phase angle convention

[Joerg]

Joerg [email protected] posted to
sci.electronics.design:

Joerg wrote:
[snip]
Shouldn't be. Power folks also consider 360 degrees full circle.
Now when they introduced a 400 degree convention I bailed ;-)
Don't worry. That convention is now gon.

I still remember many EE exams where my brain went like this: I know
it's wrong but I've got to do it anyhow so that I pass. Like
impedance matching of a large transmitter final amp which, if it was
ever done, would result in flying glass and a 5-alarm blaze.

It should not have had that effect in a correctly designed unit.
Quite the opposite in fact.

Final amps of modern transmitters are low-Z (as seen after the matching
network). On really modern ones they are essentially fast switches. If
the fiaal amp had a transformed Z of 50ohm and the antenna had the same
you'd see a whole lot of dissipation. Modern transmitters can get pretty
close to 90%.

Impedance matching is usually only done in installations like a CATV
distribution because they have to assume that the far end is rarely
terminated with the proper 75ohms. Usually done via a series resistor.
Do that in a 100kW AM transmitter ... phsssst ... BANG.

 

[Phil Hobbs]

John Larkin [email protected] posted to
sci.electronics.design:


Unfortunately true, they would rather use tables.

Given the consequences of hooking the phases up wrong in a power plant,
I'm sort of glad they do!

Cheers,

Phil Hobbs

 

[JosephKK]

Joerg [email protected] posted to
sci.electronics.design:

Joerg [email protected] posted to
sci.electronics.design:

Paul Hovnanian P.E. wrote:
Joerg wrote:
[snip]
Shouldn't be. Power folks also consider 360 degrees full circle.
Now when they introduced a 400 degree convention I bailed ;-)
Don't worry. That convention is now gon.

I still remember many EE exams where my brain went like this: I
know it's wrong but I've got to do it anyhow so that I pass. Like
impedance matching of a large transmitter final amp which, if it
was ever done, would result in flying glass and a 5-alarm blaze.

It should not have had that effect in a correctly designed unit.
Quite the opposite in fact.

Final amps of modern transmitters are low-Z (as seen after the
matching network). On really modern ones they are essentially fast
switches. If the fiaal amp had a transformed Z of 50ohm and the
antenna had the same you'd see a whole lot of dissipation. Modern
transmitters can get pretty close to 90%.

Impedance matching is usually only done in installations like a CATV
distribution because they have to assume that the far end is rarely
terminated with the proper 75ohms. Usually done via a series
resistor. Do that in a 100kW AM transmitter ... phsssst ... BANG.

Collector of drain impedances of even small RF amplifiers are rather
small, but they usually use a transformer of some kind to get to
antenna feed line impedances. I still believe in the maximum power
transfer theorem.

 

[Joerg]

Joerg [email protected] posted to
sci.electronics.design:

Joerg [email protected] posted to
sci.electronics.design:

Paul Hovnanian P.E. wrote:
Joerg wrote:
[snip]
Shouldn't be. Power folks also consider 360 degrees full circle.
Now when they introduced a 400 degree convention I bailed ;-)
Don't worry. That convention is now gon.

I still remember many EE exams where my brain went like this: I
know it's wrong but I've got to do it anyhow so that I pass. Like
impedance matching of a large transmitter final amp which, if it
was ever done, would result in flying glass and a 5-alarm blaze.

It should not have had that effect in a correctly designed unit.
Quite the opposite in fact.

Final amps of modern transmitters are low-Z (as seen after the
matching network). On really modern ones they are essentially fast
switches. If the fiaal amp had a transformed Z of 50ohm and the
antenna had the same you'd see a whole lot of dissipation. Modern
transmitters can get pretty close to 90%.

Impedance matching is usually only done in installations like a CATV
distribution because they have to assume that the far end is rarely
terminated with the proper 75ohms. Usually done via a series
resistor. Do that in a 100kW AM transmitter ... phsssst ... BANG.

Collector of drain impedances of even small RF amplifiers are rather
small, but they usually use a transformer of some kind to get to
antenna feed line impedances. I still believe in the maximum power
transfer theorem.

The transformer is used to achieve the desired power output at a given
voltage and use the current capabilities of the device to a healthy
extent and not beyond. You can easily double the output power of a
modern RF amp and then one of three things happen:

a. Nothing but the circuit breaker trips all the time.
b. The federales come because you exceeded the licensed limits.
c. The overtemp warning light comes on, followed by a shut-down or a
loud bang.

 

[Fred Abse]

Final amps of modern transmitters are low-Z (as seen after the matching
network). On really modern ones they are essentially fast switches. If the
fiaal amp had a transformed Z of 50ohm and the antenna had the same you'd
see a whole lot of dissipation. Modern transmitters can get pretty close
to 90%.

Impedance matching is usually only done in installations like a CATV
distribution because they have to assume that the far end is rarely
terminated with the proper 75ohms. Usually done via a series resistor. Do
that in a 100kW AM transmitter ... phsssst ... BANG.

Impedance matching, or the need for it, is probably one of the most widely
misunderstood pieces of EE theory. Remember the thread in here around July
2005 on the subject? I think you said then that your Prof had screwy ideas
about it.

 

[Fred Abse]

The overtemp warning
light comes on, followed by a shut-down or a loud bang.

Usually in the opposite order :-(

 

[Joerg]

Usually in the opposite order :-(

Yes, I was thinking about the tube days. Red -> cherry red -> orange ->
white -> KABLAM! (then the sound of flying glass)

With transistor amps the kablouie often comes first. But at least there
is no glass flying about.

 

[Joerg]

Impedance matching, or the need for it, is probably one of the most widely
misunderstood pieces of EE theory. Remember the thread in here around July
2005 on the subject? I think you said then that your Prof had screwy ideas
about it.

Yep, he sure did. There was actually an assistant professor who still
kept disagreeing with me. Me being the lowly student. Then I asked him
whether he could calculate the final amp stage of a modern AM
transmitter at greater than 85% efficiency. I gave him a datasheet of a
really big one so he would believe me that this was state-of-the-art.
Well, he stared at the stuff, sweat beads showing up on the forehead ...

 

[Hal Murray]

With transistor amps the kablouie often comes first. But at least there
is no glass flying about.

I've seen bits of plastic flying around. Well, I didn't actualy see it,
but we found it after turning the power off. The +12 and -12 supply
wires were swapped. A 1488 had a neat conical hole in the top of the
package. There wasn't any smoke, but there was a loud snap, loud
enough so that neither of us had any question that something bad
had happened.

If I was going to do that on a production line, I think safety
glasses would be a good idea.