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FEBRUARY 12, 2020
In-Depth Technology for Radio Engineers
Collocating AM Transmitter Facilities With Cellular Monopole Towers
Fig. 1: A slant-wire was used to feed the tower.
Conventional wisdom about the number and length of “ground radials” is far too conservative
◗WHITEPAPER BY BENJAMIN F. DAWSON III
The author is former managing partner of Hatfield & Dawson Consulting Engineers and is a senior consultant to the firm. While cell site towers and monopole masts have long been potential nuisances and sometimes severe impairments to the operation of nearby AM antennas, they can actually be useful as AM radiators in some situations. The relaxation of the AM antenna efficiency requirements in the AM Improvement rulemaking has provided flexibility by revising the rules which previously made use of electrically fair-
ly short towers and restricted ground system areas difficult (see reference  found later in this paper). Cell site antenna support structures vary widely in height, but many are tall enough to be suitable for use as antennas at AM frequencies. We at Hatfield & Dawson began to investigate the possibility of using a cell site monopole for an STA antenna for an AM station in about 2004, well before the time when the FCC promulgated its efficiency rule changes. We discussed the possibility with the tower owner, and obtained assurance that they would entertain the idea of use for a low power AM operation. The idea didn’t go further at that time because the then-licensee was not sure they would relocate and rebuild.
56.3 m (185 ft)
57.9 m (190 ft)
Slant Wire Shunt Fed
(continued on page 10)
Towers ASR: 1231476 (Existing) Site Coordinates(NAD 27) 27o 19' 26" N 82o 29' 46" W
Not To Scale
SKETCH OF ANTENNA ELEMENT RADIO STATION WSRQ SARASOTA, FLORIDA 1220 KHZ 770 W-D 15 W-N U ND du Treil, Lundin & Rackley, Inc. Sarasota, Florida
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FROM THE TECH EDITOR Is the Time Right for All-Digital AM? What has to come first: a critical mass of receivers, or a significant number of stations transmitting? BY CRIS ALEXANDER
It is the age-old question: What came first, the chicken or the egg? Arguments can be made for both, so the question remains unanswered, at least in the philosophical sense. But what does that have to do with broadcast radio? In that regard, I suppose we could ask whether the transmitter or receiver came first, and the answer would be that they both came at the same time. In the early months of this year, we are faced with a similar question: What has to come first for all-digital AM to succeed: a critical mass of HD-Radio capable receivers, or a significant number of stations transmitting in the all-digital mode? As the FCC considers allowing AM stations to convert to the MA-3 all-digital mode on a voluntary basis, broadcasters are faced with a choice as to which stations it makes sense to convert. In some situations, the choice would seem to be fairly clear. If an AM station’s programming is 100% duplicated in the coverage area by an FM signal, whether from a sister full-power station or a translator with good market coverage, chances are that the majority (if not all) of the listeners are tuning into the FM signal anyway, and there is no downside to converting the AM to the all-digital mode, at least in terms of audience impact. The all-digital signal will give listeners another high-fidelity means of getting the station’s programming. But in other situations, there may be some FM duplication of coverage and programming, but is it enough that no listeners are disenfranchised if the analog AM signal goes away? That is a decision that each licensee will have to make; only those who are intimately familiar with the market, their radio stations and audiences have sufficient information to make that determination. I suspect that this is where the vast majority of AM stations are — in a situation that is anything but clearcut one way or the other. WHERE TO START? Standalone AM stations would seem to be poor candidates for all-digital conversion. If you believe the HD Radio penetration data, that means as soon as the all-digital switch is flipped, at least 50% of the station’s listeners will get nothing but white noise. And while the statistics on receiver proliferation are undoubtedly correct on the whole, I imagine that the real numbers vary widely depending on region, demographics, the local or regional economy and other factors. All this is part of what amounts to a very local decision as to whether all-digital conversion is right for a particular AM station. And then there is the elephant in the room: cost of conversion. Since the MA-3 mode primary digital carriers fit within the spectrum occupied by the analog
signal, it is likely that most stations have sufficient antenna bandwidth to handle the all-digital spectrum. This is in contrast to the demands of the digital hybrid mode that is authorized at present. A lot of stations that got on that bandwagon had to do a significant amount of work to get their antenna systems in shape to pass the digital sidebands. I did a bunch of those myself some 15 years ago, and it wasn’t easy. So assuming few or limited antenna issues, all-digital conversion costs are primarily the Xperi licensing, and the signal generation equipment, which can run into the tens of thousands of dollars. Figure in some needed infrastructure changes for many stations and the costs will be even higher. That may not be a huge (continued on page 4)
THIS ISSUE FEBRUARY 12, 2020 Collocating AM Transmitter Facilities With Cellular Monopole Towers. . . . . . . . . . . . . . . . . . . 1 Is the Time Right for All-Digital AM?. . . . . . . . . . . . . . . . . . 3 Get Email Alerts From an RFEngineers Watch Dog Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Introduction to the Six Basic Audio Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Be Smart When Thinking About UPS. . . . . . . . . . . . . . . . 20
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February 12, 2020
ALL-DIGITAL AM (continued from page 3)
thing for a fully-duplicated AM in a profitable local cluster, but for the maand-pa AM with a translator in a small market, it may be a deal killer. And that brings us back to the “what comes first” question. SO MUCH NOISE As Ben Downs so eloquently argued in his petition for rulemaking that eventually resulted in the all-digital AM NPRM, the AM broadcast medium is in trouble. In decades past, the issue was interference. In the here and now, it is man-made noise. The interference issue remains, but in many cases it is eclipsed by the noise problem. The interference issue is, if you will pardon the pun, static (the adjective, not the noun). By and large, other than as a result of normal ionospheric variations, interference for a particular station’s signal is what it is … and what it was. The FCC’s rules and international treaties tend to keep interference from increasing significantly beyond current levels. But the noise problem, now that’s anything but static. It is ever increasing. I encountered an excellent example of this at my home a few months ago. I began experiencing a lot of new noise on the AM band as well as on the
Many modern transmitters are all-digital ready.
