Measuring FM Harmonics

Anytime a new transmitter is installed or major changes to an FM transmission system are implemented, the performance measurements described in FCC 73.317 should be completed to ensure no interference to other radio services. This is becoming a larger issue with the advent of LTE and 5G mobile data. These services along with E911 and other mobile services are often co-located at FM transmitter sites.

The FCC stipulates that emissions removed from the carrier by more than 600 KHz must be attenuated 80 dB below the carrier. These days, that is not enough. We have had issues with older tube-type transmitters interfering with cellular and mobile data service, even though they met or were far below the FCC specification. The first in, first out rule also didn’t seem to matter either. Those mobile phone providers paid a lot of money when they purchased chunks of RF spectrum at auction, and the FCC will side with them if there is any dispute.

Having a record of measurements that show compliance with the FCC regulations can go a long way in heading off any future problems. I make measurements out to the 10th harmonic.

To get the best results, I have been using a couple of high-pass filters from Mini-circuits.

Mini-Circuits NHP-200+

These attenuate the carrier power seen by the spectrum analyzer by approximately 90 dB depending on the frequency. That allows the instrument noise floor to be lowered to -130 dB which should be well below any receiver noise floor being used by other wireless services.

100.7 MHz no HPF
100.7 with two HPF-200+ High Pass Filters

The carrier is attenuated by 92.44 dB. The rest of the measurements are made with the attenuation set to zero and the preamp turned on. For the lowest FM frequency, 88.1 MHz, the filters are on the edge of their shoulder at the 2nd harmonic. I measured the return loss and found that they matched the manufacturer’s datasheet.

Mini-circuits HPF-200+ X 2, 176 – 216 MHz S21 Return loss

That loss is counted as attenuation for the second harmonic. For the rest of the harmonics, I used 0.5 dB attenuation, which represents connector loss. I could have also measured the cable loss at each harmonic, but that seems unnecessary, given several of the readings were below the noise floor.

To speed things along, I made this handy Excel spreadsheet, which does all of the calculations for me:

FM harmonics spreadsheet

A copy of that spreadsheet can be downloaded: FM Harmonic template

Once completed, I printed a copy and put it with the station maintenance log at the transmitter site.

The Rhode and Schwarz THR9 transmitter

Part II of II

This thing is on the air! There are still some tidying-up things to finish, but it is up and running and sounds great! Here are some pictures of various stages of the installation work:

Making harmonic measurements

The filing cabinets hold manuals and spare parts. There is not a lot of room left in this building, so workspace is at a premium. The filing cabinet on the left needs some Windex and elbow grease.

Main disconnect and conduit to 400-volt transformer
Outdoor coolant run
3/0 cables, 240-volt input to Hammond HPS Sentinal K transformer

The transformer does not have a neutral reference to the power company. The neutral for the transmitter is derived from the Y output connection. The transformer is also designed to suppress harmonics from non-linear loads like switching power supplies.

Wiring in Square D I line panel
Square D I-line panel rated for 600 volts
#2 SOOW cable feeding upper and lower sections of transmitter
Wiring to disconnect switch on transmitter
Pump station during system fill
Heat exchanger

The wiring on the pump station and heat exchanger needs a little more work. The client wanted to get this on the air as soon as possible because they are in a book and were running at 50% power. Once things calm down a bit, I will put the backup transmitter on for an afternoon and properly dress the wires.

FM modulation analysis

I found this FM modulation analysis function on my spectrum analyzer very useful. The station deviates slightly more than the allocated 75 KHz because of a subcarrier. Overall, it looks good. I measured the harmonics out to the 10th harmonic, most of them were in the noise floor. A few made a slight appearance, but well within FCC tolerances. It is important to document this, as this site has colocated cellular carriers and several E911 services.

FCC part 73.317 states:

(d) Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

47CFR 73.317
WHUD fundamental
WHUD fundamental with two Mini-Circuits NHP-200 high pass filters installed

The rest of the harmonics were measured down to -130 dB with the two NHP-200 high-pass filters in the circuit. The 3rd, 4th, 5th, 6th, and 8th harmonics were unmeasurable. The 8th, 9th, and 10th made slight appearances.

WHUD 6th Harmonic, noise floor
WHUD 8th harmonic makes a little appearance
Main antenna VSWR
Antenna VSWR according to the transmitter directional coupler

Pretty close, the VNA was inserted at a patch panel, which is the last thing before the transmission line leaves the building. The transmitter goes through an ERI switchless combiner, which probably gives it a slightly better load.

Backup antenna SWR

Aside from the finishing details, I need to keep an eye on this for a week or so and top off the Heat Transfer Fluid as needed. It takes a bit of time to get all of the air out of the coolant loop. Another thing; the operating pressure on this is 4 Bar, which is almost 60 PSI. That is higher than other liquid-cooled transmitter systems I have installed before.

The Rhode Schwarz THR9 transmitter

This is part I of II.

We are in the process of installing an R&S 40 KW liquid-cooled FM transmitter. My first comment; these are well-built units. A quick look at the machining of the parts indicates attention to detail is a key design feature.

As the price of electricity continues to rise, liquid-cooled transmitters for this power level make a lot of sense.

Rhode Schwarz THR9 VHF transmitter

This installation is for Pamal Broadcasting’s WHUD, Peekskill, New York. The site has undergone major upgrades in the last few years. The original 1958 World Tower Utility 80 was replaced a year ago with this Valmont 60X394. Two cell carriers, two translators, and several E911 services are now colocated on the tower.

