Alex,
I think the ARF15XX line has 300 V devices.at the kW level involved,
the output impedance is disgustingly close to 50 ohms.
Do you mean to say "delightfully close"? I would see it that way - if
only those transistors had low enough capacitances and/or would work
well enough in linear mode.
Actually when
designing for high VWSR operation, you have to take into
consideration the TOTAL power involved complete with the reflected
power from the antenna? . The amp doesn't care whether it receives
power from the power supply, or from antenna.reflection. It has to
dissipate it, besides generating fresh RF to pump into the antenna.
It turns out that an amplifier able to handle a 5:1 swr and still put
out 1 kW has to be able to output about four kW.? And that goes for
everything: power supply, cooling, antenna matching of whatever
type.....
No, no, no...! You got that wrong. The power dissipated in the
amplifier stage (mostly in the active devices) is equal to the amount
the power supply is putting out, minus the amount that the antenna is
effectively accepting. And not any sum of powers.
A high SWR indeed means that the amplifier components have to handle a
higher _reactive_ power, but _not_ necessarily a higher real power. And
the power supply definitely only needs to supply the power delivered to
the antenna plus the power being dissipated.
If an amp has to put 1kW into a 5:1 SWR load, it needs to put out a real
power of 1kW, no more. It may need to handle more reactive power, but
not necessarily. It depends on the actual impedance offered by that
antenna/feedline combination. Then the design of the amplifier, with all
of its matching, comes into play, to determine how many of the
components need to handle the additional current, voltage, or both.
Usually that will be some relays, measuring circuits, and the tuner or
matching section. In a typical amplifier designed to operate into such
poor loads, there will be some sort of tuning/matching circuit that
takes care of it, and the amplifier stages proper will run just the same
way, regardless of how high the SWR is, and what the load impedance is.
Thinking about a tuner-less amplifier intended for driving poor loads
directly, it would need to be a switchmode amplifier that can handle
certain variations of voltage, current and load phase angles without a
big increase in losses. Plain common motor controllers (called "drives"
in the industry) routinely do this up to a frequency of a few hundred
Hz, and achieve efficiencies close to 100% over a very wide range of
motor impedances. There is no fundamental reason why it couldn't be done
at RF. Only practical reasons, like the unavailability at this time of
suitable active devices...
An amplifier intended for operation into a poor SWR, that needs 5kW of
DC to deliver 1kW into a 5:1 load, would be a mighty bad design!
Manfred
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