Jerry Martes wrote:
"David G. Nagel" wrote in message
...
Tim Wescott wrote:
billcalley wrote:

We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

That's assuming you use an antenna tuner. The tuner will transform the
transmitter's output impedance* just as it transforms the line. Were the
transmitter output impedance actually at 50 ohms, on the other side of
the tuner it would have the same VSWR as the line when everything was
tuned up.

Having said that, the VSWR _does_ matter somewhat when using low loss
lines, both because the line loss is low but not zero, and the tuner loss
will tend to go up as you correct for higher and higher VSWR.

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The cherry
red plates are the reflected energy being absorbed. Transistors will just
turn to smoke under the same conditions.

Dave WD9BDZ

Hi david

Wouldnt it be OK to have a high VSWR along the transmission line if the
"tank ckt" can be adjusted to match the load to the transmitter output
impedance? That is, the VSWR along the transmission could concievely be
high, yet, with proper "tank ckt" adjustment that impedance seen by the
output circuit (plate) wouldnt result in a "cherry red plate".
What I am asking is ? is the transmission line VSWR directly related to
"plate reddening"?
I'm more asking than *telling*.

Jerry


Jerry;

The point I was trying to make is that the reflected current is
disapated as heat in the finals if the transmitter isn't matched to the
load.
In a tube radio the tank circuit is the equivilent of an antenna
match/tuner and converts the 2000 or so ohms at the plate to the 50 ohms
of the transmission line and the unknown ohms of the mis matched antenna.


Dave WD9BDZ

On Mar 12, 4:56 am, Richard Clark wrote:
On 11 Mar 2007 20:39:46 -0700, "Bob" wrote:

The active part of the transmitter output isn't 50 ohm.
That would cause half the power to be lost as heat in
the output stage.

Hi Bob,

Well, aside from the initial misunderstanding of how power gets to the
load (much less back, and then to the load again); I will put to you
a question that has NEVER been answered by those who know what the
transmitter output Z ISN'T:
"What Z is it?"

73's
Richard Clark, KB7QHC

As Tim Williams alludes, it depends on the transmitter design.
It will often be complex rarther than resistive. Since the active
device changes impedance during a single cycle of the RF
signal it may not even be adequately described by a single
value in ohms for a paticular frequency if you wish to
analyse the case of forward and reflected power.

Consider a class C or class E output stage with an
output transistor that is low impedance during
most of the positive half of a cycle of signal and mostly
somewhere near open circuit for the negative half
of the cycle. It seems to me that the effect of reflected
power is going to be different depending its phase
relative to the forward power.
I think this also applys to a lesser extent to a class
A PA with a nice hi-Q tank circuit.

As usually whan this topic comes up, It don't feel
like we have arrived at a usefull and convincing model
of what happens, possibly because simple
descriptions don't cover everything.

Bob

On 12 Mar 2007 23:00:59 -0700, "Bob" wrote:

As Tim Williams alludes, it depends on the transmitter design.

Hi Bob,

No quantifiable answer I see. It's not unexpected, everyone who knows
what it isn't has never been able to say what it is. It seems like
the stock answer you give the cop who asks if you know the speed
limit.
"No. But I wasn't speeding!"

The dependency here started with a conventional Ham transmitter, one
so ordinary as to be a commodity. The design is not so exotic as to
elude a very simple value - except for those who know it isn't 50
Ohms.

73's
Richard Clark, KB7QHC

"Cecil Moore" wrote in message
...
Of course, that was a tongue-in-cheek posting.
But if you could design a Thevenin equivalent
source with a 0.1 ohm source impedance, wouldn't
the efficiency calculate out to be pretty high?

Class D rules. (Using MOSFETs, the Thevenin equivalent is quite easy to
spot, too!)

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

"Roy Lewallen" wrote
The problem is that the idea of "reflected energy" turning the plates hot
is so easy to understand, that people aren't willing to abandon it simply
because it isn't true.
_____________

But reflected energy/power does exist.

For an easy example, such reflections are evident in the picture seen on an
analog TV receiver when the match between the transmit antenna and the
transmission connected to it is bad enough.

In analog TV transmit systems with a typical 500+ foot length transmission
line from the tx to the antenna, a 5% reflection from a far-end mismatch can
be quite visible, showing as a "ghost" image that is offset from the main
image as related to the round-trip propagation time of the transmission
line.

RF

Jimmie D wrote:
"Dan Bloomquist" wrote in message
...

billcalley wrote:


We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna.

As pointed out, VSWR does matter. A lot of bouncing means you heat the
transmission line with the power instead of radiating the power. 'Doesn't
matter', really means it can be tolerated if need be.


I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

Here is what you are missing. In the case of the output, (real/resistive
component of the transmitter), seeing the reflected wave, it is _not_
reflecting that power back up the transmission line as you think it is. It
would go back to that real impedance and heat the transmitter. Here is
what is done with a miss match in the real world.

trans-output - match - line - antenna

The 'match' is where the magic happens. All the energy coming down the
line that got reflected from the antenna 'sees' the 'trans-output -
match' as a perfect reflector and gets bounced back[*]. On the other side
of the match is the trans-output. There the trans-output sees a perfect
impedance, (technically, the conjugate of the trans-output), so that all
the power travels through the match toward the antenna.

The magic is that when the match is tuned, both of the above conditions
are satisfied.

*The reflected wave sees a purely reactive reflector not just because of
the network but also because of the output power of the transmitter.
Without transmitter power the impedance as seen from the load will
dramatically change.

Best, Dan.