As the FCC considers allowing AM stations to convert to the MA-3 all-digital mode on a voluntary basis, broadcasters are faced with a choice as to which stations it makes sense to convert. lower HF bands. The noise produced such a roar that I could not listen to any AM signals at home without at least some underlying noise. The big 50 kW signals were at least listenable, but they weren’t clean. Lower-powered signals were completely unlistenable. I tried everything I could think of to track down the noise source without success, walking around the house with a battery-operated portable radio, listening for an increase or decrease in the roar as I moved from room to room. The noise seemed to be ubiquitous. I eventually concluded that it must be coming from my neighbor’s solar charge controller or inverter. Then one day, I happened to have a radio on when I turned off the switch for the front exterior lights. We normally leave those lights on all the time, but for some reason I turned them off that day
… and instantly the noise disappeared! AM reception was clear and clean, and the S-9 noise floor on the 80, 60 and 40 meter bands dropped to S-2! I turned the exterior lights back on, but the noise remained gone. I left the lights on, thinking that the noise would eventually come back and I could investigate further, but it never did. And then later that day, as I was backing the car out of the garage, I noted that one of the front exterior lights was out. I opened the fixture and looked at the LED bulb, and I found it discolored. Clearly it had been hot. Most likely it had been arcing internally, and when I turned off the switch, the arc extinguished, and the spacing was sufficient that it did not return when I turned the circuit back on. I replaced that bulb with a new GE LED bulb, and all was well. Still no noise.
The point here is that what happened at my house with one noisy LED bulb (in a house that has 100% LED bulbs) happens all the time in other homes and businesses. It may not be an LED bulb. It may be the motor controller in a highefficiency HVAC unit. It may be the microprocessor in a washing machine or refrigerator. Or it may be solar charge controllers and inverters. Each noise source adds to the RSS interference level at every receive location, and as more and more devices are added, the noise floor goes up and up and up. Each device is okay by itself, but each one adds to the total. LET’S GET MOVING At this late date, I daresay that there is nothing that can be done about the noise issue. That train left the station a long time ago, and there is a lot of momentum. In my opinion, this noise issue spells doom for most of the AM broadcast medium. Only the strongest stations that produce a field of 10 mV/m or more throughout the coverage area have a chance at survival. This is where all-digital comes in. It has a demonstrated immunity to noise. It’s not a panacea, but it does perform well in our 21st century noisy environment. So I’m going to go out on a limb here
and agree with proponents that if AM is to survive for the long term, it has to make the jump to all-digital. But what comes first? Do we wait for a critical mass of receivers before making that jump, or do we go now? Do we drive the demand for digital receivers by going all-digital now, or is that a pipe dream? Or … is it way too late for any of this, making this a pointless discussion? I don’t have a Magic 8-Ball that I can shake and get answers, but I do believe that the AM broadcast medium has both value and a future — if we get moving now, in at least a limited way, with conversion to the noise-immune all-digital MA-3 mode. Receiver proliferation will independently continue, driven by the auto industry and FM. AM can ride that wave. But if the AM medium dies while we wait … well … it won’t much matter if there are plenty of digital AM capable receivers out there. It’s certainly something to think about. Watch a Radio World webcast about all-digital on the U.S. AM band on Feb. 19. Info is at https://tinyurl.com/ rw-sunrise. Cris Alexander, CPBE, AMD, DRB, is director of engineering of Crawford Broadcasting Co. and technical editor of RW Engineering Extra. Email him at email@example.com.
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Get Email Alerts From an RFEngineers Watch Dog Receiver A Pi can easily be used to provide a serial-to-email interface BY DAN D’ANDREA
The RFEngineers Watch Dog receiver is used by many radio stations for local and remote off-air monitoring of audio, signal level, RDS and pilot. In this installment of our ongoing Raspberry Pi project series, Dan D’Andrea, RFEngineers’ “software guy,” details a project that employs a Pi to channel alerts from the receiver. The Watch Dog receiver does not have an Ethernet port for sending out alerts over the internet. Instead, the receiver is configured, monitored and powered via its USB port. Thankfully, the USB port on the Watch Dog makes the receiver available as a serial device, which is easy to connect to in a variety of ways. We refer to this as its “Serial API” or “serial interface,” and it’s quite extensive. (Plenty of documentation can be found at http://www.RFEngineers.com/WD1.) As one demonstration of what can be achieved with the Watch Dog’s serial interface, have a look at the RFEngineers Watch Dog Dashboard for Windows. The Watch Dog Dashboard is a free program that lets one easily configure and monitor the receiver and which is based entirely on the Watch Dog’s serial interface. See Fig. 1. We began to wonder: How easy would it be to hook a Watch Dog receiver to a Raspberry Pi and have the Raspberry Pi continually monitor the Watch Dog’s status via the serial interface and send out an email any time an alarm condition is found? Perhaps a small Python program? It turns out that the Watch Dog’s serial interface makes it ideally suited for automating with Python. We were able to write a simple Python program to monitor the Watch Dog and detect alarm conditions in less than 10 lines of code! We then extended the program to include email alerts. This program is available for free and for you to adapt however you see fit in a public GitHub repository that we created: https://www.github.com/rfengineers/ Watch-Dog-Python. This article will go through the steps of setting up a Watch Dog receiver and a Raspberry Pi to work in conjunction as an Internet-enabled confidence monitor. It will use the AlarmEmail.py program referenced above and found on GitHub. We used the following equipment:
the Pi’s 4 main USB 2.0 connectors. We chose to use a Raspberry Pi for this example but any PC, Mac or other computer that can run Python would be fine as well. Note that you might ultimately gain better mileage using a powered USB hub to connect the Watch Dog to the Raspberry Pi, as we observed an Under-voltage detected! message in the system log file when first connecting the Watch Dog. The Raspberry Pi otherwise
showed no problems powering the Watch Dog receiver. FINDING THE WATCH DOG’S SERIAL PORT IN RASPBIAN Run the following Linux command to determine on which port your Watch Dog is available: dmesg | grep tty Look for a line containing a message like USB ACM device. Copy down the full tty value for a later step, e.g. ttyACM0 in our case. See Fig. 2. Next, clone the GitHub repository or simply download the AlarmEmail.py program directly from here: https://github.com/rfengineers/Watch-Dog-Python. Open up the AlarmEmail.py program in your favor-
Fig. 1: RFEngineers Watch Dog Dashboard for Windows software.
