Henry Kolesnik wrote:

Richard
This is in response to your answer of last night. Before going to bed I got
out the book REFLECTIONS II by Walt Maxwell W2DU. I'm typing verbatim from
page 2-2 and 23-1 for those who don't have the book.page 2-2 "Contrary to
what many believe, it is not true that when a transmitter delivers power
into a line with reflections, a returning wave sees an internal generator
resistance as a dissipative load. Nor is the reflected wave converted to
heat and, while at the same time damaging the final amplifier....the
reflected power is entirely conserved...." from page 23-1 "One of the most
serious misconceptions concerned reflected power reaching the tubes in the
RF amplifier of the transmitter. The prevalent, but erroneous thinking was
that the reflected power enters the amplifier, causing tube overheating and
destruction. However, I dispelled this misconception in the above mentioned
publications, using wave-mechanics treatment, discussed here in greater
detail, by showing that when the pi-network tank is tuned to resonance, a
virtual short circuit to rearward traveling waves is created at the input of
the network. Consequently, instead of the reflected power reaching the
tubes of the amplifier, it is totally re-reflected toward the load by the
virtual short circuit appearing only to waves at the network input".

Please note that when this was written, there were no solid-state transmitters
with fixed 50 ohm outputs. Walt was talking specifically about tube transmitters
with built-in adjustable pi-net antenna tuner circuits, like my old Globe Scout.
The closest present-day transmitters to match the above statements would be
something like my IC-756PRO with a built-in auto-tuner.

I'm guessing it's a virtual short because the pi-network is resonant but
what happens if it is a bit off.

If it is a bit off, reflected voltage and current flow into the finals
and superpose with the source voltage and current. Since the reflected
voltage and current are 180 degrees out of phase, the superposition
usually causes the load on the amp to become reactive. Even if the
voltages are in phase or the currents are in phase, the load on the
amp becomes something other than the designed-for load. The result
can be over-voltage or over-current or both. That's not good.

Also what happens in a transistor final with no pi?

Note: most transistor finals have a fixed pi-network designed to
operate into a fixed 50 ohm load. The results would be the same as
the detuned example above. Such commercial finals are protected
from over-voltage and over-current by decreasing the source power.
--
73, Cecil http://www.qsl.net/w5dxp



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