Richard Fry wrote:
"It doesn`t matter whether we state the result of measurement in units
of SWR, return loss, or as a reflection coefficient -- they all give the
same information -- ."

Correct. The units above are fungible. All are an expression of the
mismatch of a load to the Zo of the transmission line.

With a Bird wattmeter, the reflection coefficient (rho) is the sq. rt.
of the reflected power divided by the forward power.

SWR = 1+rho / 1-rho

Return loss in dB = 20 log (rho)

Return loss in dB = 10 log (Ref.Pwr./Fwd.Pwr.)

Rho = (ZL/Zo)-1 / (ZL/Zo)+1

None of the expressions above include the source Z, therefore it does
not apply.

Best regards, Richard Harrison, KB5WZI

"Roy Lewallen" wrote:
Sorry, it still isn't clear.

What, then, is "system SWR"? How do you define it?

System SWR is the net SWR of a component assembly present at its input
terminals. "Antenna system SWR" then is comprised of the net SWR of
everything in the RF path from the output of the SWR meter to and including
the antenna. In a transmitter, the antenna system begins electrically at
the output of the SWR meter -- physically close to the output connector of
the tx.

Obviously, if we have a voltage or current source of fixed value
and change the source impedance, the power delivered by the
source changes,

But the mechanism I've described considers the re-reflection by a mismatched
source of power not initially absorbed by a mismatched load -- not that a
change of source impedance changed the total power flowing out of the
source.

...both the "forward" and "reverse" powers
will change, but by the same fraction...

Agree. I'm not so sure that the Model 43 or equivalent methods used in/with
transmitters accurately preserves the power ratios under these conditions,
though.

RF

What's a directional coupler?
What do they look like?
Don't bother answering those questions.

Why do the arguers, when caught in a tight corner, always escape to UHF for
help from directional couplers?

There are NO directional couplers at HF. They are as scarce as real swr
meters. So they cannot be used in futile attempts to explain what really
happens at HF.

You're next move will be to drag in scattering-matrices.
---
Reg ;o)

"Reg Edwards" wrote in message
...
What's a directional coupler?
What do they look like?
Don't bother answering those questions.

Why do the arguers, when caught in a tight corner, always escape to UHF
for
help from directional couplers?

There are NO directional couplers at HF.

So what is the element in my Bird Thruline then, and how does it work?

I'd dying to hear at what frequencies directional couplers suddenly begin to
"exist". A sudden change in the laws of Physics at some arbitary frequency
named by man.

(Most SWR meters I've seen use a directional coupler, by the way. Even the
cheap ones.)

I think we've got to the root of Reg's problem. Just like the last time he
raised this nonsense.

--
Brian Reay
www.g8osn.org.uk
www.amateurradiotraining.org.uk
FP#898

"Richard Clark" wrote
To date in this matter, I have yet to see any concrete value of source
Z offered from those of the NOT 50 Ohms camp. Further, I have yet to
see any of them offer any experimental confirmation of their assertion
Richard Fry wrote:
Below is a quote from a paper titled "A Study of RF Intermodulation
Between
FM Broadcast Transmitters Sharing Filterplexed or Co-located Antenna
Systems," by Geoffrey Mendenhall. (clip). Quoting Mendenhall, "...If the
source impedance were equal to the fifty ohm line impedance, half of the
transmitter's output power would be dissipated in its internal output
source
impedance..."
Walter Maxwell wrote
The last sentence in the paragraph above is incorrect. This shows that
the writer of the quote is in the unbelievably large group that still
believes
incorrectly that half of the tx power would be lost if if it were
conjugately
matched. But we all know that efficiencies greater than 80% is achieved
by Class C amps, and greater than 60% is achieved by Class B amps
when the source impedance of the tx is 50 ohms resistive and the load
impedance is also 50 ohms resistive.
_______________

To Walter Maxwell:

1. You may be interested in reading Mendenhall's complete paper, which I
will email to you. The lab measurements reported in it used two, operating,
high-power FM broadcast transmitters -- and support his statements about
amplifier source impedance and its consequences.

2. I will ask again, if transmitters have a 50 ohm source impedance, what
accounts for the fact that TV ghosts are produced by an antenna system
reflection having a sufficient delay time? Calculations and measured data
show that the energy that produced the ghost originated by re-reflection off
the TV transmitter output stage of far-end reflections in the antenna
system. If the tx source impedance was 50 ohms, it would absorb the far-end
reflection, which would be incapable of producing a ghost image.

Further, if the tx source impedance was 50 ohms, then the RF intermodulation
measured and reported in Mendenhall's paper -- and verified in real-world
installations by the radiated interference those IM products produced --
would not occur.

RF

Reg, G4FGQ wrote:
"There are NO directional couplers at HF."

Behold the Bird wattmeter!

Why not call the SWR indicator a "mismatch meter"?

Best regards, Richard Harrison, KB5WZI

The meaning of this paragraph in my last post in this thread is more clearly
understood when two commas are added...

But the mechanism I've described considers the re-reflection,
by a mismatched source, of power not initially absorbed by a
mismatched load -- not that a change of source impedance
changed the total power flowing out of the source.

In message , Reg Edwards
writes
What's a directional coupler?
What do they look like?
Don't bother answering those questions.

Why do the arguers, when caught in a tight corner, always escape to UHF for
help from directional couplers?

There are NO directional couplers at HF. They are as scarce as real swr
meters. So they cannot be used in futile attempts to explain what really
happens at HF.

You're next move will be to drag in scattering-matrices.
---
Reg ;o)



Reg, I thought that a DC was essentially any device which sampled some
of the RF signal travelling in one direction, and (if perfect) none of
the signal travelling in the other. Over the past 40 years, I have had
lots of dealings with them. I've even designed some which were bought in
their thousands (if not millions). Some of them seemed to work from
below 5MHz to over 870MHz (well, I THOUGHT they did, and so did the
people who bought them).

Where did I go wrong?

On the positive side, I never went as far as to claim that they would
measure SWR.

Ian.
--

On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards"
wrote:

|What's a directional coupler?
|What do they look like?
|Don't bother answering those questions.
|
|Why do the arguers, when caught in a tight corner, always escape to UHF for
|help from directional couplers?
|
|There are NO directional couplers at HF. They are as scarce as real swr
|meters. So they cannot be used in futile attempts to explain what really
|happens at HF.
|
|You're next move will be to drag in scattering-matrices.

Why not. I have used an HP3577 network analyzer with an S-parameter
test set that was specified to work over the frequency range of 100 Hz
to 200 Mhz.

I guess the guys at HP didn't realize that you can't do this.

On Sun, 5 Sep 2004 07:49:06 -0500, "Richard Fry"
wrote:

if transmitters have a 50 ohm source impedance,

Hi OM,

There you go again "IF." IF indeed!

It seems you find controversy where there is none. :-)

I would again suggest you read what I wrote, and point out what
exactly your contention is with IT.

73's
Richard Clark, KB7QHC