Fig. 2: Our Watch Dog was found on ttyACM0.
• R FEngineers Watch Dog FM/AM/NOAA + RDS receiver, firmware v2.2.7 • Raspberry Pi 3 Model B • 2.5A USB power supply INITIAL SETUP We used a Raspberry Pi 3 Model B with a fresh install of Raspbian OS, but just about any Raspberry Pi should do. We connected the Raspberry Pi to wall power using a 2.5A USB power supply. We then connected the Watch Dog to the Raspberry Pi via one of
December 11, 2019
Fig. 3: AlarmEmail.py showing normal output with no alarms.
(continued on page 8)
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WATCH DOG (continued from page 6)
ite text editor and change the following parameters: ALARM_LIMIT_SECS This limits how often, in seconds, an alarm email will be sent. For instance, if set to 900 then the program will wait 15 minutes before sending another alarm email. Leave at the default value of 900 seconds if this works for you. ALARM_POLL_SECS This is how often the program will query the Watch Dog via its serial interface to get the latest alarms reading. Leaving this at the default value of 15 seconds should be fine for most uses.
Fig. 4: Simulating an alarm condition and receiving an alert email.
WATCH_DOG_PORT Put in the value that you found above in the “Finding the Watch Dog’s serial port in Raspbian” section. For example, if the value you found was “/dev/ttyACM1” then you would change this value to that. You can leave it at the default value if your Watch Dog showed up on the same port as ours. CONFIGURING ALARMEMAIL.PY EMAIL SETTINGS You will need to change several email-related settings, and possibly a few other email-related settings as well. EMAIL_SUBJECT You can leave this as it is if you are fine with the default message we chose. Otherwise change to suit your needs. EMAIL_FROM You will need to put your email address here, or the email address where you want the emails to come from. EMAIL_PASSWORD The password used to send email on your email server with your email address. I used my Gmail account, which required that I set up an App Password. More info on that here: https://support.google.com/ accounts/answer/185833?hl=en. EMAIL_TO Where the email alerts should be sent.
Fig. 5: An alert email showed up on my phone within seconds.
EMAIL_SERVER_HOSTNAME This is the hostname of the email server for the email address you are sending from. In my case, I was sending from my Gmail account, so I used smtp.gmail. com. EMAIL_SERVER_PORT The default port should be fine for most email servers. Otherwise you can change it here as needed. RUNNING ALARMEMAIL.PY Simply execute the following command to run the program: python AlarmEmail.py. See Fig. 3. Pulling the antenna from our Watch Dog receiver was enough for us to generate several alarms. See Figs. 4 and 5.
Fig. 6: Installing and starting the AlarmEmail.py systemd service.
ADVANCED: CONFIGURE ALARMEMAIL.PY FOR 24X7X365 FAIL-SAFE OPERATION We want AlarmEmail.py to stay running, even if the Raspberry Pi temporarily loses power, is rebooted, or if the program ever crashes. We will accomplish this by running AlarmEmail.py as a Linux systemd service. Copy the AlarmEmail.service file from our GitHub repository to the /lib/systemd/system/ directory on the Raspberry Pi (see Fig. 6) and then issue two more commands to start the service: sudo cp AlarmEmail.service /lib/systemd/system/ sudo systemctl enable AlarmEmail.service sudo systemctl start AlarmEmail.service You may also want to go ahead and reboot your Raspberry Pi now to verify that the service starts on boot. CONCLUSION Getting real-time email alerts from devices that don’t provide an Ethernet interface can be easily accomplished when you combine a Raspberry Pi or other computer and a bit of Python code. In this sense, the Raspberry Pi can be a great piece of “glue” for broadcast engineers. This article will hopefully get you thinking about other automation opportunities for your broadcasting infrastructure. For example, using similar methods to those outlined above, a device like the Watch Dog receiver could easily be turned into a multi-station confidence monitor. Stay tuned, as we will likely be publishing just such an article in the near future. Dan D’Andrea is an amateur radio operator, embedded systems enthusiast, Software-Defined Radio (SDR) hobbyist and professional software developer with 20 years of industry experience.
