Field Intensity Measurement Methodology

Gather Your FI Meter, GPS Receiver, Topos And Foul-Weather Gear Into the 4x4, and Read On
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Gather Your FI Meter, GPS Receiver, Topos And Foul-Weather Gear Into the 4x4, and Read On

Gather Your FI Meter, GPS Receiver, Topos And Foul-Weather Gear Into the 4x4, and Read On

Some time ago an engineer friend here in Connecticut mentioned to me that he had put a new AM transmitter online. Jokingly I told him that I would do a field intensity measurement to note the improvement and mark the occasion. Grabbing the meter, I obtained a number and e-mailed it to him in about a minute.

Did this 1.5 mV/m number mean anything? Not really. It told us only that, at my desk, inside the office, there was 1.5 mV of radiation from this station available at the terminals of a 1-meter loop antenna.
TipsMore tips from Buc Fitch for your FIM excursion:

* Wear Polaroid sunglasses when it is bright or glary.

* Drink plenty of water.

* Allot lots of time.

* Take the time to be safe (e.g., don't park the truck on a blind curve so you shorten your walking distance).

* In rattlesnake country, stomp your feet so they hear you coming.

* Relax, enjoy, make new friends, explore the beautiful or at least interesting countryside around you.

* Don't be perplexed by the numbers you take, or you will be tempted to analyze and fudge them. Review the data in the calm and quiet of an office or hotel room in the relaxing evening hours. Anomalies usually make sense if you have taken time to understand the system, topography and conductivities.

Repeat the above about 400 times at 400 locations, and you have the makings of a great day.
FIMs can, however, tell us a great deal. Taken properly, they are valuable to the integrity of our broadcast system and your station.

Therein lies the rub. Most are not taken properly.


To be valid, accurate, repeatable and useful in the grand scheme of things, FIMs for any service generally must be:

  • Taken on a calibrated and accurate instrument;
  • Taken by someone who has knowledge of the methods and physics, and can apply proper conduct and judgment to the proceeding;
  • Done methodically so that factors such as weather, time of day, time of year, ground cover and vegetation can berationalizeduniformly;
  • Taken accurately by time, location and physical spacing;
  • Well-annotated, concise and clear in data and format;
  • Repeatable for anyone who follows.

If you can't honestly say that you have met the above criteria, why waste the time taking measurements?

An MP on your side

Let's get down to AM cases.

Most FIMs are taken for one of four reasons: to check monitoring points (MPs) on a directional antenna (DA) system; to proof out a new station or antenna pattern; to arbitrate interference; or to determine ground conductivity for the purpose of an allocation study.

Although FCC rules specify the methodology for taking these measurements and hence the qualities needed in the measurement device, we know that there is really only one AM FIM available these days. That unit, by far the most widely used, is the Potomac Instruments FIM-41, which has been in near-continuous manufacture for more than 50 years, along with its broadcast band-only predecessor, the FIM-21. The industry is lucky to have such an accurate and durable device

Monitoring points for directionals usually are locations specified by the license that require periodic monitoring to confirm that the directional pattern is formed properly. Ordinarily the engineer tuning up and proofing a DA selects practical and definitive measurement locations identified during the tune-up and measurement of each pattern. These are submitted to the FCC for approval and made part of the license.

Logically and traditionally, these have been located such that, when values are correct, they demonstrate proper pattern operation, protection towards affected stations and rated antenna system efficiency.

On older directional licenses, there usually is an MP for each maxima, each protection null and any critical azimuth towards a protected station. On new and more recent DA licenses, for the most part, the FCC has only been requiring MPs in the pattern minima.

In the case of a "zero null" antenna system where the radiation in the direction of a protected station is required to come down to a very low level, MPs on the slope of each side of the null are chosen instead of the null point azimuth. This is done so that you are measuring your station instead of the incoming signal from the protected station.

A handful of simple arrays, especially for daytimers, are dinosaurs and can be way out of adjustment yet still meet specification for the small count of MPs dictated by the license.

Most directionals, however, are critical in at least one direction, and many have super-critical tolerances in several azimuths. For these, accurate MP measurements are a must; even a few feet of variance from the specified point can take you over the dreaded Maximum Expected Operating Value (MEOV).

My humble suggestion to my DA clients is to have custom surveyor stakes made up in either hard wood or concrete and install them exactly where the measurement point is located. The stake should have the station call and the MP number marked indelibly on it.

