tuner - feedline - antenna question ?
 

Please don't insult our intelligence. If the Bird reads

Of course it is nonsense, but it is a logical development based on Jeff's
words "What you are describing could be called 'transmitted' power or
power delivered into a mismatched load, but that it different from
forward power, or the power delivered by the source" and your words "For
systems


Only a logical development if you selectively snip Owen.

"What you are describing could be called 'transmitted' power or
power delivered into a mismatched load" was referring to "Pload = Pfor -
Pref".

Ok I admit that 'transmitted' power could have been better phrased.

Power may not actually be dissipated in a lossless line but that does not
detract from the fact that there is current flow and a voltage along the
line produced by two distinct and independent waves travelling in opposite
directions. True that the power can only be realised when it encounters a
load, but it is highly pedantic not to regard the reflected signal as having
'power' until it actually encounters such a load. If you extend this theory
to a radiated signal, you could equally say that there is no power
travelling through the aether until it encounters a receiver.

It is naive to believe that reflected power is not dissipated in a matched
source, or partially re-reflected at the source/line interface is not
matched. Again going back to the optics corollary you would not expect a
reflected light signal not to impinge on, and interact with a source.

If you pad out a source with a sufficiently high attenuator such that the
reflected signal will not have significant effect on it, you will see an
increase in dissipation in the attenuators when the load is mis-matched. I
am confident that an attenuator is not having its "load line" changed such
that its dissipation goes up magically just by the same amount as the power
in the reflected wave!! (Of course the dissipation in the load is measurable
as heat).

Adding a circulator to a system will not change "the load line" (if a
transmission line or circulator can have such a thing), but it will cause
the power in the reflected wave to be separated so that it can be monitored
and measured. Surprisingly power monitored in this way ties up with the
notion that power is reflected at a mis-matched load.

73
Jeff

tuner - feedline - antenna question ?
 

Cecil Moore wrote:

My favorite quotation by an antenna guru on this
newsgroup is that "a 50 ohm antenna can be replaced
by a 50 ohm resistor without changing anything".
If that were true, we don't need antennas. :-)


That sounds like a direct misquotation of me.

What I HAVE said - and often - is that if you measure the load impedance
presented by an antenna and feedline at the output socket of the
transmitter, and replace it by the same impedance made from lumped R and
L/C components, then the steady-state operating conditions of the
transmitter will not change.

If the transmitter isn't touched, it will deliver exactly the same power
as before - because that happens to be how much power it can deliver
into that particular load impedance.

That's all the RF power there is. In a lossless system, all of that
power will be radiated from the antenna. With the alternative lumped
load, exactly the same power will be delivered into the resistive part
of the load, and dissipated as heat.

Of course the transmitter is under more stress from voltage, current and
heat when it's operating into an incorrect load impedance (not what it
was designed for) but that's all it is. There is no need to invent
reflected power that is being "dumped" back into the transmitter to
cause this stress.

There is also a strong tendency to invent virtual instruments such as
"directional wattmeters" which do not actually exist. An instrument such
as the Bird 43 is calibrated in watts, but it doesn't actually sample
power. As Owen relates (and so have I) these instrument only sample V
and I on the line - they categorically DO NOT sample power.

The power scale is only a meter calibration - literally, only ink on a
meter scale. It indicates the amount of power delivered into a matched
load, when the "reverse" reading is exactly zero [1]. The instrument was
calibrated under those specific conditions, and the "forward power"
reading is only physically meaningful for that specific case.

For a mismatched load, the meter will read higher in the forward
direction than in the reverse - but that is purely a feature of the
instrument. It all looks so plausible on the meter scale, but those are
not genuine power waves flowing in opposite directions.

Everything that's happening inside the instrument can be completely
explained from the new V and I conditions on the line. Waves of power
simply don't come into it. Most people don't want to go that deep into
the theory... but, regrettably, that may be the only way to understand
that the "power" readings on the meter scale are no longer valid under
these conditions.

What IS physically meaningful is the DIFFERENCE between the forward and
reverse "power" readings. That difference will equal the net power
delivered to the load [1]. But those two readings are only meaningful as
a pair - individually they are only "intermediate results" with no
physical meaning of their own.


[1] Ignoring real-life meter errors such as directivity and scale
accuracy.


I'm going to be away from my computer for 48 hours.

But you'll be back... :-)


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

tuner - feedline - antenna question ?
 

On Feb 26, 6:02 pm, Owen Duffy wrote:
Cecil Moore wrote et:
Please don't blame me for someone else's words.

A selective partial quotation to misrepesent what was actually written Cecil!

My true statement was combined with someone else's false statement to
make the combination false. I'm asking politely that not be done
again.
--
73, Cecil, w5dxp.com

tuner - feedline - antenna question ?
 

On Feb 26, 6:11 pm, Owen Duffy wrote:
Cecil Moore wrote . net:

Owen Duffy wrote:
I suggest that if a PA / line / load situation transforms the actual
load to some arbitrary impedance Z at the PA end of the line, the PA
will peform exactly as if the PA were directly loaded by a lumped
constant load of Z.