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February 12, 2020
MONOPOLE (continued from page 1)
RON RACKLEY’S WSRQ CP APPLICATION AND LICENSE Ron Rackley’s client, WSRQ, had been 120 evenly spaced ground buried radials out to an operating with a temporary antenna under average radius of 21.7 meters Extent of Ground from tower, bonded to a STA, but evidently was unable to continue System copper strap along the building perimeter at the STA site, or to construct previously approved directional facilities. When Ron was given the problem in 2016, he knew of our previous cell tower analysis, was aware of a cell site/communications tower Building (Shown for ground of substantial height in an acceptable locaconnection detail tion, and advised his client to investigate its only – not to scale) possible use. When the result was favorable, he prepared an application for construction permit to use the site. Two technical matters regarding use of this antenna tower required resolution. The first was the feed system arrangement, and the second was the ground system and resulting antenna system efficiency analysis. 0 10 20 The cell/communications tower is 185 Scale Site Coordinates(NAD 27) feet (56.3 meters) in height, which repre(meters) 27 19' 26" N 82 29' 46" W sents an electrical height of 82.5 degrees, and therefore is tall enough to be an acceptANTENNA SITE PLAT able AM radiator. However, like nearly all RADIO STATION WSRQ cell and communications masts, monopoles SARASOTA, FLORIDA 1220 KHZ 770 W-D 15 W-N U ND and towers, it’s grounded. While detuning du Treil, Lundin & Rackley, Inc. Sarasota, Florida of grounded antenna support structures is generally accomplished with a skirt of three or more vertical wires, this was not a practi- Fig. 2: The property boundaries resulted in a shortened cal feed system method because of the mul- ground system. tiplicity of antennas mounted on the tower. A detuning skirt generally exhibits low RF burn hazard and can easily be temporarily disabled of a tower for additional load. The solution was to feed by “grounding” to the support structure in the vicinity the tower with a slant wire feed. See Fig. 1. of any necessary work. A driven skirt, however, has Precedent for use of a slant wire feed had been higher RF burn potential, and would require an off-air obtained by our office in the application for license of period while work is performed on the tower. station KFIO in April 2017. Ron used that precedent Skirts also add non-trivial amounts of wind loading, and showed a NEC-4 analysis of the essential circulardeadweight and leg stress, reducing the total capacity ity of the proposed WSRQ radiation pattern at horizono
70 mV 2.56 km
100 mV 2.19 km
105 mV 1.72 km
125 mV 1.37 km
182 mV 0.98 km
285 mV 0.64 km
400 mV 0.37 km
680 mV 0.18 km
tal and vertical angles, within 1.5 dB up to elevation 70 degrees. This result is consistent with the KFIO situation, and most all other slant wire feeds for towers of about 135 degrees or shorter, as is shown in our previous work . Since many cell and communications towers are relatively short, slant wire feeds for AM use can often be a very desirable feed solution. A NEC-4 analysis was also performed to determine the radiation efficiency of the proposed antenna, since the property parcel was very limited in size, as is typical of cell and communications installations. See Fig. 2. Geometry showed that the site allowed a radial ground system equivalent in size to a 21.7-meter radius circle, or about 32 electrical degrees. This is well below the variables allowed by the FCC’s “Figure 8” table and computer program, which are of dubious provenance in any event. Although the construction permit was granted without a requirement for any efficiency measurements, a single radial was measured, and confirmed the calculated value. See Fig. 3. The WSRQ construction permit was granted in November 2018, and the station is now licensed. THE PENDING APPLICATION FOR CONSTRUCTION PERMIT FOR KARR Our original analysis of possible cell site use for an AM station in 2004 was for KARR, a station that had lost its original transmitter site to development. The station licensee was unable to find any other possible permanent location except for, as in the WSRQ instance, a fairly tall cell tower on a small property parcel. See Fig. 4 for an idea of the constraints of the ground system. The cell monopole itself is 150 feet in height (45.7 meters), 80.1 electrical degrees, and its antenna platform adds a bit of top-loading. The site size allows a 120 radial ground system of average length 0.134 wavelength. This radial system, like the WSRQ example, is well below the correction range of the FCC Figure 8 graph and computer algorithm. (continued on page 12)
160' GROUND SYSTEM (NOT TO SCALE)
WSRQ Close-In Measurement Locations Radial = 270 deg TN Fig. 3: Close-in field measurements were done on a single radial to confirm radiation efficiency.
D STRAUME, H&D
HD_HEADERS WITH LAYOUTS 2.dwg
GROUND SYSTEM DIAGRAM KARR 1460 kHz
Fig. 4: Ground system constraints for KARR at cell site.
