On Sun, 05 Jun 2005 21:58:06 GMT, "Henry Kolesnik"
wrote:

In TV broadcasting reflections from the antenna back to the transmitter will
be reflected by the transmitter to the antenna and the signal will be
rebroadcast albeit at somewhat less power.

H. Adam Stevens, NQ5H wrote:

"Cecil Moore" wrote:
The log of the ratio of two SWRs doesn't seem to have much
meaning.

It's called db, Cecil.

The IEEE Dictionary says the ratio of power, voltage, and
current can be expressed in dB. It specifically states
that dB can only be related to power ratios or to parameters
that are proportional to the square root of power ratios.

SWR1 = [SQRT(Pfor1)+SQRT(Pref1)]/[SQRT(Pfor1)-SQRT(Pref1)]

SWR2 = [SQRT(Pfor2)+SQRT(Pref2)]/[SQRT(Pfor2)-SQRT(Pref2)]

The ratio of two SWRs will not reduce to a power ratio or
to the square root of a power ratio.
--
73, Cecil http://www.qsl.net/w5dxp

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H. Adam Stevens, NQ5H wrote:
Good Lord Roy, I thought you knew better.

If the match at the load is not perfect, energy is refleced back to the
source, are you with me so far?

I can easily build a source that absorbs all the reflected power: A zero
impedance source in series with a resistor that matches the transmission
line impedance.


Let's see, I put a 100 volt zero impedance source in series with a 50
ohm resistor, connect that to a half wave transmission line terminated
with 150 ohms. The current will be 100/200 = 0.5 amp, the power in the
150 ohm load is 37.5 watts, the power in the 50 ohm source resistor is
12.5 watts. The SWR is 3:1, the forward power is 50 watts, the reverse
power is 12.5 watts. Sure enough, the power in the source resistor
equals the reverse power. Good job. That sure must be the worst case,
all right.

Just to check, I'll change the load resistor to 16.67 ohms. Now the
current is 1.5 amps, the power in the 16.67 ohm load is 37.5 watts, and
the power in the source resistor is 112.5 watts. The SWR is still 3:1,
the forward power is 50 watts just like before, and the reverse power is
12.5 watts just like before. Hm. The reverse power is 12.5 watts, but
the source resistor is now dissipating 112.5 watts. Must be worse than
the worst case.

Well, shoot, maybe the source resistor dissipates all the reverse power
*plus* some more power that comes from somewhere else. So let's try a
200 ohm load. Now the current is 0.4 amp, the power in the 200 ohm load
resistor is 32 watts, and the power in the 50 ohm source resistor is 8
watts. The SWR is 4:1, the forward power is 50 watts, and the reverse
power is 18 watts. Oops, the source resistor is only dissipating 8 watts
but the reverse power is 18 watts. Not only isn't it dissipating all the
reverse power, but it isn't even dissipating that extra power that came
from somewhere else when we connected the 16.67 ohm resistor. Wonder
where the other 10 watts of reverse power went?(*)

So using your simple criterion of a zero impedance source and resistor
equal to the transmission line impedance, and by only changing the load
resistance, we've got cases whe

-- The source resistor dissipation equals the reverse power
-- The source resistor dissipation is greater than the reverse power
-- The source resistor dissipation is less than the reverse power

And none of these will explain the loss figure you gave earlier.

Guess I don't know better after all.

Anyone who's interested can find more interesting cases in "Food for
thought - Forward and Reverse Power.txt" at
http://eznec.com/misc/food_for_thought/. And those who aren't
interested, well, you're welcome to believe what you choose. Just don't
look too closely at the evidence.

(*) Anybody fond of the notion that reverse power "goes" somewhere or
gets dissipated in the source or re-reflected back needs to come to
grips with this problem before building further on the flawed model of
bouncing waves of flowing power.

Roy Lewallen, W7EL

Yes, SWR is a dimensionless quantity. dB is, as far as I know, defined
only for power, voltage and current ratios, as the IEEE Dictionary
implies. Since it's defined differently for power than for voltage or
current (so that an increase or reduction in one quantity represents the
same number of dB increase or decrease in the other), anyone using it
for something else would have to clarify how it would be defined in that
context.

