input vs output power

[Joe Buch]

At 08:31 2/3/98 -0800, Roderick M. Fitz-Randolph wrote:
>>Why is it that tube xmtrs are rated by input power, and SS xmtrs by
>>output power? Is it that SS rigs are expected to be operated into
>>a fixed impedance, but no such expectations accompany tube rigs?
>>(Of course, that's not true - many tube rigs terminate with an
>>SO-239 connector, unless it's a Heath!)
>>Jeff KH2PZ (22-year General still asking Novice questions)
>___________________________________________________________________
>Jeff, I believe it is a carry-over from the years when virtually
>all ratings were expressed in terms of input power.

True.  When hams actually built transmitters and experimented with new
transmitter technology, getting higher efficiency from your transmitter was
considered a worthy goal.  Thus, specifying input power limits gave an
incentive to transmitter designers to come up with more efficient
amplifiers.  The higher the efficiency, the higher the power fed to the
antenna. Also, measuring input power is easy.  Any kid could do it with an
ampmeter and a voltmeter.

With the shift to commercially made transmitters for amateur communications,
the need for stimulating efficient designs evaporated.  With the advent of
class AB linear amplifiers, parameters like distortion, harmonic
suppression, and absence of spurious emissions became more important than
efficiency.
Compared to class C amplifiers, these new linear amplifiers were at a
distinct disadvantage in terms of power output for a given input limit. If
one manufacturer's 1 KW transmitter with a class C amplifier put out 800
watts, and the next guy's class AB transmitter only put out 450 watts, they
could both be advertised as 1 KW input.  So it was thought that advertising
output power was a way to equalize things to help sell the newer transmitters.

Now if somebody would only invent an accurate way to measure output power
that cost as little as an ampmeter and a voltmeter.

-73-

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