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MONOPOLE (continued from page 10)
The detailed model of the proposed slant wire fed, grounded-base, monopole tower is shown in Fig. 5. The ground radials are modeled as #10 AWG wires buried to a depth of 0.15 meters (approximately 6 inches) in soil having a conductivity of 4 mS/m, and a relative dielectric permittivity constant (epsilon) of 15. This is the same dielectric constant used by the FCC in developing the Ground Wave Field Strength Versus Distance Curves in Section 73.184 of the Commission’s Rules and Regulations. The NEC-4 files are over 90 pages and impractical to put in this report, but can be accessed on the FCC CDBS website at: https://tinyurl.com/karr-report. Fig. 6 shows the model-predicted current distribution on the monopole. Based on the results of the NEC-4 modeling, the predicted verticallypolarized RMS attenuated electric field at one kilometer is 197.1 mV/m, assuming a soil conductivity of 4 mS/m and a dielectric constant of 15. From this attenuated value the predicted unattenuated field (antenna effi-
ciency) was determined from the Ground Wave Field Strength Versus Distance graph (1430–1510 kHz) of Section 73.184. From the graph, for a referenced radiated field of 100 mV/m at one kilometer, the attenuated field at one kilometer for a soil conductivity of 4 mS/m is 76.7 mV/m. Stated differently, the 4 mS/m soil is predicted to attenuate the field by a factor of 0.767 when compared to the 100 mV/m unattenuated field at one kilometer. Therefore, the model derived attenuated RMS of 197.0 mV/m at one kilometer can simply be divided by 0.767 to yield the predicted unattenuated RMS field of 256.8 mV/m/kW at one kilometer. Using the same NEC-4 model, the attenuated field strength in the horizontal plane varies from 196.1 mV/m to 200.8 mV/m, providing a circularity of 0.2 dB. The application for construction permit is pending as of the time of this writing . THE “FM TOWER” SPECIAL TEMPORARY AUTHORITY Another site loss case resulted in an STA for use of an unusual wire antenna supported by the grounded guyed tower of a commonly-owned FM station. The tower guys were uninsulated and WIRELESS PANEL ANTENNA
150.0' TOP OF STEEL
Fig. 5: Vertical sketch showing slant wire fed, grounded-base, monopole tower proposed for use by KARR.
WIRELESS PANEL ANTENNA
WIRE CABLE FEED MECHANICAL ATTACHMENT TO TOWER TO BE ENGINEERED BY STRUCTURAL ENGINEER
FEED ATTACHED TO TOWER SLANT WIRE SHUNT FEED HEIGHT OF FEED ADJUSTED FOR BEST TRANSMITTER MATCH ~30' AGL
REPRESENTATIONAL ONLY DRAWING NOT TO SCALE EQUIPMENT CABINET 10'
D. STRAUME, H&D
AMTOWER AND SITE.dwg
KARR AT ATC TOWER KARR (AM) 1460 kHz
February 12, 2020
Fig. 6: The NEC-4-predicted current distribution on the monopole.
grounded at their outer ends as well, as is the usual case for towers not designed for AM use. The grounded 300-foot guyed uniform cross-section antenna tower of was fed with an “umbrella spoke” wire,
mounted and configured as shown in Fig. 7. The base of this wire was terminated on the existing equipment building, and the matching network and transmit(continued on page 14)
Fig. 7: Slant-wire orientation.
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MONOPOLE (continued from page 12)
ter were installed inside the building. Figs. 8, 9 and 10 show the details of the installation. This installation also had a very cursory limited ground system. Six radial “ground” wires, extending to a distance of about 200 feet from the antenna tower (about 70 degrees) were installed. The radials were barbed wire laid in the snow (it was December). Barbed wire makes excellent radials for use in some specialized situations, and is far cheaper and less susceptible to theft than copper or even copperweld. The efficiency for this antenna installation, based on reasonable ground conductivity assumptions and a moment method model, is about 200 mV/m/kW/km. The radiation pattern is modestly directional. Connection of the “umbrella spoke” at its upper end to the tower would result in a less directional pattern, but also a higher drive impedance.
Fig. 8: Mechanical termination of the slant wire on the corner of the FM transmitter building.
USE OF MOMENT METHOD ANALYSIS FOR SHORT OR ODDLY-SHAPED GROUND SYSTEMS Dave Pinion, Steve Lockwood and Joe Overacker in our Fig. 10: The VSWR sweep of the antenna tuning unit office have just cominput. pleted an extensive NEC-4 study showing that considerable ground system area, but with an irregumodification of the ground system in lar configuration. a complex diplexed DA has essentially Similar studies have been performed NO effect on the system efficiency. This on situations with extensive reducsituation will be an increase in total tion of outer ground system area, and
Fig. 9: The antenna tuning unit circuitry was assembled on Unistrut attached to a wall inside the FM transmitter building.
Vertical Bridge – Broadcast Facility Dual Use Analysis – Hillsborough County, Florida
structure being modeled. 5.
No wire segment in the model may exceed 10 electrical degrees in length at the WFLA
and WHNZ operating frequencies of 970 and 1250 kHz.
Case 1 - Ground System as Installed NEC Model
REFERENCES:  1 st R&O, FNPRM, NOI; MM Docket 13-249, at paragraphs 41–48 (October 23, 2015)  D awson, B., “The Slant Wire Fed Monopole, a Neglected but Invaluable Technique,” paper presented at the 60th Annual Institute of Electrical and Electronic Engineers Broadcast Symposium, October 2010.
Case 2 - Modified Ground System NEC Model
Fig. 11: NEC-4 model of an irregular ground system.  O ther examples of the use of moment method analysis to determine effective field of unusual antennas or antennas in unusual situations include: WRGC BP-20190130ABH, KSSK BP-20180921AAW, KIKI BP-20180921AAS and KHVH BP-20180921AAV.  D awson, B and S. Lockwood, “Revisiting Medium-Wave Ground System Requirements,” IEEE Antennas and Propagation Magazine, August 2008. Trainotti, V and L. Dorado, “Accurate Evaluation of Magnetic- and Electric-Field Losses in Ground Systems.” IEEE Antennas and Propagation Magazine, January 2008.  C hristman, A and C. Beverage, “The AM Umbrella Antenna,” IEEE Transactions on Broadcasting, June 1999.
