On Fri, 21 Nov 2003 06:57:23 +0100, "Meindert Sprang"
wrote:

"Gary Schafer" wrote in message
.. .
Easiest way to visualize it is with open wire line which has very low
loss. Feeding a non resonant antenna the SWR can be very high on the
line. You can have 50% reflected power on the line. With 100 watts
forward you would then have 50 watts reflected but just about all of
the 100 watts will reach the antenna and be radiated. The only loss
will be the very small loss in the line. (typically a few tenths of a
db loss)

But if all energy is radiated by the antenna, how on earth can you have any
reflected power???? I thought the essential part here is that reflected
energy is the energy that os NOT radiated.

Meindert


Think of it this way: You have the same mismatch to the feed line on
both ends.

If your antenna provides a 70 ohm load for a 50 ohm feed line you will
have reflected power from the antenna. That mismatch of impedance is
reflected back down to the transmitter end of the line. The feed line
at the transmitter NO LONGER LOOKS LIKE A 50 OHM LOAD. If a matching
device is placed between the transmitter and the feed line it must
provide the same match at the transmitter end as the antenna does at
the other end of the line. Called a conjugate match. (you can read
about it in your handbook) It is usually a complex impedance made up
of resistance and capacitance or inductance.

Having the same mismatch on each end of the feed line will provide the
same amount of reflected power at both ends of the line. So any power
that gets reflected from the antenna because of the mismatch there
will get re-reflected back toward the antenna again because of the
similar mismatch to the line at the transmitter end.

Regards
Gary

On Fri, 21 Nov 2003 20:12:32 +0100, "Meindert Sprang"
wrote:

"Gary Schafer" wrote in message
.. .
You need to tune your tuner only to provide the proper impedance match
to your transmitter. A 50 ohm SWR bridge at the transmitter will tell
you when you have achieved that.

Swr on the feed line matters very little.
snip
If you have an antenna and a feed line that has a 3:1 SWR on it at the
transmitter end the transmitter will not perform properly. If you
place a matching device (tuner) between the transmitter and that feed
line and tune it so that the transmitter sees a pure 50 ohms, then the
transmitter is completely happy.

But the feed line still has that same 3:1 SWR on it. 25% reflected
power from the antenna.

You obviously don't understand how this works. The combination of antenna
and feedline presents a certain impedance at the end of the feedline. What
the tuner does is transform the 50 Ohm on the TX side to the impedance on
the feedline. And when this all matches, there is no SWR. Neither on the TX
side of the tuner, nor on the feedline side.

Meindert
pe1grv



Wann bet? :)

Try it some time. Tune the tuner so there is no SWR at the
transmitter. Then place the SWR meter between the tuner and the
transmission line. You will still see the same SWR that you started
with!

There is nothing you can do at the transmitter end of the transmission
line to change the impedance or the SWR on the transmission line. That
can only be done at the antenna end of the line.

Regards
Gary

I suspect that there is a big misunderstanding of TERMS, going on here.
Definitions are being misused on both sides......

Lets get the TERMS Fixed and Understood first.

Transmitter = Rf Source that has an impedance of 50 Ohms

Transmission Line = Coaxial Feedline between Transmitter and Antenna
Tuner.
Antenna Tuner = Device that matches impedance of it's input (50 Ohms)
to that of the antenna that connects to the tuners
output.
Antenna = Effectivly a wire, or pipe, or other conducting structure
used to radiate RF Energy supplied by the Transmitter to
the feedline.

NOW that we have these definitions settled, Please continue the
discussion.

Bruce in alaska
--
add a 2 before @

Humm. I guess I need to contact the ARRL and make arrangements
to return all the Antenna Handbooks and Radio Amateur Handbooks
I've accumulated over the past 30 years.

Doug, k3qt
s/v Callista

"Gary Schafer" wrote in message
...
You need to tune your tuner only to provide the proper impedance match
to your transmitter. A 50 ohm SWR bridge at the transmitter will tell
you when you have achieved that.

Swr on the feed line matters very little. SWR at the fixed 50 ohm
output port of your transmitter matters allot. The networks inside
your transmitter that provide the impedance match between your final
amplifiers and the 50 ohm output port are fixed. They are not
adjustable to match any other load. If you do not provide the proper
50 ohm load to the transmitter then the finals can draw excessive
current and damage them. At the same time you may not have the proper
impedance match to the finals that allow the transmitter to put out
its designed power.

As long as the transmitter with its fixed 50 ohm output sees a 50 ohm
load it is happy. It doesn't matter what is happening in the feed
line.

