
Fig.
1: It may not be necessary to bring the consulting engineer and his
magic bag
of tricks for the biennial recertification. Here, Ben
Dawson of Hatfield & Dawson uses a network analyzer to adjust a
seventower directional array.

In
our business we tend to operate on deadlines. The FCC imposes some of
these, things like weekly log checks, quarterly tower
lighting/control inspections, quarterly issues/programs lists and
annual occupied bandwidth measurements. Our calendars are loaded with
reminders for these recurring items, and we hope we do a good job of
keeping up with them.
Since
the moment method rules went into effect several years ago, engineers
for stations that have taken that option and licensed their stations
pursuant to a moment method proof of performance have had one other
item on their calendars: the biennial sample system recertification.
It’s
been my experience that this particular item tends to fall off the
radar. There is evidently some widespread confusion about what is
required and when. As we settle into the routines of a new year, this
would seem like a good opportunity to revisit the sample system
recertification issue.
Let’s
start with the rule, which is contained in 47 CFR §73.155. Here are
the high points:
• Recertification
is required once within every 24 months.
• Purpose
is to verify the continuing integrity of the sampling system.
• For
basesampled arrays, disconnect and calibrate the sample
transformers or voltage samplers.
–
Must be within the manufacturer’s stated specifications.
• For
basesampled arrays, check the resonant lengths of the
sample lines.v
–
Use the same frequencies as in the full proof
–
Characteristic impedance must be within 2 ohms
–
Electrical length must be within 1 degree
• For
loopsampled arrays, measure the terminated impedances of
the sample lines.
–
Must be within ±2 ohms and ±4 percent resistance and reactance of
the proof values.
• Repeat
reference field strength measurements.
• Place
the results of the recertification measurements in the public file.
From
the earliest days of MoM proofing, there has been confusion as to
what starts the 24month clock on recertification. Does this mean 24
months from the date of the original proof? Is it 24 months from the
date of the actual measurements?

Fig.
2: A good RF impedance bridge such as this one can be used with a
calibrated oscillator to make the sample system recert measurements.

The
FCC has simplified this for us in a policy statement. The 24month
clock starts on the grant date of
the station license. That gives licensees a little more
time on the initial recertification clock, and the reference date is
affixed in a permanent place where you can find it easily.
So
pull out your station license — you’ll know you have the right
one if it has a file number prefix of “BMM” — and note the
grant date, marking your calendar both with the twoyear anniversary
date and with a reminder six or so weeks prior so you can have the
recertification wrapped up and in the file by the anniversary date.
The
idea behind biennial recertification of the sample system is to make
sure the daily measurements the station uses to adjust its
directional antenna parameters are accurate.
It’s
not hard for most of us to imagine a situation where perhaps a little
water might get into a sample line, skewing the antenna monitor
indication a bit. The engineer, unaware of the fault, could attribute
the shift in that one tower’s parameters to normal drift and
“chase” the varied parameters with phasor controls.
That,
of course, would result in a misadjusted pattern and perhaps
interference to a spectrum neighbor.
Recertification
measurements are intended to ensure that this kind of thing does not
happen, at least not over a long period of time. Because in an
MoMproofed array, we are relying solely on antenna monitor
indications for pattern verification, we had better be certain those
indications are correct.
TOOLS
OF THE TRADE
So
what’s required in a set of recertification measurements?
Let’s
say that a consulting engineer came in with his network analyzer,
broadband amplifier, attenuators, directional coupler and magic bag
of tricks to do the original measurements and MoM proof, but here we
are, coming up on a couple of years later. Do we need to get that
consulting engineer and his equipment back out for the
recertification? The answer is probably not.
If
a station engineer has access to a field intensity meter, good
impedance bridge, a calibrated oscillator and detector of some sort
(such as a Potomac SD31 or Delta RG), he or she can perform the
recertification easily.
Why
don’t you really need a network analyzer? Because you’re only
interested in one frequency at a time; it’s not necessary in the
recertification process to “sweep” the lines over a broad
frequency range. Simply make accurate measurements on the original
proof frequencies, which would include for each sample line in a
basesampled system: (1) the resonant frequency for the line; (2) the
+45 degree frequency
for the line; and (3) the –45
degree frequency for the line.
Consider
that the resonant frequency will be at some odd multiple of 90
degrees, i.e. 90, 270, 540, etc. At a frequency that is 45 electrical
degrees either side of resonance, the impedance of an opencircuited
transmission line will be the same as the characteristic Z of the
line. So what we do is measure the Z at plus and minus 45 degrees
from the resonant frequency and take the geometric mean of the values
to get the measured characteristic Z of the line.
For
example, say the line is 90 degrees long at a resonant frequency of
1,000 kHz. The +45
degree freq. would be 135 / 90 x 1000 = 1500 kHz. The –45
deg. freq. would be 45 / 90 x 1000 = 500 kHz.
So
we would measure the Z at 1500 kHz, measure the Z at 500 kHz and take
the geometric mean to get the measured characteristic Z of the line.