Saying that SWR doesnt matter is a rather broad statement(like saying never
or always) but I have know of antenna systems having an SWR of 30:1 and his
was normal. The feedline was balanced line made of 1 inch copper. Of course
an SWR lie this on coax could be fatal to coax and equipment. A more common
example of this is the 1/4 wl matching section on a J-pole antenna. It
matches 50 ohms to a few Kohms so an SWR of 60: 1 or so would not be unusal
here.S oas long as the feedline can handle the current and voltage peaks
without much los it doesnt matter much as long as the source impedance is
matched to the impedance at the input to the transmission line.Im sure there
is a practical limit though.

Hi Jimmie,
Keep in mind I'm answering in the context of the op's post. And the
theoretical SWR on a stub is infinite. The point of the stub at the
antenna is to keep the SWR on the transmission line in a reasonable
range, to make a match if you will. To put high SWR on the feedline
instead of matching at the antenna isn't a great idea in my book.

OTOH.
I finally did some sidebanding a couple of months ago. (First time on
HF.) I got my hands on an old swan 500c. After changing the 6je6's and
supply caps, I had to find out what it was like to get on the air. I ran
outside and hung a wire between the lab and the shop. 40-50 feet. Put a
couple of alligator clips on the end of a chunk of rg-58 and into the
window. I started looking for the antenna through the trans-match with
an antenna bridge. The tuning was very sharp, lots of Q. I don't know if
I could have found it without the bridge I was willing to tolerate
the miss match to get on the air.

Well, it worked out. I made some great QSLs across the mid west and into
northern CA. I live in Vernon AZ. I'm pleased this turned out to be as
great a radio location as I thought. It shouldn't be long before I get a
beam on a tower. By then I'll look to match at the antenna and keep the
SWR off the feed line as much as possible.

Best, Dan.

Roy Lewallen wrote:

Yes! All that matters to the transmitter is the impedance it sees. It
doesn't know or care that you've mathematically separated the delivered
power into "forward" and "reverse" components. It doesn't know or care
what the SWR is on the transmission line connected to it, or even if a
transmission line is connected at all.

Well, without a line, you don't have a real component to tune into.
Drawing arcs on a smith chart from an open line with capacitors and
coils will only get you to another purely reactive point.

Best, Dan.

In rec.radio.amateur.antenna Roy Lewallen wrote:
David G. Nagel wrote:

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The
cherry red plates are the reflected energy being absorbed. Transistors
will just turn to smoke under the same conditions.

Unfortunately, you'd be learning the wrong lesson.

The cherry color is due to the transmitter being loaded with an
impedance it's not designed for, causing the final to run at low
efficiency. You can disconnect the antenna and replace it with a lumped
RC or RL impedance of the same value and get exactly the same result.
Alternatively, you can attach any combination of load and transmission
line which give the same impedance, resulting in a wide variation of
"reflected energy", and get exactly the same result. All that counts is
the impedance seen by the transmitter, not the VSWR on the line or the
"reflected power".

The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.

See http://eznec.com/misc/Food_for_thought.pdf for more.

Roy Lewallen, W7EL

The fact that any transmission line and antenna combination can be
replaced with an RLC lumped load at the transmitter output and the
transmitter can't tell the difference is something that a lot of
hams seem to have a problem understanding.

What I've never understood is why so many hams have a problem with
the concept of equivalent circuits only when antennas and transmission
lines are involved.


--
Jim Pennino

Remove .spam.sux to reply.

"Dan Bloomquist" wrote in message
...
Jimmie D wrote:
"Dan Bloomquist" wrote in message
...



Hi Jimmie,
Keep in mind I'm answering in the context of the op's post. And the
theoretical SWR on a stub is infinite. The point of the stub at the
antenna is to keep the SWR on the transmission line in a reasonable range,
to make a match if you will. To put high SWR on the feedline instead of
matching at the antenna isn't a great idea in my book.

Sure yoiu can, that stub is a transmission line. It would matter if it s a
1/4 wl long or 21 1/4 wl long. If it is designed to handle the current and
voltage peaks it can transmit power with low loss when a high VSWR is
present. Its just that most people dont make there feedlines out of inch
copper tubing. Even with 450 ohm ladder line 10:1 VSWR is very acceptable.

Richard Fry wrote:
"Roy Lewallen" wrote
The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.
_____________

But reflected energy/power does exist.

For an easy example, such reflections are evident in the picture seen on
an analog TV receiver when the match between the transmit antenna and
the transmission connected to it is bad enough.

In analog TV transmit systems with a typical 500+ foot length
transmission line from the tx to the antenna, a 5% reflection from a
far-end mismatch can be quite visible, showing as a "ghost" image that
is offset from the main image as related to the round-trip propagation
time of the transmission line.

RF

Richard,

You are undoubtedly correct, but you have also demonstrated what is
really the lifeblood of many arguments in RRAA. You have introduced both
transient behavior and multi-frequency behavior.

Clearly these are important in the real world. However, the vast
majority of models and calculations used as support for RRAA postings
are steady-state and monochromatic. Anyone who stayed awake through
calculus and differential equations might recall that the equations for
steady-state and transient behavior are often quite different.

No one denies the existence of reflections. Some people get confused by
the mathematics of power and voltage. But a big argument is about the
round-trip travel of energy in the steady-state. Some people seem to
believe that energy continues to flow back and forth from one end of a
(mismatched) transmission line to the other under steady-state
conditions, even simultaneously traveling in both directions. (Passing
like ships in the night?) Since energy is a scalar quantity, and any
given joule is not distinguishable from another, it is not clear how the
proponents keep track of the bookkeeping, but they muddle through somehow.

73,
Gene
W4SZ