 0.01

What does intuition affirm? *What is preferable? * * * *

73's
Richard Clark, KB7QHC

Thanks Richard. Intuition is that the Zs is near 50 ohms for as many
frequencies as the designer can manage. I am on record (including in
this group) of letting intuition lead me down the path to ruin.

I get from you that there's a presupposition that I know the source
impedance or can easily establish it. Hm-m-m ... not so. One big
problem I see is the need to try to measure power delivered in a
non-50-ohm system with my existing instruments that depend on a 50-ohm
system. I don't have a nominal 75-ohm power meter. Won't putting a
50-ohm meter into a 75-ohm circuit not only read wrong but introduce
reflection losses, too?

I think I'd need a collection of non-inductive load resistors and an
accurate rf ammeter. I'd need to connect them and calculate power at
a few points in every band.

Maybe the papers that Cecil cited for me will fill in the gaps or
suggest other approaches.

"Sal"