Yes, that is true for the performance of the PA.
Certainly not true for the performance of the
transmission line or antenna.

Please explain?

If the transmission line and antenna are replaced by a lumped constant
load, the transmission line and antenna cease to function. IMO, that's
not "performing exactly as".
--
73, Cecil, w5dxp.com

tuner - feedline - antenna question ?
 

On Tue, 27 Feb 2007 09:10:12 +0000, Ian White GM3SEK
wrote:

As Owen relates (and so have I) these instrument only sample V
and I on the line - they categorically DO NOT sample power.

Hi Ian,

We've been through this before. No instrument operates in the absence
of power. Simply because you and Owen are graced with instruments
that demand so little, does not negate what power they do rob from
what is available. Even the humble electrometer has to overcome the
force of gravity to open its foil leaves, and climbing that potential
energy hill is work over time - power.

Hammer down the directivity as much as you want, and it will still
resolve to some diminution of power available to the load. To wave a
hand and say NOTHING does not make it so.

73's
Richard Clark, KB7QHC

tuner - feedline - antenna question ?
 

On Feb 27, 3:10 am, Ian White GM3SEK wrote:
Cecil Moore wrote:
My favorite quotation by an antenna guru on this
newsgroup is that "a 50 ohm antenna can be replaced
by a 50 ohm resistor without changing anything".
If that were true, we don't need antennas. :-)

That sounds like a direct misquotation of me.

Nope, it wasn't you, Ian. You are usually more careful than that.

For a mismatched load, the meter will read higher in the forward
direction than in the reverse - but that is purely a feature of the
instrument. It all looks so plausible on the meter scale, but those are
not genuine power waves flowing in opposite directions.

But they are genuine energy waves flowing in opposite directions.
Standing waves require two coherent energy waves flowing in opposite
directions. Can you explain how to create a standing wave without
two energy waves flowing in opposite directions? And remember, the
two EM wave components in the standing wave cannot stand still.

There's truth in what you say, Ian, but it is not the whole truth.
There's
no difference except frequency (and all that frequency implies)
between
an RF electromagnetic wave and a visible light electromagnetic wave.
In fact, RF waves are covered in many physics textbooks whose subject
is light. There is a wealth of information available from the field of
optics
that is applicable to RF waves.

Visible light physicists don't have the luxury of measuring voltage or
directly measuring phase. They have to rely on a power measurement
of irradiance. As a result, visible light measurements are actually
power measurements so we indeed do know how EM waves behave
at the joules/second level.

Visible light physicists found that when they superpose two coherent
light waves, Ptotal = P1 + P2 + 2*SQRT(P1*P2)cos(A)
where 'A' is the angle between the electric fields of the two waves.
That exact same equation applies to coherent RF waves. Phasor
addition is used for the superposition of two coherent RF voltages.
The power equation is used to find out what happens to the power
during that voltage superposition. P1 = V1^2*Z0 and P2 = V2^2*Z0
The last term in the power equation is known as the interference term
and is either constructive, destructive, or zero.

Since antenna radiation patterns depend upon constructive and
destructive interference of EM waves in space, we hams could
learn a lot from the field of physics known as optics.

I'm going to be away from my computer for 48 hours.

But you'll be back... :-)

Yep, I'm posting from my sister's computer through my Google account.
--
73, Cecil, w5dxp.com

tuner - feedline - antenna question ?
 

On Feb 27, 2:53 am, "Jeff" wrote:
Adding a circulator to a system will not change "the load line" (if a
transmission line or circulator can have such a thing), but it will cause
the power in the reflected wave to be separated so that it can be monitored
and measured. Surprisingly power monitored in this way ties up with the
notion that power is reflected at a mis-matched load.

Yes, and a little modulation added to the source signal will prove
that the
signal being dissipated by the circulator resistor has made a round
trip
to the load and back. That's hard to explain if reflected energy
doesn't
actually exist.
--
73, Cecil, w5dxp.com

tuner - feedline - antenna question ?
 

Richard Clark wrote:
On Tue, 27 Feb 2007 09:10:12 +0000, Ian White GM3SEK
wrote:

As Owen relates (and so have I) these instrument only sample V
and I on the line - they categorically DO NOT sample power.

Hi Ian,

We've been through this before. No instrument operates in the absence
of power. Simply because you and Owen are graced with instruments
that demand so little, does not negate what power they do rob from
what is available. Even the humble electrometer has to overcome the
force of gravity to open its foil leaves, and climbing that potential
energy hill is work over time - power.

Hammer down the directivity as much as you want, and it will still
resolve to some diminution of power available to the load.

Richard, that argument is otiose.

What I said (in full context) was that their principle of operation as
measuring instruments does not involve sampling traveling waves of power
from the line.

Of course they must incidentally consume some power to move the meter
needle, but that is not part of their operating principle.