If you have an antenna and a feed line that has a 3:1 SWR on it at the
transmitter end the transmitter will not perform properly. If you
place a matching device (tuner) between the transmitter and that feed
line and tune it so that the transmitter sees a pure 50 ohms, then the
transmitter is completely happy.

But the feed line still has that same 3:1 SWR on it. 25% reflected
power from the antenna. Place your SWR bridge between the tuner and
the feed line and you will see it. Does that mean that 25% of the
power is wasted? Where does it go? We just adjusted the tuner so that
there was no SWR on the transmitter so it isn't going there.

Put a watt meter at the antenna end of the feed line and guess what?
You will see the full transmitter power there. Providing of course you
don't have a high loss coax line.

If the SWR on a coax line gets too high then there are going to be
more cable losses due to excessive currents in the line. That is why
open wire feed line has much less loss than coax. Because its
impedance is higher the current is less for a given amount of power
and you have less IR loss. RG 8 cable has less loss than RG 58. The RG
8 has bigger conductors so less resistance.

If you use a cable with larger conductors or one with a higher
impedance you will have less loss.

As I mentioned before, with open wire line you can feed a dipole
antenna on many bands and have very high SWR on the fed line and have
negligible loss on the feed line. Look in your hand book on multi
band antennas fed with open wire line.

I keep mentioning open wire line only because it serves as a good
example of systems often used with high SWR on the line with almost no
loss. The same holds true for coax line but there is less written on
it. And coax being a lower impedance will have higher currents and
thus more inherent loss for a given amount of power.

Regards
Gary



On Fri, 21 Nov 2003 09:18:38 -0500, "Doug Dotson"
wrote:

If SWR doesn't matter, then why do I need to tune my antenna
or use an automatic tuner if all the reflectedd power eventually gets
radiated? Why did my finals burn up when I keyed up into a very
poorly mismatched antenna.

Doug, k3qt
s/v Callista

"Larry W4CSC" wrote in message
...
Ok, you go on believing that.........

On Thu, 20 Nov 2003 18:02:21 GMT, Gary Schafer
wrote:

Simply not true?? You need to do some more reading Larry. A good
start would be Walter Maxwell's book "reflections". It is explained
there well. Even the later handbooks touch on the subject.

First, impedance does not "absorb any reflected power".
Reflected power on the antenna line DOES NOT go back into the final
amplifier and get dissipated. That is an old wives tale that is
probably older than all of us.

The reason for "being careful" on a high power transmitter with
reflected power is that the voltages can become very high due to the
high impedance's involved in the tank circuit. Also circulating
currents can become high in the matching components. Thus stressing
the circuit components. But no great amount of reflected power is
absorbed by anything.

Ever look at the color of the plates on a high power transmitter
working into a normal load verses a high SWR load? When tuned for the
same power level in both cases there is no difference in plate color.
If reflected power were being dissipated in the final plates they
would be hotter, indicated by a hotter color.

If you think that the tank coil in your 50 kw transmitter is going to
dissipate 5 kw in heat,, then watch it glow red. But we both know it
doesn't, right?

With solid state amplifiers there is the problem of transistors not
liking to work into complex impedance's. It causes them to draw very
high currents. Nothing to do with absorbing reflected power.

Have you ever used open wire feeders to an antenna? The SWR on the
feed line can be very high. It can be in the order of 15 or 20:1 on
the line depending on the antenna type and frequency being used. But
there is almost no additional loss on the line over the line being
1:1.
What do you think happens to all that reflected power on that feed
line?
Where do you think it gets dissipated? Hint, it all gets radiated.

Regards
Gary


On Thu, 20 Nov 2003 12:38:22 GMT, (Larry W4CSC) wrote:

On Thu, 20 Nov 2003 03:50:41 GMT, Gary Schafer

wrote:

SWR in itself is not necessarily bad. Power reflected back toward
the
transmitter is not lost as a result of the reflection itself. When
that reflected power hits the transmitter it is re-reflected back up
to the antenna.

Simply not true. The source impedance of the output power amplifier
is, ideally 50 ohms to match the cable. This impedance absorbs
reflected power, converting it into heat in its resistive component
which is lost. The output matching network of the transmitter is
tuned to make it look resistive. Almost nothing is reflected, again.

At 150W with a couple of watts reflected, it's a no-brainer.
However,
if you are running a 50KW broadcast transmitter, reflected power
greatly increases the transmitter's output amp heating problems so
they are very careful with it. A 2:1 SWR means we have another 5000
watts of heat to cool off the finals, cooking them. The normally hot
finals simply cook themselves.

