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  #31   Report Post  
Doug Dotson
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

I guess you have the proverbial Iron Constitution

"Leanne" wrote in message
...
I've never seen a boat with metal plumbing.


Doug, I was referring to my internal plumbing and not the boat...

Leanne
S/V Fundy










  #32   Report Post  
Jim Woodward
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

"Doug Dotson" wrote in message
...

snip

I just have to ask what the thinly vailed insult has to do with anything?

I think
Larry's explanation of SWR was quite valuable and others who chimed in
had valuable contributions as well.


snip
Doug


Doug:

If the comment above is aimed at me, we have a misunderstanding and I
apologize. To clarify the comment I made (see below):
1) Honest thanks to Larry and the rest of the posters in this thread. I
understand how much time it takes to write good posts such as Larry's
original explanation here (I sometmes wonder why any of us do it). While I'm
often a giver-of-advice in other newsgroups, on this one I'm usually a
taker; and when that happens I say "thank you".
2) Often there is little solid information in the marine world (or why would
we all be spending time here trading information)
3) You guys don't always agree -- that's OK, on some subjects I disagree
with myself.
4) We usually get a spectrum of views -- "different ships, different long
splices" -- with a spectrum we all get to decide which long splice we'll use
on our ship.
5) Asking antenna manufacturers or dealers, for example, what antenna to
use, is not always the best way to choose. Mostly, the people here don't
benefit directly from their advice. That's good. When they do benefit, it's
usually disclosed, which is also good.

I try very hard never to insult -- correct, yes; disagree with, often -- but
insult, never. If, however, I do, it won't be thinly veiled -- I'm very
direct, often to a fault.

Again, sorry for any misunderstanding.


--
Jim Woodward
www.mvFintry.com


..




The only reason you need the power is to overcome noise
and the damned marinas docking boats from a 70' tower with a 9 dB
antenna running 25 watts to get to the end of the dock. Why the FCC
doesn't restrict marinas to 1W and 10' AGL has always been a mystery
to me. They're NOT part of any rescue party, manned by teenage girls.


Actually, don't the rules say that we all MUST use 1W when possible?

Thus
the marinas are actually in violation if they're using 25 watts. Of

course,
the FCC has better things to do than enforce its own rules.

And thanks to all for comments in this thread. So many decisions, so

little
solid information available on subjects like this. While you guys don't
always agree, at least I get a spectrum of views from people who mostly
don't have a horse in the race.


--
Jim Woodward
www.mvFintry.com


.






  #33   Report Post  
Doug Dotson
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

I agree with all of your points. It just didn't seem to be the
case of this thread. Nuff sed!

Doug

"Jim Woodward" jameslwoodward at attbi dot com wrote in message
...
"Doug Dotson" wrote in message
...

snip

I just have to ask what the thinly vailed insult has to do with

anything?
I think
Larry's explanation of SWR was quite valuable and others who chimed in
had valuable contributions as well.


snip
Doug


Doug:

If the comment above is aimed at me, we have a misunderstanding and I
apologize. To clarify the comment I made (see below):
1) Honest thanks to Larry and the rest of the posters in this thread. I
understand how much time it takes to write good posts such as Larry's
original explanation here (I sometmes wonder why any of us do it). While

I'm
often a giver-of-advice in other newsgroups, on this one I'm usually a
taker; and when that happens I say "thank you".
2) Often there is little solid information in the marine world (or why

would
we all be spending time here trading information)
3) You guys don't always agree -- that's OK, on some subjects I disagree
with myself.
4) We usually get a spectrum of views -- "different ships, different long
splices" -- with a spectrum we all get to decide which long splice we'll

use
on our ship.
5) Asking antenna manufacturers or dealers, for example, what antenna to
use, is not always the best way to choose. Mostly, the people here don't
benefit directly from their advice. That's good. When they do benefit,

it's
usually disclosed, which is also good.

I try very hard never to insult -- correct, yes; disagree with, often --

but
insult, never. If, however, I do, it won't be thinly veiled -- I'm very
direct, often to a fault.

Again, sorry for any misunderstanding.


--
Jim Woodward
www.mvFintry.com


.




The only reason you need the power is to overcome noise
and the damned marinas docking boats from a 70' tower with a 9 dB
antenna running 25 watts to get to the end of the dock. Why the FCC
doesn't restrict marinas to 1W and 10' AGL has always been a mystery
to me. They're NOT part of any rescue party, manned by teenage

girls.

Actually, don't the rules say that we all MUST use 1W when possible?

Thus
the marinas are actually in violation if they're using 25 watts. Of

course,
the FCC has better things to do than enforce its own rules.

And thanks to all for comments in this thread. So many decisions, so

little
solid information available on subjects like this. While you guys

don't
always agree, at least I get a spectrum of views from people who

mostly
don't have a horse in the race.


--
Jim Woodward
www.mvFintry.com


.








  #34   Report Post  
Gary Schafer
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

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.

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.


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.
This is because not only is there less power to reach the antenna that
is causing the reflection, but also there is less of the reflected
power that gets back to your SWR meter. It gets lost in the coax both
ways.

So if you have 3 db of line loss and your antenna has a 3:1 SWR, you
will only read it as about 1.4:1.

WHY:
With 3 db of line loss only 12.5 watts will make it to the antenna.
With a 3:1 SWR at that point 25% of the 12.5 watts (or 3.125 watts)
will be reflected back towards the SWR bridge. But the 3.125 watts
coming back down the cable will also undergo the 3 db of cable loss so
only .78 watts will show up as reflected power back at the SWR bridge.
That gets compared with the full 25 watts that the input of the SWR
bridge sees. The bridge will tell you that you have an SWR of only
about 1.4:1 when it is really 3:1!

