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

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