[AMRadio] FCC's AM power

Gary Schafer garyschafer at largeriver.net
Tue Sep 27 13:11:19 EDT 2016

The Bird APM-16 is about 3 times the price of the Telepost unit and you
still have to buy slugs at close to $300.00 apiece for the Bird unit.

Measuring power with a directional coupler is common practice. Even the Bird
43 uses a directional coupler and it measures forward current and voltage as
well as reflected current and voltage.
It uses a diode as a peak detector, calibrated in average power. The line
impedance does not have to be exactly 50 ohms in order to have accurate
measurements same as other watt meters using a directional coupler.

Gary K4FMX

> -----Original Message-----
> From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of
> Donald Chester
> Sent: Tuesday, September 27, 2016 11:08 AM
> To: amradio at mailman.qth.net
> Subject: Re: [AMRadio] FCC's AM power
> > Gary Shafer wrote:
> > Actually most watt meters that have a "peak power" position are
> intended
> to
> > 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
> (PEP)".
> > 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
> > 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."
> http://www.birdrf.com/Products/Wattmeters_Line%20Sections/PortableWattme
> ters
> /APM-16_Average-Reading-Power-Meter.aspx
> 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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