So a 3:1 swr with 6.25 watts of reflected power and 25 watts of
forward power, still delivers 25 watts to the antenna to be
radiated.
That is of course when there is no feed line loss.

Too bad this isn't true. If the final amp were purely reactive, it
would be, but then there would be no match between the transmitter
and
feed line to begin with so there'd be no power output if it were
purely reactive.

With feed line loss involved (as there always is) you will get a
false
SWR reading. The more loss your cable has the better your SWR will
look.

Finally something that is true. SWR should be measured at the
antenna
if the line is long and lossy. However, this isn't that important in
a boat with 50' of RG-58 at VHF.



73 de Larry W 4 C S C
h h o a
i a u r
s r t o
k l h l
e e i
y s n
t a
o
n

NNNN


Larry W4CSC

NNNN

I think we all kind of sort of knew this Bruce.

Doug

"Bruce in Alaska" wrote in message
...
I suspect that there is a big misunderstanding of TERMS, going on here.
Definitions are being misused on both sides......

Lets get the TERMS Fixed and Understood first.

Transmitter = Rf Source that has an impedance of 50 Ohms

Transmission Line = Coaxial Feedline between Transmitter and Antenna
Tuner.
Antenna Tuner = Device that matches impedance of it's input (50 Ohms)
to that of the antenna that connects to the tuners
output.
Antenna = Effectivly a wire, or pipe, or other conducting structure
used to radiate RF Energy supplied by the Transmitter to
the feedline.

NOW that we have these definitions settled, Please continue the
discussion.

Bruce in alaska
--
add a 2 before @

One exception, in this case we are talking about an antenna tuner with
its output going to a coax line that then goes to the antenna. The
antenna being something near 50 ohms but far enough from it to produce
a 3:1 SWR on its feed line.

So in this case we have 2 transmission lines. One between the
transmitter and the tuner and one between the tuner and the antenna.

Regards
Gary


On Fri, 21 Nov 2003 21:40:42 GMT, Bruce in Alaska
wrote:

I suspect that there is a big misunderstanding of TERMS, going on here.
Definitions are being misused on both sides......

Lets get the TERMS Fixed and Understood first.

Transmitter = Rf Source that has an impedance of 50 Ohms

Transmission Line = Coaxial Feedline between Transmitter and Antenna
Tuner.
Antenna Tuner = Device that matches impedance of it's input (50 Ohms)
to that of the antenna that connects to the tuners
output.
Antenna = Effectivly a wire, or pipe, or other conducting structure
used to radiate RF Energy supplied by the Transmitter to
the feedline.

NOW that we have these definitions settled, Please continue the
discussion.

Bruce in alaska

"Gary Schafer" wrote in message
...
On Fri, 21 Nov 2003 20:12:32 +0100, "Meindert Sprang"

You obviously don't understand how this works. The combination of antenna
and feedline presents a certain impedance at the end of the feedline.
What
the tuner does is transform the 50 Ohm on the TX side to the impedance on
the feedline. And when this all matches, there is no SWR. Neither on the
TX
side of the tuner, nor on the feedline side.

Meindert
pe1grv



Wann bet? :)

Try it some time. Tune the tuner so there is no SWR at the
transmitter. Then place the SWR meter between the tuner and the
transmission line. You will still see the same SWR that you started
with!

Of course you will. That's because you put the SWR meter on a place where
the impedance is not 50 Ohm anymore. So now you have a mismatch between the
feedline and 50 Ohm directional coupler in the SWR meter.

Meindert

No, you just need to read the ones that you have. :)

See if you can find in any of them where it tells you that reflected
power gets dissipated in the finals of the transmitter. I don't think
you will.

Regards
Gary



On Fri, 21 Nov 2003 17:25:33 -0500, "Doug Dotson"
wrote:

Humm. I guess I need to contact the ARRL and make arrangements
to return all the Antenna Handbooks and Radio Amateur Handbooks
I've accumulated over the past 30 years.

Doug, k3qt
s/v Callista

"Gary Schafer" wrote in message
.. .
You need to tune your tuner only to provide the proper impedance match
to your transmitter. A 50 ohm SWR bridge at the transmitter will tell
you when you have achieved that.

Swr on the feed line matters very little. SWR at the fixed 50 ohm
output port of your transmitter matters allot. The networks inside
your transmitter that provide the impedance match between your final
amplifiers and the 50 ohm output port are fixed. They are not
adjustable to match any other load. If you do not provide the proper
50 ohm load to the transmitter then the finals can draw excessive
current and damage them. At the same time you may not have the proper
impedance match to the finals that allow the transmitter to put out
its designed power.

