Updated June 14,
here to go to our page on maximum power transfer theorem
On this page we'll guide you
through some simple matching networks. This is a huge topic, and
there are multiple solutions to every problem. Matching networks
are what keep microwave engineers employed, so don't give all of
our secrets away or we'll all end up down at the dog track!
For now, here's the content we
have on matching networks:
transformers (separate page)
tricks (separate page)
taper (new for May 2008!)
Matching network overview
Joseph Henry is attributed as
the first engineer or scientist that determined that power is maximized
if the load is "matched" to the generator. In microwave
engineering, this is one of the basic concepts. In our case, we
are more often than not trying to match all manner of loads to a
generator impedance of 50 ohms. Henry is a member of the Microwaves101
Hall of Fame!
A word of advice... if you are
designing a matching network, and don't know what elements to incorporate,
find a similar design and see what the last guy did! That's how
you learn what to do, and what not to do. Soon you'll notice there's
a lot of great material out there, and not much new under the sun!
Matching networks are used to
reduce VSWR between source and load that are not of the same characteristic
impedance (like Klopfenstein's taper
of quarterwave transformers),
or match an arbitrary real/imaginary load to an arbitrary real/imaginary
generator (like an interstage in matching network in an amplifier).
We usually try to keep matching networks as lossless as possible,
it is very easy to cheat and create an impedance match using resistors
like the L-pad attenuator!
The photo at the top of the page
shows a power pole like you might see if you live in either the
Third World, of anywhere in the United States (why we don't bury
this stuff defies logic... but that's a different topic.) On this
particular pole you can see three widgets that are connected by
three wires to the three-phase power. Too small to be transformers,
what the heck could they be?
These are power factor correction
capacitors, a type of matching network used by the local power company.
A good fraction of the load that the power company drives is inductive
(think about all of those motors out there running equipment). Even
though power is delivered at 60 Hz (or 50 Hz in other parts of the
world), an inductive load can have a significant reactance to it.
Capacitors are placed in shunt across the line to compensate for
inductive reactance, three capacitors are usually configured in
a "Wye" network across three-phase transmission lines.
The power factor is a number
that is less than one, it's is the ratio of real power delivered
to the "apparent" power delivered. A power factor of 1
means that a perfect resistive load has been achieved. Power factor
correction is not the same as a true impedance matching, power factor
correction means that the reactive component has been minimized
compared to the real component of the load. Impedance matching implies
that the real parts of generator and load are matched too.
Why doesn't the power company
take care of this problem on the generator side of the power line?
By fixing the power factor at the load, the loss in the transmission
line is minimized.
The capacitors are specified
in "kvar", short for kilovolt-amps (reactive). Maybe someone
could help us out with a conversion between Farads and kvar sometime
that would be appreciated. Also, math to support statement in the
preceding paragraph would save us the trouble...
Here's another photo of a power
pole with capacitors on it. We're not sure, but we think that maybe
in this case the power company has the ability to switch these in
and out depending on the load.
Tons more to come... send us
some content please!