Valmont 60X394 tower, WHUD Peekskill, NY

The transmitter building is also the original cinder block structure from 1958. When it signed on, the station had a Gates FM5B 5 KW transmitter, an RCA BFA-7, 7-bay horizontally polarized antenna with an ERP of 20 KW. In 1970, that antenna was changed out to a 6-bay circularly polarized ERI with a Harris FM20H transmitter, increasing the ERP to 50 KW. As of now, the station has a 4-bay ERI SHP-4-A-C main antenna and the TPO is 28 KW for the same 50 KW ERP. As the station’s power increased, the building became a little bit smaller than optimal. We needed to rearrange some equipment to gain space for the pump station and step-up transformer.

Pump Station
Heat Exchanger

Rhode Schwarz recommended installing a step-up transformer for the incoming AC mains. The power supplies run most efficiently with 400 volts AC.

Hammond HPS Sentinel K dry core transformer
The Rhode Schwarz RF connection to an ERI switchless combiner

We decided to reuse the ERI switchless combiner left over from the Nautel V-40 installation. There are two Nautel V-10 transmitters with a hybrid combiner that are to be used as a backup. We won’t be running this as a combined transmitter operation, it is a way to save money rather than install a separate 3-inch coax switch. I will build a simple control panel to move the combiner position either all the way up (THR9) or all the way down (V-10s).

2.5 inch core drilled holes for coolant supply and return

Working on the liquid cooling system. I used a core drill to make the supply and return lines to the outdoor heat exchanger. I made sure that I had the shop vac (with a HEPA filter) running while drilling so that all of the concrete dust was captured. That stuff can get everywhere and has a bad tendency to destroy motor bearings. Whatever plant made these blocks in 1958, they used some hard material. It took a while for my masonry drill to get through them.

Excel spreadsheet formulas for Broadcast Engineers

There are many times when some mathematics is needed in this profession. For one-off situations, the calculator applications found on most smartphones will work just fine. However, sometimes the calculation is complex or is needed to be repeated many times. Excel Spreadsheets have many mathematical functions built in. Plugging a formula into an Excel spreadsheet is a handy tool.

I recently acquired this rather nice precision power meter:

Mini Circuits precision power meter

It has an input power range of -60 to +20 dBm with a stern warning not to exceed +23 dBm. Since we will be using this for a variety of applications, I thought it might be useful to know approximately how much power will be presented to the instrument in any given situation. For example, we are installing a 30 KW FM transmitter soon. The directional coupler that will be used has a coupling factor of -48.5 dBm. The TPO is 28,000 watts.

The formula to convert Watts to dBm is dBm=10 X Log10(Pw) + 30, where Pw is power in Watts. Thus dBm=10 X log10(28000) + 30 or 74.4715 dBm minus the 48.5 dBm coupling factor which is 25.9715 dBm. That is too much input for this power meter. A 20 dB attenuator will need to be used.

Since I will be using this meter in other places, rather than doing that calculation over and over again, why not build an Excel spreadsheet? That would make it easy to check.

A simple Watts to dBm calculator in Excel looks like this:


This is copied into cell C11. C6 is the cell in which the Transmitter output power in watts is entered. The other cells contain the coupling factor (C5) and external attenuation (C7) In application, it looks something like this:

Excel spreadsheet power meter calculations

You can arrange these any way you like, just change the cell numbers to suit your needs.

I like to make the data entry cells green. You can lock the formula cells so that the formulas don’t get changed accidentally. Below the Approximate port power cell, is the IF statement that will return either a “LOW”, “HIGH”, or “OK” depending on the result value in C11. That looks like this:


The spreadsheet itself is downloadable: Power meter port calculator

It would be very easy to make a system gain/loss calculator for using the licensed ERP to calculate the proper TPO.

Other examples of useful Excel spreadsheet formulas:

To convert from dBm to watts:


B22 is the cell in which the power in dBm is entered. These can be any place you want on the spreadsheet.

Radio Frequency to Wavelength in Meters:


Where B10 is the cell in which the frequency in Hz is entered. 299792458 is the speed of light (Meters per second) in a vacuum. If you wanted the input frequency to be in kHz, simply move the decimal point for the speed of light three places to the left, e.g. 299792.458. For MHz move the decimal four places to the left, GHz five places, etc.

Convert electrical degrees to Meters:


Where B10 is the frequency in kHz and B11 is the number of electrical degrees in question.

An example of that in an Excel Spreadsheet can be downloaded: Frequency to Wavelength converter

Audio Frequency to Wavelength in Meters:


Where B11 is the air temperature in degrees Celsius and B12 is the frequency in Hz. Room temperature is normally about 21 degrees Celsius (about 70 degrees Fahrenheit). Humidity and altitude can also affect the sound wave velocity, which will affect the wavelength.

Base (or common point) current calculator using base impedance and licensed power:


Where B12 is the License power in watts and B11 is the measured base impedance of the tower (or common point impedance of the phasor).

Convert meters to feet:


Where B11 is the length in meters

Convert feet to meters:


Where B12 is the length in feet.

Convert degrees F to degrees C:


Where B11 is the degrees Fahrenheit

Convert degrees C to degrees F:


Where B12 is the degrees Celsius. In this case, the order of operations will work without the prentices but I kept them in place for uniformity.

Convert BTU to KW:


Where B11 is the BTU/hr

Example of an Air Conditioner load estimation:


Where B11 is the TPO, B12 is the transmitter AC to RF efficiency. The output is in BTU.

There is an entire list of Excel functions here: Excel Functions (alphabetic order)

You get the idea. Yes, there are smartphone applications as well as online calculators for most of these functions. However, I have found smartphone apps are becoming more painful to deal with as time goes on, mostly due to the ads. App developers need to make money, and you can buy apps for things that are often used. However, it is nice to have these types of calculators available offline. Besides, it is fun to play around with Excel formulas.