Roy Lewallen, W7EL

Cecil Moore wrote:
H. Adam Stevens, NQ5H wrote:

"Cecil Moore" wrote:
The log of the ratio of two SWRs doesn't seem to have much
meaning.

It's called db, Cecil.


The IEEE Dictionary says the ratio of power, voltage, and
current can be expressed in dB. It specifically states
that dB can only be related to power ratios or to parameters
that are proportional to the square root of power ratios.

SWR1 = [SQRT(Pfor1)+SQRT(Pref1)]/[SQRT(Pfor1)-SQRT(Pref1)]

SWR2 = [SQRT(Pfor2)+SQRT(Pref2)]/[SQRT(Pfor2)-SQRT(Pref2)]

The ratio of two SWRs will not reduce to a power ratio or
to the square root of a power ratio.

On Sun, 05 Jun 2005 21:58:06 GMT, "Henry Kolesnik"
wrote:

In TV broadcasting reflections from the antenna back to the transmitter will
be reflected by the transmitter to the antenna and the signal will be
rebroadcast albeit at somewhat less power.

Hi Hank,

That would pretty much reveal the SWR if we knew, wouldn't it? If
"somewhat less power" was in 1.2:1 ratio, we wouldn't care so much,
but how would the viewer feel about such service?

Then depending on the length of
transmission line the viewer may see ghosting.

I think we, or another correspondent and I have dealt with that at one
time. At the time I believe it was called "fringing," not "ghosts."
The difference being that what were called ghosts at the dawn of the
TV era were separated by fractions of an inch rather than fractions of
a mm. As such, ghosts couldn't have been originated by anything
shorter than mile length transmission lines that were poorly
terminated at both ends. Instead, ghosts were actually transmission
path length differentials in a multipath situation.

In audio I don't know why and I have run my Collins 30S-1 into ladder line
with a 14 to SWR with no one except me knowing!

Well, if this is meant to be analogous to fringing/ghosting, I suppose
its because a microsecond blur at AF is entirely inaudible. Or are we
speaking of SSTV? However, this begs the question, How did you know?
All the Collins equipment I taught at school didn't come with a SWR
meter. It was wholly unnecessary if you performed the standard
tune-up. Matter of fact, back then the only SWR meter I saw was for
Ham gear. The finals' tank performed every function of matching as
any tuner.

However, with the KWT-6, we did use an external tuner, 180-V1
(although I may have this mixed up with another model), for coax
feedlines. This was more for its automatic feature where the
transmitter could be tuned up with a 50 Ohm load, and the automatic
tuner simply did the job of presenting it with the transformed load.

However, returning to the point of a transmitter rereflecting a
reflection; I know the bare KWT-6 into ladder line employs its tank to
protect its final tubes. Without that safeguard, I have seen plates
melt - something no one here wants to call dissipation lest it be
evidence of an internal resistance. The bare tubes with their native
very hi Z would rereflect like nothing else - and this begs the
observation - how could you get original any power out of them, past
the tremendous mismatch? The tuner/final tank comes back into the
equation, and rereflection goes out the window as a property of the
transmitter and returns to the domain of matching.

If anyone wants to constrain the entire crusade of the rereflecting
transmitter to the tube set feeding ladder line - then feel free to do
so. However, I don't think I've ever seen a mobile tube rig feeding
ladder line - no doubt one day I will. We will probably talk about
efficiency. :-)

73's
Richard Clark, KB7QHC

However, I don't think I've ever seen a mobile tube rig feeding
ladder line - no doubt one day I will. We will probably talk about
efficiency. :-)

It was done in 1936.
http://web.wt.net/~nm5k/mobile36.jpg
Cover pix from a 1936 QST...Forgot
what month...
But I still think I prefer coax...
Their "ladder line" looked to be a twisted wire feeder.
The call on that vehicle was W9MSY...
With the short feedline run on a mobile,
even coax is pretty low loss...
I never used an SWR meter when I was a
novice...I had an old viking valiant that would
tune nearly anything...You didn't need a meter...
You just loaded it up to full plate current and went
with it...My TS 830 is like that to a lesser extent..
If it loads within the loading range, it's good nuff...
No point in even putting a meter on it...Adding a
tuner, would just add some loss...MK

Roy Lewallen wrote:
(*) Anybody fond of the notion that reverse power "goes" somewhere or
gets dissipated in the source or re-reflected back needs to come to
grips with this problem before building further on the flawed model of
bouncing waves of flowing power.

