[Amps] Measuring RF Power

[Harold B. Mandel]

In the cellular telephone RF world measuring power
has become a critical function because the difference of
1 decibel can affect the operation of a 'cell site with
three sectors, eight channels per radio, three radios
per sector, & etc.

With the upgrading of the cellular system towards 3G,
in the phase called "EDGE," and in the gyration known as
Multi Carrier Power Amplifier (MCPA) upgrades, the
RF engineer needs to know the effected radiated power
along with the mechanical and electrical downtilt, the
beam spread and the azimuth beam center, because an
adjacent transmit antenna cannot overly interfere with
its receive neighbor or desensitization screws up the
revenue to the carrier as calls are missed and perhaps 
grabbed elsewhere.

So the installing contractors are given adjustment parameters
to set power output at the amplifier bays. These amps are
oftentime combined with diplexers and duplexers so that
feedlines share power and multi-frequency antenna get
split energies at the top of the tower.

Before EDGE, like in 2G, we used BIRD 43's primarily,
and relied upon the slugs to put us in a ballpark, e.g., 
10 watts, 45 watts, 150 watts.  With the MCPA upgrade,
and the power level increasing we needed to determine
what was accepted as a power measurement. PEP is different
than RMS, and RMS is different than Average. When you're
talking about digital cellular signals, the phase angle of 
pseudo-random waveforms from 12 channels has to be taken
into account because the quanta is different with revenue traffic
than with let's say all zeroes.

So the BIRD 43's were scrapped throughout the worldwide project
and either HP or Agilent power meters are now accepted. The meters
need to have a NIST calibration sticker that is current, and a set of
attenuators that also carry NIST stickers. The meters need to prove
their calibration against field standard loads. All this so that everyone
agrees on the difference between 41 and 42 dB.

The point of this letter is to say that there is no end to splitting the 
calibration hair with electronic test gear. As the last gentleman pointed
out, a statistical profile of a carton of BIRD elements wouldn't have
that much deviation. If you were looking at them from the point of
view that cellular radios require, you would see a complete mess. If you
were looking at them from the point of restored tube-type WWII mil
surplus you would see gemstones.

If you require a particular level of calibration then it might be wise to
rent a calibrated instrument to perform your measurement and to compare
its
results with the instrumentation on hand. 

In the 'phone world, it seems that no one owns anything anymore. The
technology
changes so fast that obsolete test equipment piles up faster than the
revenue
that pays for it.

Respectfully,

Hal Mandel
W4HBM