[AMRadio] Power Levels

[Bob Bruhns]

Hi Dave,

It's nothing personal, Dave.  Elmer is just tired of this
subject.  He was tired of it when the thread began, and he's
REALLY sick of it now.  Sorry, Elmer.

Modulation does interesting things in AM.  500W input, 375W
output is roughly true of the CW carrier, but then we apply
AM to the carrier.  With 100% symmetrical modulation, this
varies the carrier amplitude from its normal level up to
twice carrier voltage on positive peaks, and down to zero on
negative peaks.

With a resistive load (which we have), doubling the voltage
results in doubling the current.  But when both the voltage
and current are doubled, the power is multiplied by four.
Hence with 100% symmetrical modulation, and likewise with
any AM modulation waveform producing 100% positive
modulation, PEP = 4 * carrier.   .

In fact, the average overall output power level increases to
1.5 times carrier power level with 100% sinusoidal AM.  This
is where the extra power goes that the modulator provides.

It's an interesting subject.  Sometimes it doesn't seem to
make sense, but it really does.

Another fine point: the carrier itself is a sine wave.  When
we modulate it, it is slightly distorted from a sine wave
because it is changing in amplitude.  But these change are
usually very slow compared to the carier frequency, so we
ignore them.  The carrier is essentially a sine wave.  This
is what the FCC is talking about when they say that the peak
RMS power of the RF envelope shall not exceed 1500 watts,
averaged over a carrier cycle.  This was to avoid ambiguity
about the meaning of peak level.  It could have meant the
peak voltage that the carrier sine wave reached, which would
be 1.414 times the RMS; in this 1500W PEP example, the power
at that instant would be 3000 watts.  But that's not what
they meant; they meant the peak RMS level on a one-cycle RF
timescale.

  Bacon, WA3WDR

----- Original Message -----
From: <Wt8r at aol.com>
To: <amradio at mailman.qth.net>; <w5omr at w5omr.shacknet.nu>
Sent: Saturday, June 29, 2002 4:17 PM
Subject: Re: [AMRadio] Power Levels


> In a message dated 6/25/02 5:58:30 PM Eastern Daylight
Time, w5omr writes:
>
> > Subj:Re: [AMRadio] Power Levels
> > Date:6/24/02 8:44:58 PM Eastern Daylight Time
> > From:<A
HREF="mailto:w5omr at w5omr.shacknet.nu">w5omr at w5omr.shacknet.n
u</A>
> > Reply-to:<A
HREF="mailto:amradio at mailman.qth.net">amradio at mailman.qth.ne
t</A>
> > To:<A
HREF="mailto:amradio at mailman.qth.net">amradio at mailman.qth.ne
t</A>
> > Sent from the Internet
> >
> > :     I guarantee you that those running T-368s and
BC-610s that are
> > properly
> > : tuned ARE running illegal.  Then there are those that
goes steps farther.
> > : There are a few that run in the neighborhood of 10KW
output, and some
> > that
> > : are running just a few KW.
> >
> > How do you figure?
> >
> > At 2kVDC on the plate of the 250TH, drawing 250mA of
Current equals (last
> > time I
> > checked Ohm's Law of P=I*E) 500WDC Input power.
> >
> > Efficiency of a Class C amplifier is *typically* 75%,
right?
> >
> > 75% of 500 = 375W of Output Carrier power.
> >
> > 375 * 4 = (drum roll, please?) 1500W PEP Output.
> >
> > 73 = Best Regards,
> > -=Jeff/W5OMR=-
>
> Jeff,
>
>        I'm new to the list and would like to know more
about how you arrive
> at your numbers.
>
>        75% of 500W is 375 Watts output of carrier power
alright, but where
> did the "X4" come from to get the PEP figure?
>
>        I also wonder what the relationship is between RMS
power and PEP
> power?  that is, if I tune my rig up to 1500 W PEP output
and without
> changing anything, switch to CW, what is the reading on my
wattmeter?
>
> 73 de
> Dave in Dayton, WT8R
>
>
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