A previous post called attention to an amplifier being sold that
had a reference to "bird watts."
Yesterday this term arose in another conversation and I asked the
broadcast engineer I was speaking with what it meant.
In the eleven-meter circle, aficionados of QRO do not cheapen
themselves with such expressions as "Peak Power," or "Average
Power," or even "R.M.S." Dummy loads are strictly declasse,
as power to a radiator is what counts.
There are many different RF Power wattmeters, but the Bird 43
with a 5 or 10KW slug connected in-line to the nearby antenna
is the hands-down final say of True Power in that group.
All seriousness aside, "bird watts" neatly shoves all those
tedious calculations into a convenient drawer, as no one
needs to bother with such folderol at 27.3MHz.
However, there is a respectable albeit very narrow use
of the term "bird watts:"
In the cellular world, PCS and TDMA made way for GSM
waveforms as the digital age entered. On cellular sites
there are BTS (Base Transmitting Stations) cabinets
with analog and digital signals. The analog variety
of transmitters require Multiple Channel or Single
Channel Power Amplifiers (M.C.P.A.'s or S.C.P.A.'s)
in rural areas when coverage is scant. Not surprisingly,
these are called "boosters" in the generic language when
RF Engineers talk about them.
RF power measurement in the analog domain is conducted
with Bird 4310 wattmeters. This gives the amplifier output
in decibels so that the boosters may be attenuated to the
proper setting so adjacent Main Receive (Rx1) and
Diversity Transmit/Receive (TRx2) channels do not
become desensitized from nearby radiation.
When GSM boosters began making appearances, especially
on the 1900MHz band, this desensitization became a real
problem. The RF Engineers said to space radiators a certain
distance and to effect physical and electrical downtilts to
greater isolate the booster Tx1 signal from the Receives, but
coverage declined and more calls were being dropped after
booster calibration.
Then Agilent published their famous paper regarding the
phenomenon whereby it was seen that GSM signals, measured
with a wattmeter similar to the Bird 4310, produced an entirely
false reading, as the GSM signal, with 128 phase angles, when
measured with an instrument that could perform the simultaneous
equations and correlate the magnitude and angle of all the
vectors, produced an Average Power reading, that when applied
as an attenuation guide, re-sensitized the Main and Diversity
Receives!
So today, when we go out to cell sites here on the Cingular
project we need to carry both Bird Wattmeters for the
calibration of the Nortel analog (PCS) BTS bays, and
an Agilent (HP) Average GSM Power meter ($5,000.00)
so we can set things right.
Hence, "Bird Watts" versus "HP Watts."
Hal Mandel
W4HBM
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