On Sun, 5 Sep 2004 07:49:06 -0500, "Richard Fry" wrote:
snip
Walter Maxwell wrote
The last sentence in the paragraph above is incorrect. This shows that
the writer of the quote is in the unbelievably large group that still
believes
incorrectly that half of the tx power would be lost if if it were
conjugately
matched. But we all know that efficiencies greater than 80% is achieved
by Class C amps, and greater than 60% is achieved by Class B amps
when the source impedance of the tx is 50 ohms resistive and the load
impedance is also 50 ohms resistive.
_______________

To Walter Maxwell:

1. You may be interested in reading Mendenhall's complete paper, which I
will email to you. The lab measurements reported in it used two, operating,
high-power FM broadcast transmitters -- and support his statements about
amplifier source impedance and its consequences.

2. I will ask again, if transmitters have a 50 ohm source impedance, what
accounts for the fact that TV ghosts are produced by an antenna system
reflection having a sufficient delay time? Calculations and measured data
show that the energy that produced the ghost originated by re-reflection off
the TV transmitter output stage of far-end reflections in the antenna
system. If the tx source impedance was 50 ohms, it would absorb the far-end
reflection, which would be incapable of producing a ghost image.

Further, if the tx source impedance was 50 ohms, then the RF intermodulation
measured and reported in Mendenhall's paper -- and verified in real-world
installations by the radiated interference those IM products produced --
would not occur.

RF
Thank you Rick, I've received the Mendenhall paper, which I'll review and
comment later. However, before reading it I have one comment. On the condition
that the tx has tubes (and I assume it does) with some sort of LC output
coupling network, then if the source impedance of the tx is 50 ohms it will not
absorb the far-end reflection, because the source impedance of this type of tx
is not absortive. The source impedance of a resonant tank circuit is a
resistance determined by the voltage-current ratio in the tank--high resistance
at the tank input and low resistance at the output. The resistance being
proportional to the load line there is no dissipating resistor involved. The
only dissipative resistance in the system is the cathode-plate resistance, which
is separate from the output resistance. And contrary to what I've skimmed in the
Mendenhall paper, the output circuit of the tx is linear, not non-linear as
Mendenhall says, because the energy storage of the tank isolates the non-linear
input from the linear output. Remember, the tx output is a nearly perfect sine
wave.

I'll not comment further on this point, Rich, until I've reviewed the Mendenhall
paper.