[AMRadio] FCC's AM power

Donald Chester k4kyv at charter.net
Tue Sep 27 12:08:27 EDT 2016

> Gary Shafer wrote:

> Actually most watt meters that have a "peak power" position are intended
> show PEP and not peak power. Some are better than others at doing so.
> Although most watt meters do detect peak power but are calibrated in
> average power...

> ...Note that true "peak power" is different than "Peak envelope power
> Peak power is measured from the actual peak voltage of the signal where as
> PEP is measured from the RMS voltage of the signal which gives us AVERAGE
> power (some incorrectly call this RMS power).
> With modulation PEP is the average RF power at the crest or peak of the
> modulation envelope.

Now how many of to-day's licensees, including Extra Class, do you think
would read that and have even the slightest clue?

> Although most watt meters do detect peak power but are calibrated in
> average power. Peak envelope power is just a fast responding average
> power meter.

Very few instruments are capable of reading true average power.  Average
power = RMS voltage × RMS current  (there is NO SUCH THING as RMS power).
Average voltage × average current yields a meaningless figure. So-called
"watt" meters actually read RF voltage or RF current, and, on the assumption
of a defined resistive load (usually 50 ohms), the dial scale is calibrated
for the number of watts that would be working into that load at that voltage
reading.  Diode rectifier meters like the Bird 43 read average voltage.  It
takes more sophisticated active circuitry to measure true RMS voltage.
OTOH, a thermocouple RF ammeter is inherently a true RMS-reading device.
The dial scale of the Bird 43 and similar meters are calibrated to indicate
the average power of a steady unmodulated sinusoidal carrier into the
prescribed resistive load, so they are useful for FM, CW, RTTY and other
steady carrier modes with no amplitude modulation, since the average voltage
of a sine wave is a known function of RMS voltage. (Vavg = 0.9 Vrms).  

The "average" power function of a Bird 43 falters when reading the power of
a SSB transmitter.  The meter reading will always be substantially lower
than the true reading.  With AM, the meter indicates average CARRIER power,
but ignores sideband power. The reason for this is that the AVERAGE
rectified RF voltage of an amplitude modulated carrier is the same as that
of an unmodulated  carrier; the positive peak and negative peak voltages
cancel, leaving the meter to indicate the remainder, the carrier voltage.
For a true average power indication, a square-law instrument (RMS voltage or
RMS current reading) is required, since power is a function of voltage
squared or current squared.  OTOH, the thermocouple RF ammeter is inherently
a square-law instrument that measures the actual heating effect of the
current, which is, by definition, what RMS current is.

An accurately calibrated diode rectifier type meter like the Bird 43 is
useful for indicating AM carrier power, since the pointer does not kick up
with modulation the way a thermocouple ammeter does.  This will also reveal
if carrier shift occurs with modulation. A rectifier type RF ammeter has an
advantage when taking base-current readings at an AM broadcast, since the
technician does not have to wait patiently for a few seconds pause in the
program audio. A Bird 43 would unlikely be useful for this measurement,
since few broadcast tower base impedances are precisely 50 +j0 ohms.

> Here is a wattmeter that is excellent at reading PEP. Yes it will read AM
> accurately.
> http://www.telepostinc.com/lp100.html Price is around $465.00, not much
> different than a Bird and a couple of slugs.

A better choice for that kind of money, according to Bird, the
self-proclaimed "RF Experts", would be the APM-16, Average Reading Power
Meter, which "is designed to keep pace with the ever growing complexity of
digitally-based communication systems. Bird's model 43 and most other
wattmeters available today were designed to measure power of CONSTANT

"Modern wireless communication systems can use a variety of digital
techniques to combine many voice data channels into a complex, composite RF
signal. Measurement of such signals with a conventional wattmeter may yield
unacceptable errors. The APM-16 employs active circuitry to deliver accuracy
of ± 5% for multiple-access technologies such as CDMA, TDMA, FDMA and other
digitally-encoded communication systems.

"Designed especially for RF power measurement in PCS, cellular, ESMR, paging
and similar communication systems, EQUALLY EFFECTIVE for measuring RF power
in CONVENTIONAL ANALOGUE SYSTEMS, uses APM-series plug-in elements to cover
a wide range of frequency and power levels. Simple Thruline® style operation
for instant forward or reflected power readings." 


A true direct-reading RF power meter would have four terminals, one pair in
series with the load and another pair bridged across the load, to sample RF
current and RF voltage.  It would still be accurate only when working into a
purely resistive, non-reactive load, unless it contained built-in circuitry
to correct for power factor.  Such instruments exist for 60~ a.c., but I'm
not sure about RF.

Don k4kyv

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