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

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Larry W4CSC

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