As long as the transmitter with its fixed 50 ohm output sees a 50 ohm
load it is happy. It doesn't matter what is happening in the feed
line.

If you have an antenna and a feed line that has a 3:1 SWR on it at the
transmitter end the transmitter will not perform properly. If you
place a matching device (tuner) between the transmitter and that feed
line and tune it so that the transmitter sees a pure 50 ohms, then the
transmitter is completely happy.

But the feed line still has that same 3:1 SWR on it. 25% reflected
power from the antenna. Place your SWR bridge between the tuner and
the feed line and you will see it. Does that mean that 25% of the
power is wasted? Where does it go? We just adjusted the tuner so that
there was no SWR on the transmitter so it isn't going there.

Put a watt meter at the antenna end of the feed line and guess what?
You will see the full transmitter power there. Providing of course you
don't have a high loss coax line.

If the SWR on a coax line gets too high then there are going to be
more cable losses due to excessive currents in the line. That is why
open wire feed line has much less loss than coax. Because its
impedance is higher the current is less for a given amount of power
and you have less IR loss. RG 8 cable has less loss than RG 58. The RG
8 has bigger conductors so less resistance.

If you use a cable with larger conductors or one with a higher
impedance you will have less loss.

As I mentioned before, with open wire line you can feed a dipole
antenna on many bands and have very high SWR on the fed line and have
negligible loss on the feed line. Look in your hand book on multi
band antennas fed with open wire line.

I keep mentioning open wire line only because it serves as a good
example of systems often used with high SWR on the line with almost no
loss. The same holds true for coax line but there is less written on
it. And coax being a lower impedance will have higher currents and
thus more inherent loss for a given amount of power.

Regards
Gary



On Fri, 21 Nov 2003 09:18:38 -0500, "Doug Dotson"
wrote:

If SWR doesn't matter, then why do I need to tune my antenna
or use an automatic tuner if all the reflectedd power eventually gets
radiated? Why did my finals burn up when I keyed up into a very
poorly mismatched antenna.

Doug, k3qt
s/v Callista

"Larry W4CSC" wrote in message
...
Ok, you go on believing that.........

On Thu, 20 Nov 2003 18:02:21 GMT, Gary Schafer
wrote:

Simply not true?? You need to do some more reading Larry. A good
start would be Walter Maxwell's book "reflections". It is explained
there well. Even the later handbooks touch on the subject.

First, impedance does not "absorb any reflected power".
Reflected power on the antenna line DOES NOT go back into the final
amplifier and get dissipated. That is an old wives tale that is
probably older than all of us.

The reason for "being careful" on a high power transmitter with
reflected power is that the voltages can become very high due to the
high impedance's involved in the tank circuit. Also circulating
currents can become high in the matching components. Thus stressing
the circuit components. But no great amount of reflected power is
absorbed by anything.

Ever look at the color of the plates on a high power transmitter
working into a normal load verses a high SWR load? When tuned for the
same power level in both cases there is no difference in plate color.
If reflected power were being dissipated in the final plates they
would be hotter, indicated by a hotter color.

If you think that the tank coil in your 50 kw transmitter is going to
dissipate 5 kw in heat,, then watch it glow red. But we both know it
doesn't, right?

With solid state amplifiers there is the problem of transistors not
liking to work into complex impedance's. It causes them to draw very
high currents. Nothing to do with absorbing reflected power.

Have you ever used open wire feeders to an antenna? The SWR on the
feed line can be very high. It can be in the order of 15 or 20:1 on
the line depending on the antenna type and frequency being used. But
there is almost no additional loss on the line over the line being
1:1.
What do you think happens to all that reflected power on that feed
line?
Where do you think it gets dissipated? Hint, it all gets radiated.

Regards
Gary


On Thu, 20 Nov 2003 12:38:22 GMT, (Larry W4CSC) wrote:

On Thu, 20 Nov 2003 03:50:41 GMT, Gary Schafer

wrote:

SWR in itself is not necessarily bad. Power reflected back toward
the
transmitter is not lost as a result of the reflection itself. When
that reflected power hits the transmitter it is re-reflected back up
to the antenna.

Simply not true. The source impedance of the output power amplifier
is, ideally 50 ohms to match the cable. This impedance absorbs
reflected power, converting it into heat in its resistive component
which is lost. The output matching network of the transmitter is
tuned to make it look resistive. Almost nothing is reflected, again.

At 150W with a couple of watts reflected, it's a no-brainer.
However,
if you are running a 50KW broadcast transmitter, reflected power
greatly increases the transmitter's output amp heating problems so
they are very careful with it. A 2:1 SWR means we have another 5000
watts of heat to cool off the finals, cooking them. The normally hot
finals simply cook themselves.

