Don’t Forget Those Recertification Measurements!
     

 
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 seven-tower 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 base-sampled arrays, disconnect and calibrate the sample transformers or voltage samplers.
   – Must be within the manufacturer’s stated specifications.
For base-sampled 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 loop-sampled 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 24-month 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 24-month 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 two-year 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 MoM-proofed 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 SD-31 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 base-sampled 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 open-circuited 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 J-plug, 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 ±45-degree 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 ±45-degree frequencies:


Then with those numbers in hand, find the geometric mean:

 

where ZO is the characteristic impedance of the line, Z1 is the +45-degree Z and Z2 is the –45-degree Z.

The rules require that the ZO of the lines be within two ohms. If you find a line with a ZO 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 ZO compares to the rated line characteristic ZO. 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 loop-sampled 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 base-sampled 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 J-plug 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 J-plug 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 50-ohm 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 professional-looking 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.


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Comment List:

I've not done an AM proof in many years, but imagine the luxury of GPS coordinates for a monitoring point and points along a radial! I remember reading descriptions 50 feet west of the tall oak tree... um, ya in 1948 when the station was built the tall oak tree was there but in 1975, 1985, where's the tall oak tree?
By Hal Kneller on 3/5/2014

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