& Dawson Consulting Engineers WITH NO GROUND14RADIALS the results confirm that Hatfield the “normal” SYSTEMS 120-radial 90-degree ground system is A system using a relatively tall tower overly conservative . that does not have a base insulator but Fig. 11 shows the situation in the is grounded only with a few driven “before” and “after” ground system rods and has no radial ground system configurations. has been employed in two or three The conclusion is that the lack of implementations which were licensed scaling for frequency in the Brown and by FCC, originally at WNTF. Epstein experiment analysis, the source This arrangement uses slanted wires of the original 120 ninety degree radial from close to mid-height on the tower, requirement, has cost hundreds of miles slanting in an umbrella arrangement of copper wire to be unnecessarily to locations a short distance from the (continued on page 16) wasted.
WHITE PAPER Introduction to the Six Basic Audio Measurements RADIOWORLD ENGINEERING EXTRA
February 12, 2020
Let’s take a look at the main tests that lie at the heart of all audio testing BY DAVID MATHEW
The author is technical publications manager and a senior technical writer at Audio Precision in Beaverton, Ore. When reduced to its basics, the process of audio test and measurement is concerned with a small number of performance benchmarks. At my company, we call these “the Big Six,” and they are as follows: • Level • Frequency Response • THD+N (Total Harmonic Distortion plus Noise) • Phase • Crosstalk • Signal-to-Noise Ratio (SNR) Fig. 1 shows a typical test setup for the Big Six audio measurements. LEVEL Any Device Under Test (or DUT, as often referenced in the world of test and measurement) may have a number of level measurements that (continued on page 18)
Fig. 1: Test setup for the Big Six audio measurements of a device under test.
MONOPOLE (continued from page 14)
tower base. The wires are fed against the grounded tower . See Fig. 12. The complete lack of a radial ground system isn’t damaging to the efficiency, since the lower portion of the skirts or the “umbrella” together with the tower itself create a quasi-dipole, and, there is a path for the displacement current return, thus satisfying Kirchoff’s law. This result is similar to the use of elevated radial systems, with as few as four or six radials. A quasi-dipole can also be created by a pair of skirt-wire assemblies on a grounded tower of sufficient length. A NEC-4 model using a pair of 50-degree skirts shows this configuration in Fig. 13. This example falls slightly short of the FCC’s minimum efficiency requirement, but would be valuable for an STA. It could be easily modified to meet the FCC minimum value if a slightly taller grounded tower were available. CONCLUSION The conventional wisdom about the necessity of “ground radials” of substantial number and length is simply far too conservative. Innumerable examples of antennas with restricted or convoluted ground systems have been in operation for many years, based on simplistic analysis or field measurement confirmation. But modern analysis methods clearly show that the efficiency of unusual restricted area and unusual
Fig. 12: Sketch of the feed system on an electrically tall tower in which no ground radials were used.
geometry ground systems — or in some cases no radial ground system at all — can meet the current minimum efficiency requirements of the FCC. Benjamin F. Dawson III, P.E. has over 60 years of experience with broadcast antenna systems and other applied radio physics matters.
Fig. 13: Current distribution from a NEC-4 model of a pair of 50-degree skirts on an electrically tall tower with no ground radials.
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MEASUREMENTS (continued from page 16)
are of interest. You must choose which level you are seeking. Target levels include: • an input level that produces a given output level, such as 1 volt, or 1 watt, or unity gain (see below for a discussion of DUT gain); • an input level that produces a certain output distortion, such as 1% THD+N; • a level that provides good noise performance with comfortable headroom, often called the operating level; • an input or output level specified in a testing document. Any of these levels may be used as a reference level on which we can base further measurements. Frequency response measurements, for example, are expressed relative to the level of a mid-band frequency; THD+N measurements are made at specified levels, which should be reported in the results. The ratio of a DUT’s output voltage level to its input voltage level is the voltage gain of the DUT. For example, in a DUT with a gain of 2, an applied input of 2 volts will produce an output of 4 volts. A gain of 1, where the output voltage equals the input voltage, is called unity gain. Some DUTs offer no gain adjustments, and are said to have fixed gain. The gain may be fixed at unity, or at some other value. A DUT with a volume control or other setting that affects gain is a variable gain device. When setting and measuring level, it is essential to consider whether or not the DUT gain is variable (not only volume controls, but tone controls and other settings can change gain), and, if it is, how to set the DUT controls for the desired test results. FREQUENCY RESPONSE A frequency response measurement reports the output levels of a DUT when stimulated with different frequencies of known level. The
Fig. 2: An example of a frequency response sweep of a device.
Fig. 3: Screenshot of a THD+N measurement of a device at maximum operating level.