If you cannot do this, be creative and find some way to make sure the same spot is used all the time, every time. One MP of a 50 kW DA had an MP on a golf course using the 17th green as the point. We got the groundskeeper to reposition the hole at the exact MP.

Once you get to your measurement point, first and foremost, look up! If there are overhead wires nearby, they will affect the field strength measurement adversely. Find a point clear of overhead wires. Always check overhead before each measurement.

The G2 on the PI FIM

Next, open the lid and turn on the meter. It will be a Potomac Instruments FIM, so we'll be specific.

Start by always checking the batteries using the first detent on the FUNCTION switch. If you are going to do a long series of measurements, install fresh batteries when beginning.

Calibrate the meter every time you take a measurement. Begin by turning the FUNCTION switch to FI (CAL TUNE) and setting FULL SCALE (the signal level range) switch to an appropriate level for the anticipated signal strength that you expect to obtain. Trust me, you will not wear these gold-contact switches out.

Carefully tune in the station that you will measure using first the MHz switch to select the proper band, then the RCVR (tuning) control. Rotate the FIM as necessary or position it horizontally to ensure that the indicated signal strength from the station being measured is less than 10 mV. If the signal strength is higher, the FIM will not calibrate properly.

Turn the FULL SCALE (range) switch to CAL and "zero beat" the reference oscillator either aurally via the speaker or by peaking the meter with the CAL OSC knob. Turn the FUNCTION switch to CAL NULL (reference adjust) and null the signal from the reference oscillator with the GAIN knob, which essentially sets the calibrated gain of the unit.

Now return the FUNCTION switch to FI and the FULL SCALE switch to the appropriate range for your measurements.

Lift the FIM to just above your head with the meter face perpendicular to the ground. Rotate your body through 360 degrees several times until you have identified the peak FI on the meter (adjust FULL SCALE range if needed to keep the needle on scale). Note the approximate maximum reading, then rotate the meter horizontally for a null and note its value.

If the maximum-to-minimum ratio does not exceed 10:1, some local phenomenon such as re-radiation may be affecting the measurement. If this occurs at a licensed MP, it may be necessary to choose and license a new point.

Again rotate the meter for maximum and determine as best you can if the plane of the meter is oriented toward the station. If it is not, a local phenomenon may be affecting the measurement.

Specific data

Once you've determined that the measurement is uncontaminated (and once you are no longer dizzy), step carefully over the MP, maintaining the same address towards the signal source, and note the value accurately in your daybook.

(A 1986 letter to the FCC on the subject of FI meter quadrature to the source, and the response from the FCC, are online. Go to and click on Mass Media. Under Databases, select documents_collection and download the PDF 960110.)

In the case of contentious measurements, have witnesses. Let them see you go through the exercise and the numbers displayed. Have them record the field strength independently.

Each point measured should be referenced to the name of the measurement taker, date, time, weather and relevant factors, such as snow cover, cranes working nearby or changes in surroundings such as new houses, etc.

Unlike MPs, which are fixed in your license until you turn it in or apply to have it changed, surveying the signal levels on a new antenna system is in your hands except for the non-directional (ND) portion of the submitted measurements.

This ND survey is a series of close-in measurements on evenly spaced radials to determine just how non-directional the individual typical tower in your antenna system is and the efficiency of the non-directional radiator. (In the next issue of Radio World, Cris Alexander will write on this topic; also see Jack Layton's book "Directional Antennas Made Simple.")

Similarly, you must measure the directional as well. These measurements are taken in the form of a series of radials. While it would be nice to do 120 evenly spaced radials, like your ground wires, none of us is getting younger. These radials are long and have many points; our time to make measurements is limited. We'll select just a few meaningful azimuths.

The choice of radials sometimes is driven by outside factors such as the presence of water (rivers, lakes and bays). Major land obstacles such as tall mountains, cliffs or secured federal land - in one case in Hawaii, a combination of rain forest, cliffs and federal land - can force us into compromise in locating measurement radials.

At the least, we want to measure the azimuths towards protected stations and other evenly spaced directions to prove that the antenna is running properly and all protections have been furnished.

Natural wanderlust makes it tempting to just grab the FIM, the compass and some manly junk food, and take off in a 4x4 SUV on our odyssey of measurement. Don't waste your time. Once again, "If you fail to plan, you plan to fail."

'Art of the doable'

So let's plan. I am sure every competent consultant has a derivation of the following system. But the fundamentals are as follows.