So a 3:1 swr with 6.25 watts of reflected power and 25 watts of
forward power, still delivers 25 watts to the antenna to be
radiated.
That is of course when there is no feed line loss.

Too bad this isn't true. If the final amp were purely reactive, it
would be, but then there would be no match between the transmitter
and
feed line to begin with so there'd be no power output if it were
purely reactive.

With feed line loss involved (as there always is) you will get a
false
SWR reading. The more loss your cable has the better your SWR will
look.

Finally something that is true. SWR should be measured at the
antenna
if the line is long and lossy. However, this isn't that important in
a boat with 50' of RG-58 at VHF.



73 de Larry W 4 C S C
h h o a
i a u r
s r t o
k l h l
e e i
y s n
t a
o
n

NNNN


Larry W4CSC

NNNN

"Gary Schafer" wrote in message
...
No, you just need to read the ones that you have. :)

See if you can find in any of them where it tells you that reflected
power gets dissipated in the finals of the transmitter. I don't think
you will.

On page 10.1 of my VHF-UHF manual by G.R. Jessop it says:
"a certain amount of power is reflected at this mis-termination [the
antenna] and is propagated back down the line again by means of further
waves of voltage and current travelling in the opposite direction, to be
either absorbed or re-reflected at the generator according to whether the
generator terminates or mis-terminated the line."

Meindert

On Sat, 22 Nov 2003 08:11:49 +0100, "Meindert Sprang"
wrote:

"Gary Schafer" wrote in message
.. .
On Fri, 21 Nov 2003 20:12:32 +0100, "Meindert Sprang"

You obviously don't understand how this works. The combination of antenna
and feedline presents a certain impedance at the end of the feedline.
What
the tuner does is transform the 50 Ohm on the TX side to the impedance on
the feedline. And when this all matches, there is no SWR. Neither on the
TX
side of the tuner, nor on the feedline side.

Meindert
pe1grv



Wann bet? :)

Try it some time. Tune the tuner so there is no SWR at the
transmitter. Then place the SWR meter between the tuner and the
transmission line. You will still see the same SWR that you started
with!

Of course you will. That's because you put the SWR meter on a place where
the impedance is not 50 Ohm anymore. So now you have a mismatch between the
feedline and 50 Ohm directional coupler in the SWR meter.

Meindert



Then how do you know that there ISN't any reflected power there when
you tune as you say? What do YOU measure it with??

You are right about the impedance seen at that point being different
because of the reflection but the CHARACTERISTIC IMPEDANCE of the
line has NOT changed and that is the important point.

Here is something for you guys with Bird watt meters to do to prove to
yourself what happens with reflected power. Get out your little
manual that came with your Bird through line meter. If you don't have
one go on line and look. I think Bird has that manual online.

It will tell you how to determine how much power is delivered to the
load (antenna).

"The characteristic impedance of the coax line must be the same as
that of the Bird meter (50 ohms). Subtract the reflected power reading
you get from the forward power reading you get. That is the power
delivered to the load." This holds true for any load as long as your
line is a 50 ohm line.

Still don't believe? Try this. With no tuner in your transmission
line tune your radio to a frequency where you see a high SWR. It
doesn't matter what it is. 80 meters if you have it is a good place to
do this as antenna bandwidth is rather narrow there. You can do this
at any power level that your radio is comfortable with.

Hook up your antenna tuner in the coax line as I have described
before. Place your Watt meter between the transmitter and tuner. Tune
the tuner for minimum reflected power to the radio. Note the forward
power reading on the watt meter. Don't touch the power level on the
radio. Replace the watt meter with any SWR bridge you might have
around. Set the control on the SWR meter for full scale indication.
This will be so you can achieve the same power out for the next step.

Now place the watt meter between the tuner and the antenna. Turn on
the transmitter and be sure that the SWR bridge reads the same as it
did before. This tells you that you still have the same power out of
the transmitter and that the tuner is still matched to the radio.

Now look at the watt meter! If you have a 3:1 SWR on the antenna line
and you had set up the radio for 100 watts out at the radio previously
with the tuner in line, the watt meter at the antenna side will read
125 watts forward with 25 watts reflected!!

Now do as the Bird manual tells you: Subtract the reflected reading
from the forward reading. It should equal 100 watts. The same amount
of power that the transmitter is putting out.

All you doubting Larry's please try this and report back what you
find.

Then tell us why reflected power is not re-reflected back to the
antenna if you can. :)

Regards
Gary