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simplest of all frequency response measurements consists of only two or three tones, the first near the middle of a DUT’s usable frequency range, and followed by a tone near the higher extreme of the range and sometimes a tone near the lower extreme. Assuming the tones are all generated at the same level, the DUT’s output levels describe its response to these different frequencies. Full-range frequency response measurements can be made by a number of different methods, the classic being a sweep of a sine wave from the lowest frequency in the range to the highest, with the results plotted on a graph. A “flat” response describes the shape of a graph where the DUT responds equally at all frequencies, producing a trace with a slope of 0 and with minimal variations. Fig. 2 shows a typical result. THD+N THD+N stands for Total Harmonic Distortion plus Noise. Harmonic distortion is the unwanted addition of new tones to the audio signal. These tones are harmonically related tones to the original signal: when the signal is one sine wave of frequency f1, harmonic tones are f2, f3 and so on, at integral multiples of the original tone. Total harmonic distortion is the sum of all of the harmonics measured in the DUT’s bandwidth. Why THD+N? Why not just mea-
sure THD (the distortion) and N (the noise) individually? Well, in the preFFT days of audio measurement, it was difficult to measure the THD by itself, without the noise, but it was relatively simple to measure the THD and the N together. So the accepted techniques handed down from years past specify THD+N, because that’s what was practical. In addition, THD+N is a convenient and telling single-number mark of performance, widely understood and accepted. The measured THD+N of a device will vary with the measurement bandwidth. You will almost always want to restrict the measurement bandwidth using high-pass and low-pass filters, and you must include the bandwidth used when you state the result. THD+N is typically measured and reported in a 20 Hz–20 kHz bandwidth. The measured THD+N of a device will also vary with level and the frequency of the applied signal. Audio THD+N is typically measured and reported at a mid-range frequency (1 kHz or so) at either the device’s nominal operating level or at its maximum output level (MOL). Fig. 3 shows a typical THD+N measurement result at MOL. PHASE In audio engineering, phase measurements are used to describe the positive or (continued on page 20)
RADIOWORLD ENGINEERING EXTRA
February 12, 2020
Be Smart When Thinking About UPS When managing a radio facility, sometimes saving money can turn out to be costly
Production room with APC SMX UPS
BY FRANK ELIASON
We all love cool, new technology, but sometimes we need to talk about technology that is not as sexy yet still extremely important. I want to talk a little bit about uninterruptible power supplies (UPS). It is not a conversation we like to have, but we all know how important it is to protect expensive equipment and improve uptime. I expect many smaller stations will have a similar experience to mine, so I wanted to take the time to help drive the right ways to think about this important equipment. Working at Holy Spirit Radio in the Philadelphia area, we operate two small non-profit stations. Although I have been involved with the stations since the foundation was formed 20 years ago, my involvement over the last three years has increased tremendously as we prepare for the next 20 years. Equipment in radio has changed dramatically, from much larger analog equipment of yesteryear to the alldigital equipment of today. In my view, some of the older equipment could handle power fluctuations that would damage or destroy some of the newer equipment that is used today.
MEASUREMENTS (continued from page 18)
negative time offset in a cycle of a periodic waveform (such as a sine wave), measured from a reference waveform. The reference is usually the same signal at a different point in the system, or a related signal in a different channel in the system. This choice of references defines the two most common phase measurements: device input/output phase, and interchannel phase. Phase shift varies with frequency, and it is not uncommon to make phase measurements at several frequencies or to plot the phase response of a frequency sweep. Phase is expressed in degrees. CROSSTALK In audio systems of more than one channel, it is undesirable for the signal in one channel to appear at a reduced level in the output of another channel. This signal leakage across channels is called crosstalk, and in practical devices it is very difficult to eliminate. Itâ€™s expressed as the ratio of the undesired signal in the unstimulated channel to the signal in the stimulated channel.
Crosstalk is largely the result of capacitive coupling between channel conductors in the device, and usually exhibits a rising characteristic with frequency. Itâ€™s often expressed in the form of a single-number result; however, a crosstalk versus frequency sweep will show how a DUT performs across its operating bandwidth. SIGNAL-TO-NOISE RATIO How much noise is too much? That depends on how loud your signal is. Signal-tonoise ratio (or SNR) is a measure of this difference, providing (like THD+N) a singlenumber mark of device performance. The signal is usually Fig. 4: A typical SNR measurement result. set to the nominal operating level or to the maximum operating Fig. 4 shows a typical SNR measurelevel (MOL) of the DUT. When SNR ment result. is made using the MOL, the result can Using traditional methods, SNR also be called the dynamic range, since requires two measurements and a bit it describes the two extremes of level of arithmetic. First you measure the possible in the DUT. (Dynamic range signal level, then turn off the genin digital devices has a somewhat diferator (and often, terminate the DUT ferent meaning). SNR is usually stated inputs in a low impedance as well, to in decibels, often shown as negative. fully reduce the noise in the device).
Then the noise level (often called the noise floor) is measured, using filters to restrict the measurement bandwidth. The ratio between the two is the SNR. David Mathew has worked as both a mixing engineer and a technical engineer in the recording and filmmaking industries. He was awarded an Emmy for his sound work in 1988.