Either on computer or on paper hardcopy USGS topos, plot the station location and draw in the ideal radials that you want to measure. This is a case of "the art of the doable." You must make adjustments to these radial locations such that the plethora of measurement points are in locations you can reach practically and legally.

Here's the procedure that has served us well over the years. On the radials you've drawn, circle the most likely measurement locations, such as anywhere a road crosses the radial. Arrange this material along with any support data (such as local maps and helpful directions from the locals) in a form and size that you can handle in the cockpit of a car and without blinding the driver. Then strike out, searching for each point using map and GPS.

Often you will find that many of the points you thought you'd use are inaccessible due to changes in topography, development or map inaccuracies. Perhaps they are unsuitable because of obstructions, overhead wires or other phenomena that contaminate the indication.

Don't note a point's location, descriptor or coordinates until you have made a successful measurement. This saves a lot of work.

Also, don't worry too much about interval. Just try to hit every accessible location. As mentioned, it's the art of the doable.

One reason DOEs and consulting engineers get paid the big bucks is that they have the ambassadorial skills to get things done. They know how to obtain permission to take measurements on private property that is normally off-limits. Learn from the masters.

(Some states have extended eminent domain power to allow land surveyors, consulting engineers, registered hydrologists and the like to take legal measurements for the public good on otherwise-private or governmental properties. Check your state law to see if you come under these auspices, should you be cut off from critical locations.)

Once your points are selected, determine your measurement interval and mark each carefully on the map(s). Accurately calculate each point's coordinates and note these along with a descriptor, how to reach it by road or foot and associate it with whatever numbering scheme you choose to use.

Ideally, for consistency and validity, your AM DA measurements on each radial should be taken with the same FI meter, on subsequent days, in similar weather and between the daylight hours, starting no earlier than one hour after dawn and one hour before sunset.

If using multiple FIMs in a proof or measurement project, be sure to compare the indications on all the meters and ensure that they agree with one another before embarking. When returning to re-measure a point or radial, use the same meter.

Put your name on it

Take very good care of your FIM. Yes, the PIs are rugged; but a craftsman takes the best care of his tools. The cost of having a unit reevaluated, refurbished and recalibrated is a minor one within a big project. If your last recalibration was over a year ago and the measurements are important, remove this doubt from the project and have it back to the factory for a visit.

Hint: Mark your meter as to ownership well (spray on your station call letters or add durable "decalcomania"). Ask the factory to put your ownership ID inside the unit when it is apart and before the calibration sticker goes on. These valuable meters must like to grow little feet when we're not looking, because they're always walking off. Mark them ostentatiously as a deterrent to theft.

These measurements take time and diligence, so maintain consistent work habits. Tank up the 4x4 the night before; check the oil and windshield washer fluid, tire pressure and Perrier supply. Get a solid night's sleep. The next morning, eat a good breakfast and put fresh batteries in the GPS.

Drive to the transmitter and personally check the power output and DA operating parameters so you know they are exactly right. While at the site, zero your GPS unit at the array center coordinates. Let affected personnel know your whereabouts and goals for the day; be at your first point at the appointed time.

For safety and convenience, do this work with a partner. Because of the expense and complications involved, both of you should keep records of the data taken. My favorite horror story in this sort of work was a cup of coffee spilling into my daybook, blowing away two days of notes and labor as well as a nice backseat cover job in the rental car.

The whole experience, however, can be gratifying. In "Apocalypse Now," Robert Duvall's character Col. Kilgore says that there is nothing more pleasing than the smell of napalm in the morning. Personally, I think it's getting that first FIM logged while the dew is still on the ground.

In the footsteps of giants

No one is well served by inaccurate FIs or those taken under dubious circumstances. Accurate data on your station's field strengths is an excellent tool, probably the only reliable one you have to ascertain performance, including long-term deterioration. It behooves you to do your best job in taking and preserving this data.

Further, extensive FIM data contained in elaborate or detailed proofs will be used again and again over the years by other engineers, drawn from the FCC's records, as a basis for calculating local conductivities and expected propagation.

It is comforting and instills confidence to know the data was taken by a giant such as Jules Cohen, Bill King, John Battison or Donald Everist, to name a few. If your work is of their caliber, you have done a superior job.

The author thanks W.C. "Cris" Alexander for his help with this article. Fitch calls Alexander "the Jedi master of DAs."