[Lowfer] Matching transmitters to transmission lines

[Johan Bodin]

Bill,

> I think I am going to have to respectfully disagree with your =
statement
> that there is no problem in driving a coax with a 0 ohm square wave.
> I have tried this numerous times and large ringing occurs at the
> transmitting end even though the signal at the receiving end looks =
fairly
> close to a square wave.

Are you sure the load is purely resistive 50 ohms at all frequencies?
Square wave contains a lot of harmonics and a "tuned" load will
probably be mismatched on all frequencies except the fundamental.


> I just did a search in the ARRL handbook and I can find no example of =
a=20
> class D PA driving a coax unless there is a impedance matching network =

> between.

That is probably because the goal is to produce a sine wave. "Voltage
switching" class D amplifiers, such as Lyle's complementary "totem =
pole",
a Decca style H-bridge or even a HCMOS gate all need a series resonant
circuit at the output. The fundamental frequency passes straight through =
the
series circuit while the harmonics will see a very high reactive =
impedance
(depending on Q). There will be practically no harmonic current in the
finals, only voltage.


> In fact in every example I can find relating to transmission line=20
> theory the source impedance is always matched to the
> transmission line with the same impedance.

I disagree. That would limit amplifier efficiency to 50% maximum, =
wouldn't it?

73
Johan SM6LKM