Further, most SWR bridges are quite inaccurate at low readings. In the
area of 10 to 40%.

Even the revered Bird watt meter is not very accurate when reading on
the low end of the scale. It has a published accuracy of +- 5% of FULL
scale. Measuring a 25 watt radio, a 50 watt slug is normally used. 5%
of 50 watts is +-2.5 watts. Trying to read that 1.4:1 reflected power
level of .78 watts with a meter that has an error of +- greater than 3
times the level that you are trying to read leaves you guessing at
best!

Users of watt meters measuring reflected power often make the mistake
of seeing a few watts of reflected power and thinking things are "ok"
and not really calculating what they have, as the "reflected part is
rather low compared to the forward power". They equate it to an SWR
meter position comparison.

Bottom line is when using a watt meter on VHF to look at reflected
power, if the reflected indication is more than a division or two on
the meter you probably have to high an SWR.

With a short antenna cable, 20 or so feet, 1.5:1 SWR is about the
limit. With a longer cable, depending on its loss, The acceptable
limit may be much lower as seen above.

Some transmitters start shutting down their output at around 1.8:1
SWR.
So the problem with high SWR is not so much one of added loss as it is
a problem of the mismatch that the transmitter sees and reduces its
output power. Although with a high loss feed line you end up with less
of the reflected power to be re-reflected back up to the antenna.

Regards
Gary










  #35   Report Post  
Larry W4CSC
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

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


  #36   Report Post  
Gary Schafer
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

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


  #37   Report Post  
Meindert Sprang
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

"Gary Schafer" wrote in message
news
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.


And where it is reflected back into the cable again. And this continues
until the power is completely dissipated in by the losses in the path.

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.


No. Power that is reflected the first time, will be reflected the secon time
it 'hits' the antenna. It wil NEVER be radiated.

Meindert


  #38   Report Post  
Doug Dotson
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

Wouldn't phase be a problem? Hard to believe that a signal can
be reflected back from the antenna and then reflected back from the
transmitter to the antenna and will be in phase well enough to actually
do any good.

Doug, k3qt
s/v Callista

"Gary Schafer" wrote in message
news
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.

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.


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.
This is because not only is there less power to reach the antenna that
is causing the reflection, but also there is less of the reflected
power that gets back to your SWR meter. It gets lost in the coax both
ways.

So if you have 3 db of line loss and your antenna has a 3:1 SWR, you
will only read it as about 1.4:1.

WHY:
With 3 db of line loss only 12.5 watts will make it to the antenna.
With a 3:1 SWR at that point 25% of the 12.5 watts (or 3.125 watts)
will be reflected back towards the SWR bridge. But the 3.125 watts
coming back down the cable will also undergo the 3 db of cable loss so
only .78 watts will show up as reflected power back at the SWR bridge.
That gets compared with the full 25 watts that the input of the SWR
bridge sees. The bridge will tell you that you have an SWR of only
about 1.4:1 when it is really 3:1!

Further, most SWR bridges are quite inaccurate at low readings. In the
area of 10 to 40%.

Even the revered Bird watt meter is not very accurate when reading on
the low end of the scale. It has a published accuracy of +- 5% of FULL
scale. Measuring a 25 watt radio, a 50 watt slug is normally used. 5%
of 50 watts is +-2.5 watts. Trying to read that 1.4:1 reflected power
level of .78 watts with a meter that has an error of +- greater than 3
times the level that you are trying to read leaves you guessing at
best!

Users of watt meters measuring reflected power often make the mistake
of seeing a few watts of reflected power and thinking things are "ok"
and not really calculating what they have, as the "reflected part is
rather low compared to the forward power". They equate it to an SWR
meter position comparison.

Bottom line is when using a watt meter on VHF to look at reflected
power, if the reflected indication is more than a division or two on
the meter you probably have to high an SWR.

With a short antenna cable, 20 or so feet, 1.5:1 SWR is about the
limit. With a longer cable, depending on its loss, The acceptable
limit may be much lower as seen above.

Some transmitters start shutting down their output at around 1.8:1
SWR.
So the problem with high SWR is not so much one of added loss as it is
a problem of the mismatch that the transmitter sees and reduces its
output power. Although with a high loss feed line you end up with less
of the reflected power to be re-reflected back up to the antenna.

Regards
Gary












  #39   Report Post  
Ron Thornton
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

I started out in hf transmitters, 250kw ones. I don't remember SWR
being anything but a subject in theory for us. In practice we tuned
for max radiated power which was as good as it got.

Ron

  #40   Report Post  
Gary Schafer
 
Posts: n/a
Default How to use a simple SWR meter and what it means to your VHF

Yes a small portion of the re-reflected power that gets back to the
antenna is again reflected back toward the transmitter.

There are two ways to look at it. One is the steady state and the
other the dynamic. Once the line is charged with energy, if it a loss
less line, that energy that charged the line will bounce back and
forth continuously. The energy that is put into the line after that
will all reach the antenna and be radiated.

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)

If all the reflected power were to continually bounce back and forth
on the line you would only have half the power reaching the antenna to
be radiated. That doesn't happen except for the first instant that
power is applied and the line is charged with energy.

Regards
Gary



On Thu, 20 Nov 2003 23:47:27 +0100, "Meindert Sprang"
wrote:

"Gary Schafer" wrote in message
news
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.


And where it is reflected back into the cable again. And this continues
until the power is completely dissipated in by the losses in the path.

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.


No. Power that is reflected the first time, will be reflected the secon time
it 'hits' the antenna. It wil NEVER be radiated.

Meindert


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