Roy, none of my textbook authors think the reflection model
is flawed. Walter Johnson goes so far as to assert that there
is a Poynting (Power Flow Vector) for forward power and a
separate Poynting Vector for reflected power. The sum of those
two Power Flow Vectors is the net Poynting Vector.

Here's my earlier thought example again.

100w----one second long lossless feedline----load, rho=0.707

SWR = (1+rho)/(1-rho) = 5.828:1
Source is delivering 100 watts (joules/sec)
Forward power is 200 watts (joules/sec)
Reflected power is 100 watts (joules/sec)
Load is absorbing 100 watts (joules/sec)

It can easily be shown that 300 joules of energy have been
generated that have not been delivered to the load, i.e.
those 300 joules of energy are stored in the feedline.
The 300 joules of energy are stored in RF waves which
cannot stand still and necessarily travel at the speed of
light. TV ghosting can be used to prove that the reflected
energy actually makes a round trip to the load and back.
A TDR will indicate the same thing.

Choosing to use a net energy shortcut doesn't negate the
laws of physics.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Roy Lewallen wrote:

(*) Anybody fond of the notion that reverse power "goes" somewhere or
gets dissipated in the source or re-reflected back needs to come to
grips with this problem before building further on the flawed model of
bouncing waves of flowing power.

Roy, none of my textbook authors think the reflection model
is flawed. Walter Johnson goes so far as to assert that there
is a Poynting (Power Flow Vector) for forward power and a
separate Poynting Vector for reflected power. The sum of those
two Power Flow Vectors is the net Poynting Vector.

Sorry, I misquoted there. Walter Johnson doesn't say anything
about Poynting Vectors. The above is from: "Fields and Waves ..."
by Ramo, Whinnery, and Van Duzer, page 350, where they assert:

Pz-/Pz+ = |rho|^2

The reflected power Poynting Vector divided by the forward
power Poynting Vector equals the power reflection coefficient.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil,

I will presume that your reference to Walter Johnson is with regard to
his book, "Transmission Lines and Networks", published in 1950.

I have been unable to find any mention of Poynting Vectors or Power Flow
Vectors in my copy.

Would you be so kind as to identify the page number(s) describing these
concepts?

73,
Gene
W4SZ


Cecil Moore wrote:

[snip]

Roy, none of my textbook authors think the reflection model
is flawed. Walter Johnson goes so far as to assert that there
is a Poynting (Power Flow Vector) for forward power and a
separate Poynting Vector for reflected power. The sum of those
two Power Flow Vectors is the net Poynting Vector.

[snip]

Gene Fuller wrote:

Cecil,

I will presume that your reference to Walter Johnson is with regard to
his book, "Transmission Lines and Networks", published in 1950.

I have been unable to find any mention of Poynting Vectors or Power Flow
Vectors in my copy.

Would you be so kind as to identify the page number(s) describing these
concepts?

Gene,
My next posting admitted my senility. I was quoting Ramo & Whinnery,
not Walter Johnson. In "Fields and Waves in Communication Electronics",
page 325, an equation is given for Pz+, "The Poynting vector for the
positive traveling wave ...". It continues: "Similarly, the Poynting
vector for the negatively traveling wave is always in the negative 'z'
direction except when it is zero."

On page 350 it gives the ratio of the forward Poynting vector to the
rearward Poynting vector as the power reflection coefficient. Sorry
for my faulty memory.
--
73, Cecil http://www.qsl.net/w5dxp


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