For that matter, almost all measuring instruments abstract some energy
or power from whatever they are measuring - but that is usually
incidental. It certainly does not make every instrument into a power
meter.

Can you not see this?


To wave a
hand and say NOTHING does not make it so.

To wave another hand and say traveling waves of power exist does not
make that so, either.




--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

tuner - feedline - antenna question ?
 

"Cecil Moore" wrote in message
oups.com...
On Feb 27, 2:53 am, "Jeff" wrote:
Adding a circulator to a system will not change "the load line" (if a
transmission line or circulator can have such a thing), but it will cause
the power in the reflected wave to be separated so that it can be
monitored
and measured. Surprisingly power monitored in this way ties up with the
notion that power is reflected at a mis-matched load.

Yes, and a little modulation added to the source signal will prove
that the
signal being dissipated by the circulator resistor has made a round
trip
to the load and back. That's hard to explain if reflected energy
doesn't
actually exist.
--
73, Cecil, w5dxp.com

Indeed; TDR's would have areally hard time (;-))

Jeff

tuner - feedline - antenna question ?
 

Richard Clark wrote in
:

On Tue, 27 Feb 2007 01:07:12 GMT, Owen Duffy wrote:

Richard Clark wrote in
Let's treat this like the Chinese Box problem.

If you didn't know what the load was, could you explain it any
differently? No. Apriori knowledge is not a proof.

Richard, I content that:

Contend or offer in contention.

Richard

Yes, my spelling mistake.


- the power output of the PA; and
- the efficiency of the PA may be (and usually are) sensitive to the
load impedance.

This is not contending nor contention and is content only for a non
sequitur. The line following a tuner exhibits considerable loss (poor
efficiency) that can only occur on the basis of power and mismatch.
You yourself offered in other correspondence that it exceeds cable
attenuation specifications found only in a matching condition. To

I am being picky, but "it *may* exceed cable attenuation specifications
found only in a matching condition, it may also be lower". If I said it
as you stated, I made an error. The common statement (and I have no doubt
made it) that VSWR exacerbates line loss is actually wrong in the general
case. (Having Googled my own web site I see one statement along those
lines which needs further qualification!)

suggest that a PA's sensitivity is somehow exhalted in the face of
identical, ordinary behavior of a passive component is hardly
seperable. Consider the simple substitution to your quote:
- the power output at the terminus of the line; and
- the efficiency at the terminus of the line may be (and usually are)
sensitive to the load impedance.

I meant the output at the PA terminals where an lumped constant load
would be attached for comparison.


....

Though it is often asserted that the PA will get hotter as a result of
"reflected power" being dissipated in the dynamic output impedance of
the PA, and that this may / will damage the PA, the power explanation
doesn't work numerically in the general case.

Heat is the outward proof of power and is always demonstrable in both
specific and general cases. Occurrences of other, significant
radiation from the source (as long as that source physically occupies
a substantially minor region of wavelength) is exceedingly difficult
to achieve.

You don't offer a numerical proof of a general case, and given that
the general case must allow for the specific cases already allowed in
your discussion above - that may be an untenable assertion for you.
Those specific cases are demonstrably caloric and must follow the same
math you suggest.

I suspect you are trying to argue differences by degree (no pun
intended as to heat); but I seriously doubt you can produce the math
to do that. The arguments that flow from that involve what is called
source resistance, and those arguments are legion in this forum (where
naysayers embrace a refusal to accept or name ANY value - a curious
paradox and an engineering nihilism I enjoy to watch).

PAs can be designed to behave as an equivalent fixed voltage or current
source with fixed source impedance of Zo, but HF PAs are not usually
designed in that way.

I know that there is a vein of thought that the process of adjusting a PA
for maximum output always, somewhat magically, creates a match condition
where the source impedance is the conjugate of the load at the PA
terminals, but it is contentious. What of broadband PA designs with no
such adjustment, are they source matched over a broad range of
frequencies? Observations are that experiments to discover the source
impedance by incrementally changing load current can produce a range of
values for the same PA on different frequencies, and at different power
levels. Why do amplifiers with say tetrodes and triodes which exhibit
such different dynamic plate resistance but requiring the same load
impedance deliver the same equivalent source impedance?

I am also aware that supporters of the inherent source match position
assert that you must be selective in choosing tests for source impedance.
It is all rather unconvincing when only some of the implications of a
particular source impedance are effective.

It is my view that modelling the PA as a fixed voltage or current source
with fixed source impedance of Zo, and where reflected waves on a
transmission line are absorbed by the matched source is not a good
general model for HF PAs.

The application of small signal analysis to amplifiers that sweep from
near cutoff to near saturation is suspect.

I believe that it is sound (in the steady state) to resolve the forward
and reflected wave voltages and currents at the source end of the
transmission line, calculate the complex impedance, and predict the
effects of that impedance as a PA load using the same techniques that
were used to design the PA.

Owen