On Tue, 7 Oct 2003 01:29:43 +0000 (UTC), "Reg Edwards"
wrote:

Walt wrote -
Well, Reg, the reason I asked for an expression that includes the source
resistance in measuring SWR is that you said above that the internal
impedance
of the transmitter is ASSUMED to be 50 ohms. This implies that the SWR is
dependent on the internal impedance of the source, does it not ?

===============================

Walt, there's just a slight misunderstanding.


It is not I who makes that silly assumption - it is the stupid so-called
SWR + |rho| meter!


The meter indication is linearly proportional to |Rho|, from 0-to-1, from
zero to full-scale.


In effect, by means of its scale-calibration, it calculates SWR and Refl
power from |Rho|.


I understand conversion tables even appear in the handbooks.


The number 50-ohms appears in meter design calculations, and therefore
affects the meter indication of |Rho|.


One expression involved is |Rho| = (50-ZL)/(50+ZL) -


The meter therefore ASSUMES the impedance seen looking back towards the
transmitter from the meter is exactly 50 ohms.

But, as you well know, in the usual amateur situation, this assumption can
be, and often is, is wildly incorrect.


There are only a few remaining old-wives who still think it's true. But the
biased arguments still habitually remain.


As I have been saying for years, the solution is to change the name of the
undoubtably useful meter to TLI (Transmitter Loading Indicator). Forget
about SWR on an imaginary, non-existent transmission line and get into the
real world.


Don't shoot the messenger of apple-cart-upsetting news. It's not new to the
rigid old-wives Establishment.


Walt, please direct any criticism towards the education-disrupting meter
indications.


For design of so-called HF SWR meters download in a few seconds and run
immediately program SWRMETER from website below. Copious design notes are
included. I have just re-read them. After 3 years their readability could
be improved and updated but I have no intention of doing so.

-----------------------------------------------------
Walt sez,
This concept is foreign to me, so if I'm wrong I'd like to have some proof
that
the source impedance can have any influence on SWR.

----------------------------------------------------

Walt, you must have been familiar with the incorrect concept for years. As
we both know, changing the internal impedance of the transmitter cannot
possibly have any effect on SWR on the imaginary transmission line, if there
is one between transmitter and the meter. It is the meter itself which gives
silly answers because it, and its users, assumes a line of exactly 50-ohms,
longer than 1/2-wavelength actually exists. It doesn't!

----
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================

No Reg, I've never been familiar with the incorrect concept. I had never thought
anyone was that uninformed until you and Richard C brought it up here. I was
flabbergasted to think you held that position, so I'm relieved to know that you
aren't among the uninformed. And Richard's last two posts seemed
contradictory--I haven't yet totally understood what his position really is.

I'm sure you're aware that the voltage applied to the meter movement of the SWR
meter is actually making the indicator hand respond to the value of rho, but
with the scale graduated in units of SWR. To verify this we adjust the forward
reading for full scale for the reference reading. We then switch to the
reflected reading. Let's say the mismatch is 3:1 for rho = 0.5. If the SWR
indicator is accurate a voltmeter will now read 0.5. exactly half scale, where
the full-scale reading is 1.0.

Reg, I'm not lecturing you, because I know that you know this--this is my way of
telling you that I also know it.

Walt, W2DU