[Amps] RMS-PEP & 2x8877

[garyschafer@attbi.com (Gary Schafer)]

You are right! It does get confusing.  There are two factors that come into
play. The average RF power and the average modulation power. You could call it
the average of the average power. :>)

With a single tone sine wave modulation the average power of a SSB transmitter
is the same as the PEP power. Also the same as a CW signal from the same
transmitter.

With two tone modulation of the same transmitter the PEP power is the same but
the average power is 1/2 of the PEP power.
Now remember that PEP is the average power contained in 1 RF cycle (or many RF
cycles) at the crest (or maximum peak or the biggest modulation peak) of the
modulation envelope. There are many more RF cycles than there are modulation
cycles.
With two tone modulation it works out that one audio sine wave is modulating the
other. The two are multiplied together making the average of the two 50%. This
makes the PEP to average ratio 2:1.

Another way to look at it would be with a keyed CW signal that had a 50% duty
cycle. When the transmitter is keyed on it is putting out max power. The average
power (what some call rms power) is the same as the PEP power. Because the
transmitter is only on 50% of the time the average power with respect to the
modulation duration would be 1/2 of full power or PEP. Remember that the
definition of PEP is the average power contained in 1 RF cycle at it's maximum
amplitude (or crest of the modulation wave).

The average power that we look at with voice or two tone modulation has a much
longer time than an RF cycle. In other words 1 modulation cycle can contain many
thousands of RF cycles.

To answer your question of why two tones are used for measuring:  It simulates
voice modulation but at a known fixed ratio. It allows us to observe the
transmitter reaching it's full PEP level with a known average power ratio
(50%).  With an AM transmitter we need only use a single tone to do the same
thing. With SSB, a single tone modulation would only give a steady carrier (CW)
out.

Antennas are sometimes limited by the amount of average power that the traps can
handle before they get hot and melt.  A 1500 watt SSB signal PEP would have an
average power of only around 200 to 300 watts with voice modulation. More with
speech processing.

CW could have higher average power depending on how long you hold the key down.

RTTY is on all the time at a steady power. It would cause the most heating.

73
Gary  K4FMX


on4kj wrote:

>  Adding a bunch to the confusion :
> Why antenna or baloon constructors say ;
> Max power ( as an example )
> ssb/1500 w
> cw/750 w
> rtty/500 w
>
> Just rember something ( 1950 ) like first derivation and the second
> derivation of a sine........
>  Why two-tone is used when measuring .....?
>
> Jos  on4kj
>