radioworld.com | RADIOWORLD ENGINEERING EXTRA
February 12, 2020
Right before I started to increase my involvement at the stations, the engineer replaced all our UPS equipment. The prior equipment was not nearly powerful enough to meet the needs, and the batteries did not last long in a power outage event. Like many of us, he was frustrated by the higher costs for some of the name-brand equipment, even though the technology they used was relatively unchanged. At the time, he was determined to avoid APC and a few other brands. GENERATOR/UPS CONFLICT The UPS units purchased back in 2016 have been doing their job effectively over the past few years, although I am certain some of the batteries are in need of replacement. I only had one frustration, and that was discovered during a long-term power outage during a hurricane. Due to the cost, our stations currently do not have an automatic backup generator, but instead we use a manual generator during the few times we require it. During this particular incident, the UPS units would not power on with the generator due to the ups and downs (or “dirty” energy) that they produce. In order to get the power to the equipment, we had to bypass the UPS units, which can hurt some of the equipment. One of the key protections of any UPS equipment should be to even out the power, ultimately preventing surges. The challenge is that once you get through an issue like that,
sometimes it is moved to the backburner in favor of other issues that seem to be more pressing. We should never hold off on issues that can destroy thousands of dollars in equipment! Earlier this year, we had a few incidents that brought the issue with our UPS equipment to the forefront. Over the years, the vast majority of our electrical outages have been at night, so we did not witness how well our UPS equipment handled the outage. The equipment typically stayed working, or if the battery died, it came up as soon as the utility power returned. NOT ENOUGH PROTECTION Recently, there was an electrical fire right down the street from our studios. It eventually caused the electricity to go out, but my concern started at that moment the fire started. As I watched the lights dim or go bright as the electrical pattern went up and down, I noticed some of our equipment acting in a similar manner. I immediately made sure they were plugged into the UPS, and they were. Ugh! I was now worried about the thousands of dollars we’d spent on Wheatstone equipment as the UPS was obviously not working properly. Since I could see the incident from my window, I brought down all non-essential equipment and lowered our transmitter power. Over the next few hours, our electricity went up and down. (continued on page 22)
Rack UPS with extra batteries
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UPS (continued from page 21)
Control room with APC UPS
A few days later, we had another electrical issue due to weather and a car accident. This time, the power went fully down, and when it returned, the UPS equipment did not turn on. Even though the equipment was not three years old, I decided I had to replace each of our UPS units. We may have saved money with the UPS purchase three years ago, but it almost became a costly mistake. Luckily, none of our equipment showed lasting impact, so I started the process to purchase new UPS units. I outlined what our needs would be, including ability to handle “dirty” energy, relatively quiet operation (especially for our studio), communication in the event of an outage, the ability to add battery power or hot swap batteries during an outage, the ability to program various levels to shut down equipment that is not needed, as well as guarantees for equipment. MAKING THE RIGHT INVESTMENT THIS TIME Since we do not have an automatic backup generator, batteries can help extend the time for us. If we did turn on our portable generator, I wanted equipment that would not require rewiring to get it back online. I decided to go with the SMX series by APC. Even prior to ordering the equipment, I started mapping out the electrical needs in each area that would require a UPS. It is important to understand your inputs and outputs, as well as determine what must be protected, but may not be necessary in an outage situation. I then went and placed my order. I purchased online from a variety of sources, depending on the price of the specific equipment. Between both of our stations, I knew I would have to purchase six UPS units and two additional batteries. I decided we should start with just two and make sure it was the right equipment for our needs as well as determine if changes needed to be made regarding the necessary equipment. SHOP FOR DEALS I found a great deal for an additional battery from NewEgg. It was a return that they were selling with original warranty. It had a huge cost savings, so I was worried. The box arrived at my home with the outer layer held together with tape covering virtually every square inch. It was obvious to me that the item’s original box was beyond repair. As I cut through the tape, I was able to find a perfect condition battery unit inside. I would not always advise making such a purchase from unknown vendors, but I have had good experience with NewEgg, so I trusted them. The
We may have saved money with the UPS purchase three years ago, but it almost became a costly mistake. battery worked flawlessly. Some of the equipment had the best price on Amazon, so I ordered it there. This caused me to understand why someone had to return the battery to NewEgg, because it is easy to make mistakes! I did. I accidentally purchased the SMT series instead of the SMX. The SMT series is probably a smarter UPS, but they do not allow add on batteries (although you can hot swap them). When I searched the model number, Amazon showed the SMT22000 instead of the SMX2200. I was able to use it, just not in a place where I would have an extra battery. Another mistake I made was not checking out dimensions of the equipment. I simply assumed the server rack mount would fit easily within our servers. Well, the 2,200-watt model has a much greater depth than our server rack
(APC does offer a shorter, double height version). It was not a big deal, but I had to change where I would mount it and removed a door on the back of the rack. In our main studio, I realized after the fact that a few of our rack areas do not have the same depth. This caused me to have to reposition the UPS backup. It is always a learning process! The APC devices were fantastic but certainly far from perfect. As I maneuvered these devices, I was able to reduce the equipment required by two UPS units. It required some rewiring of the racks, but not much work. I cleaned up each rack. I then added two Tripp-Lite network grade power strips (one 15-amp and the other 20-amp) as well as utilizing the existing strip built into the rack. One power strip is used for primary or always on power, another is secondary, which would stay on for part of the time and the other was equipment that would not be needed in the event of an outage. The amount of time I would program in would vary by rack. With some racks, it immediately shuts down non-essential items, while others allow non-essential items to run for 15 minutes or so. BEWARE OF REQUIRED UPGRADES I was frustrated that the APC equipment required separately-purchased network cards for some of the functionality I wanted. I was surprised this was true for the SMT, which has an app that can
monitor the device through a different network connection, but if you want that functionality, you have to buy the $300 card. It is stupid that the app does not offer the broader functions. Anyway, I was able to locate a used network card on Amazon for $88, so no big deal. We did find out in our testing process that our new UPS equipment can power even our backup transmitter (it is small). So, after testing, I reduced our number of UPSes from six to four, but purchased an additional battery to allow our equipment to feed our other station as well as operate our backup transmitter for over seven hours. Today, we have a sophisticated backup power system, even when an automatic generator is still out of reach for our nonprofit. We have programmed smart ways of using our power to help protect the equipment but also allow key equipment to be available longer in an outage. Did we make mistakes? Sure, especially with purchasing no-name equipment in the past. We learned from it and changed gears. In the future, we will be mindful of our cost-savings effort but we will consider the ramifications of those decisions. Frank Eliason is a consultant helping Fortune 500 brands with customer experience and digital disruption. He is an author and director of operations for Holy Spirit Radio in the Philadelphia area.
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February 12, 2020
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