Jerry Stuckle wrote:
On 7/5/2015 5:37 PM, Roger Hayter wrote:
Jerry Stuckle wrote:
On 7/5/2015 3:58 PM, Roger Hayter wrote:
Jerry Stuckle wrote:
On 7/5/2015 9:23 AM, Ian Jackson wrote: In message
,
writes
Wayne wrote:
"Jeff Liebermann" wrote in message
...
On Sat, 04 Jul 2015 19:04:01 -0400, Jerry Stuckle
wrote:
Think of it this way, without the math. On the transmitter side of
the
network, the match is 1:1, with nothing reflected back to the
transmitter.
So you have a signal coming back from the antenna. You have a
perfect matching network, which means nothing is lost in the
network. The feedline is perfect, so there is no loss in it.
The only place for the signal to go is back to the antenna.
Wikipedia says that if the source is matched to the line, any
reflections that come back are absorbed, not reflected back to the
antenna:
https://en.wikipedia.org/wiki/Impedance_matching
"If the source impedance matches the line, reflections
from the load end will be absorbed at the source end.
If the transmission line is not matched at both ends
reflections from the load will be re-reflected at the
source and re-re-reflected at the load end ad infinitum,
losing energy on each transit of the transmission line."
Well, I looked at that section of the writeup.
And, I have no idea what the hell they are talking about.
Looks like a good section for a knowledgeable person to edit.
If the termination matches the line impedance, there is no reflection.
Both the antenna and the source are terminations.
This is a bit difficult to visualze with an RF transmitter, but is
more easily seen with pulses.
Being essentially a simple soul, that's how I sometimes try to work out
what happening.
You'll be better off if you killfile the troll. You'll get a lot less
bad information and your life will become much easier.
The wikipedia entry is correct as written.
In the real world, the output of an amateur transmitter will seldom
be exactly 50 Ohms unless there is an adjustable network of some
sort.
I've always understood that the resistive part of a TX output impedance
was usually less than 50 ohms.
If a transmitter output impedance WAS 50 ohms, I would have thought that
the efficiency of the output stage could never exceed 50% (and aren't
class-C PAs supposed to be around 66.%?). Also, as much power would be
dissipated in the PA stage as in the load.
Fixed output amateur transmitters are a nominal 50 ohms. It can vary,
but that is due to normal variances in components, and the difference
can be ignored in real life.
But output impedance has little to do with efficiency. A Class C
amplifier can run 90%+ efficiency. It's output may be anything, i.e.
high with tubes but low with solid state. But the output impedance can
be converted to 50 ohms or any other reasonable impedance through a
matching network.
A perfect matching network will have no loss, so everything the
transmitter puts out will go through the matching network. Of course,
nothing is perfect, so there will be some loss. But the amount of loss
in a 1:1 match will not be significant.
Also, the amplifier generates the power; in a perfect world, 100% of
that power is transferred to the load. The transmitter doesn't
dissipate 1/2 of the power and the load the other 1/2. It's not like
having two resistors in a circuit where each will dissipate 1/2 of the
power.
BTW - the resistive part of the impedance is not the same as resistance.
For a simple case - take a series circuit of a capacitor, an inductor
and a 50 ohm resistor. At the resonant frequency, the impedance will
be 50 +j0 (50 ohms from the resistor, capacitive and inductive
reactances cancel). But the DC resistance is infinity. Again, a simple
example, but it shows a point.
The resistive part of the impedance is exactly the same as a resistance
as far as the frequency you are using is concerned. And if the
amplifier output impedance *and* the feeder input resistance were *both*
matched to 50 ohms resistive then 50% of the power generated (after
circuit losses due of inefficiency of generation) would be dissipated in
the transmitter. It would, at the working frequency, be *exactly* like
having two equal resistors in the circuit each taking half the generated
power. So the amplifier has a much lower output impedance than 50ohms
and no attempt is made to match it to 50 ohms.
OK, so then please explain how I can have a Class C amplifier with 1KW
DC input and a 50 ohm output, 50 ohm coax and a matching network at the
antenna can show 900 watt (actually about 870 watts due to feedline
loss)? According to your statement, that is impossible. I should not
be able get more than 450W or so at the antenna.
Because it doesn't have a "50 ohm output" it has an output designed for
a 50 ohm load.
No, it has a 50 ohm output - as did the transmitter I designed back in
my EE class days. But if what you say is correct, how is it designed
for 50 ohms? If you claim it as a low output impedance, then it should
work equally well on a 75 ohm antenna, or even a 1,000 ohm antenna, as
long as the feedline matches. I can assure you that is NOT the case.
It is true that if the transmitter is thought of as a fixed voltage
generator in series with, say, a 5 ohm resistor then the maximum power
transfer in theory would occur with a 5 ohm load. But to achieve this
the output power would have to be 100 times higher (ten times the
voltage) and half of it would be dissipated in the PA. Not the best way
to run things, it is better to have a voltage generator chosen to give
the right power with the load being much bigger than its generator
resistance.
So why do all fixed-tuning amateur transmitters have a nominal 50 ohm
output instead of 1 or two ohms? And why do commercial radio stations
spend tens of thousands of dollars ensuring impedance is matched
throughout the system?
See comment above. If you look at the spec. it probably will not
specify the output impedance, just the load impedance.
See comment above. What makes the load impedance special? Why
shouldn't it work with any sufficiently high load impedance - in fact,
the higher, the better?
What makes the load resistance special is that the voltage output of the
transmitter will drive the correct load resistance with just the right
amount of current to provide the design output power without dissipating
too much heat. Too high a load resistance may simply not take enough
power, but also may upset the operating conditions of the PA in
exact-ciruit dependent ways. Too low a load resistance will draw too
much current and overheat the amplifier. The design has absolutely
nothing to do with making the output impedance equal to the load
resistance.
The maximum power transfer at equal impedance theorem only applies if
you started with a *fixed* output voltage generator. We don't; we
start with a load impedance (50 ohm resistive), then we decide what
power output we want, and we choose the voltage to be generated
accordingly. (Thank you for giving me the opportunity to think about
this!)
Actually, it doesn't matter if it's a fixed or a variable output voltage
- maximum power transfer always occurs when there is an impedance match.
Maximum power transfer for a given voltage generator, not maximum power
transfer for a given dissipation available.
You know electronics. Just do two simple circuits on the back of an
envolope, or cigarette pack if available. A voltage generator in
series with a 5 ohm internal resistance and a 50 ohm load. And another
one with 50 ohm internal resistance and a 50ohm load. Make the output
power 100 W, that is an RMS voltage of 70 volts across the load. Now
calculate the voltage of the generator, and the total power produced,
for both cases.
So if what you say is true, I should be able to feed a 300 ohm antenna
through 300 ohm feedline and a 1:1 balun with no ill effects.
You may think you know electronics - but you do not understand
transmission theory. I don't need to draw circuits on a cigarette pack
- all I need to do is hook up my wattmeter to my transmitter to prove
you wrong.
The output impedance of a practical RF power amplifier has exactly zero
to do with transmission line theory. (The *effect* of a practical PA
output impedance on the transmission line is where we came in, but that
only arises *after* we've sorted out the PA output impedance.)
--
Roger Hayter