On Jun 13, 5:04*pm, Richard Fry wrote:
On Jun 13, 2:58*pm, walt wrote:



To Richard Fry,

Richard, it appears that you have ignored my post where I reported
data that proves the output resistance of Class B and C RF amps
is non-dissipative.

I know your conclusion, thanks Walt.

The impedance of a perfect transmission line also is non-dissipative,
but that does not change the power dissipated in a termination of that
transmission line equaling its Zo.

Do you agree that the Mendenhall paper I have quoted shows that the
source impedance of an operating Class C vacuum tube PA and its output
tuning/matching network designed for a 50 ohm load is not, by itself,
a functional 50+j0 ohms?

RF

No Richard, I've not made that inference. The point I'm emphasizing
is that because the output resistance of these amps is non-
dissipative, no reflected power from a mismatched load enters the amp,
but is totally re-reflected in the forward direction. I make no
comment on whether the output resistance is 50 +j0, because when all
the available power is being delivered at given grid drive the output
resistance equals the load resistance, which doesn't have to be 50 +
j0. Proof of that is that when the load resistance is either increased
or decreased the output power decreases.

Walt, W2DU

On Jun 13, 4:33*pm, walt wrote:
The point I'm emphasizing
is that because the output resistance of these amps is non-
dissipative, no reflected power from a mismatched load enters the amp,
but is totally re-reflected in the forward direction.

Walt, wouldn't that non-dissipative output resistance also reflect
reverse power on other close frequencies, such as those coupled from a
co-located transmission system within the bandwidth of the tx output
network?

If so, such external signals would never enter the transmitter, and
could not cause r-f intermodulation by mixing with the main signal in
the (non-linear) PA plate circuit of that transmitter to produce a 3rd
order product at 2F1 - F2, and other combinations.

But such r-f intermods are a fact of life, as shown in the Mendenhall
paper, and in the existence of elaborate and expensive r-f filtering
hardware needed to enable several stations to operate from one
location while meeting a radiated r-f intermod spec of -80 dBc -- even
from one antenna!

RF

On Jun 13, 6:09*pm, Richard Fry wrote:
On Jun 13, 4:33*pm, walt wrote:

The point I'm emphasizing
is that because the output resistance of these amps is non-
dissipative, no reflected power from a mismatched load enters the amp,
but is totally re-reflected in the forward direction.

Walt, wouldn't that non-dissipative output resistance also reflect
reverse power on other close frequencies, such as those coupled from a
co-located transmission system within the bandwidth of the tx output
network?

If so, such external signals would never enter the transmitter, and
could not cause r-f intermodulation by mixing with the main signal in
the (non-linear) PA plate circuit of that transmitter to produce a 3rd
order product *at *2F1 *- F2, and other combinations.

But such r-f intermods are a fact of life, as shown in the Mendenhall
paper, and in the existence of elaborate and expensive r-f filtering
hardware needed to enable several stations to operate from one
location while meeting a radiated r-f intermod spec of -80 dBc -- even
from one antenna!

RF

Richard, I'm not qualified to answer your question about intermods, so
I won't even try. However, interference to the amp from radiation from
an antenna driven by another amp will not be phase or frequency
coherent with the tx receiving the interference. Perhaps that's the
reason the interfering signal gets through while the waves reflected
from a mismatch don't? In other words, could the coherent relationship
between the forward and reflected waves result in the total re-
reflection at the non-dissipative output resistance of waves returning
from the mismatched load?

Walt

On Jun 13, 5:25*pm, walt wrote:
However, interference to the amp from radiation from
an antenna driven by another amp will not be phase or frequency
coherent with the tx receiving the interference. Perhaps that's the
reason the interfering signal gets through while the waves reflected
from a mismatch don't?

Measurements made of broadcast transmitters in the engineering test
lab at Harris Broadcast Division show that power reflected from a
mismatched load does indeed enter the PA plate output circuitry, and
change the amount of power dissipation and cooling requirement for the
transmitter.

That is the reason why virtually every modern manufacturer of
broadcast transmitters specifies a maximum value of load VSWR for
which their maximum rated powers are permissible.

For Harris FM transmitters that value is 1.7:1 (any phase angle). As
the load VSWR exceeds that value the transmitter progressively "folds
back" its output power to protect itself from component damage.

RF