input vs output power

Roderick M. Fitz-Randolph w5hvv at AENEAS.NET
Tue Feb 3 11:31:20 EST 1998

>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.  I was absent
from the active ham ranks (kept my license!) for a twenty year period
and when I came back to active participation in 1991, I noticed that
output power ratings had become the prime way to express power.  I
suspect that it is due to the way in which the FCC expresses the
regulations.  At one time, at least, the regulations were expressed
in terms of input power and now it is output power.  It has even changed
to the extent that at one time (when 1 KW was allowed) that an AM
rig could have an input power for the carrier of 1 KW as determined by
the plate voltage times the plate current.  This gave it a peak envelope
power of 4 KW during modulation peaks.  That is why (under the power
output method of measurement and peak output power being limited to
1500 watts) the carrier output power is now regulated on an AM rig
at 375 watts.  I suspect that power output is now considered to the
the manner in which a "level playing field" is maintained.... although
that is sort of a laugh now that some amateurs use "Tubes with handles"
like the 4CX5000J in their finals.....capable of 15,000 watts output
peak envelope power in SSB mode.

Rod, N5HV
w5hvv at

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