Fig.
3: A Jplug, in series with a part of the phasor circuitry carrying
RF current,
is a convenient place to connect voltage or current
samplers for calibration

Make
sense? Just for fun, take a 50 ohm Smith chart and plot it out. Make
a point at the R=0, X=0 point (by definition, this is resonance),
then rotate that point about the Smith chart to 12 o’clock (–45
electrical degrees) and see what the magnitude of the Z is. Yup, 50
ohms!
WHAT
TO EXPECT
With
the far end of the line unterminated, measurements on the resonant
frequency should show a zero reactance or close to it. It’s as
simple as that. If you measure on the established resonant frequency
of the line and find a bunch of reactance one way or the other, the
line is no longer resonant and you will need to investigate to figure
out why.
Once
the resonant frequency has been confirmed, measure the impedance on
the ±45degree frequencies. Find the magnitude of the impedances,
then take the geometric mean to get the characteristic impedance.
To
find the magnitude of the impedance for the ±45degree frequencies:
Then
with those numbers in hand, find the geometric mean:
where
Z_{O }is the characteristic impedance of the line, Z_{1} is
the +45degree Z and Z_{2}
is the –45degree
Z.
The
rules require that the Z_{O} of the lines be within two ohms.
If you find a line with a Z_{O} at variance with those of the
other lines by more than that amount, you will need to investigate to
find out why. In other words, the measured delta of the
characteristic impedance of all the lines must be within two ohms. It
doesn’t really matter how the measured Z_{O} compares to
the rated line characteristic Z_{O}. Typically, higher values
are caused by bad connections, perhaps a splice not making good
connection or an improperly installed connector on the transmitter
building end. Lower values are often caused by water in a connector
or saturating the dielectric.
If
you have a loopsampled array, the process is much simpler. Just
measure the terminated impedance of the sample line at the carrier
frequency (although in some special cases a different frequency is
used — check the original proof!). If each line measures within ±2
ohms and ±4 percent resistance and reactance of the proof values,
you’re golden; if something is off, it is time to investigate.
For
basesampled arrays you will have to remove the sample transformers
or voltage samplers and bring them to the transmitter building. If
they are not already marked, use a permanent marker to note on each
which tower it came from.
You
will need two calibrated lengths of transmission line. Any line of
the same characteristic impedance as the sample devices and antenna
monitor inputs will do — just trim the lengths for the same
resonant frequency, or if they are very short (just a few feet long),
make sure they came off the same reel and are exactly the same
physical length. For longer lines, using the generator and bridge to
resonate them at the same frequency is one easy way to make sure
these two test lines are the exact same length or if one line needs
to be trimmed to match.
Start
with a calibration of the antenna monitor using the manufacturer’s
instructions. Then connect the reference tower’s sample devices to
the reference channel of the monitor using one of the calibrated test
lines. Then in turn connect each of the other sample devices to
another channel using the other calibrated test line.
For
sample transformers, put the two transformers side by side and loop a
piece of insulated wire through them (AWG #10 or #12 THNN or
something along those lines works fine). Connect each end of the wire
across a Jplug that is in series with the phasor input or dummy
load, then operate the transmitter so that it produces a few hundred
watts of power. Note the indications on the antenna monitor for the
reference and channel with the transformer under test. Repeat for
each of the other sample transformers.
For
voltage samplers, do the same thing except connect both samplers
under test to a single RF voltage source with a short piece of wire
or strap (a convenient Jplug or other terminal with significant RF
potential in the phasor). Operate the transmitter at low power and
observe the antenna monitor for the reference tower and channel with
the sampler under test.
Compare
the results for both phase and amplitude with the manufacturer’s
published specifications. If the results are within spec, you’re
done. If not, investigate.
A
common failure mode in sample transformers is a burnt 50ohm
terminating resistor in the transformer, but unless something is
externally obvious (and repairable), the defective sampling device
likely will have to go back to the manufacturer for repair and
calibration.
RADIALS
STILL REQUIRED
Finally,
make the reference field strength measurements. The rule says there
should be three points for each lobe and null radial, so a complex
pattern with lots of nulls and minor lobes can have quite a few
radials, but most have just a few. It’s a lot less work than a
conventional partial proof, which requires at least 8 points per
monitored radial.
The
original MoM proof should contain GPS coordinates for each point.
Most aftermarket automotive GPS units will allow input of
coordinates, and I’ve found that is a good way to locate the points
in an automobile quickly, using a handheld GPS to pinpoint the exact
measurement location once out of the car.
I
start with a map of all the radials and points to see how they can be
most efficiently run (hint: It may not be “out one radial and back
in on the next”). Group points on the same side of obstructions
(freeways, rivers, etc.) and run those together.
Note
the date, time and field for each measurement. There is no
requirement in the rules for what the fields should be, so I suppose
in a sense it doesn’t matter what you measure. Prudence, however,
would call for investigation of any significant departure of the
fields from the proof values. Your own experience should tell you
what’s “normal” and what’s not consistent with environmental
conditions, frozen/thawed soil, snow cover, etc.
Wrap
up the recertification process with a professionallooking writeup.
Document what you did, what test equipment you used and tabulate the
results of the measurements. Use the original proof as a template.
Print the document up and place a copy in the public file and another
at the transmitter site.
And
before you quit, mark your calendar for next time.
Cris
Alexander is the director of engineering at Crawford Broadcasting and
a past recipient of SBE’s Broadcast Engineer of the Year award.
