Paul Thurst – Page 125 – Engineering Radio

The Crown FM2000A transmitter

I had the opportunity to work on one of these recently, thought I’d post a few observations. The transmitter itself comes in three parts, the FM100 which serves as the exciter and driver, the PA2000, which holds the RF amplifiers and combiner and the PS2000 with supplies the DC voltages to run the PA.

Crown FM2000A transmitter running at half power

That configuration has some advantages and disadvantages. First, it takes up much more rack space than the comparably powered Nautel VS2.5. Second, because the unit does not come with slide-out rack rails, each part needs to be removed from the rack for servicing, which makes things a little difficult when working alone as the PS2000 weighs quite a bit. As far as the rest of the design, the PA2000 is very modular, all of the PA modules, controller card, fuse board, and RF combiner easily come out of the chassis for service.

This unit had been in service at WBEC in Pittsfield, MA for an undetermined amount of time. As such, there was quite a bit of dirt and bugs inside the PA chassis. I used an air blower to clean everything out. Checked the fans for bad bearings, checked all RF connections for signs of overheating, etc. I also cleaned out the power supply and rinsed all of the air filters.

My other minor complaint is the power adjust pot is under the front cover. When making adjustments and such, the LED display indicates operating constants based on a little LED light next to the display. The legend is on the cover, which has been removed to adjust the power. Minor thing, but slightly annoying, nonetheless.

There are four RF modules in the PA2000, each one generating 500 watts. This particular transmitter has a bad device in PA3. When the transmitter is running the DC fault LED flashes and the PA3 reading shows no current. The device is a BLF278, which is a fairly common, inexpensive RF MOSFET. According to the factory tech, they can be replaced in the field provided one can solder. After replacement, there is no special tune-up or anything needed as the module is wide-band.

The four modules are combined and then sent to the RF output filter which has the low pass harmonic filter and directional coupler.

Crown PS2000 output combiner — Crown PS2000 output filter

It is a pretty simple transmitter, no bells or whistles or fancy things like IP connectivity. Overall, it seems to be well-made, robust, modular, and efficient. The remote control interface is via DB-25 connector on the back of the PA2000.

I did not get a chance to hear it on the air, I was just cleaning and testing the RF sections. The exciter is an FM100 transmitter, which I had to change frequencies on. I found that to be self-explanatory.

It would be fun to compare this to some of the other broadband FM amplifiers like PTEK and Armstrong.

Tower lighting transformers, isolation chokes, etc

Series excited AM towers require some way to get standard AC across the base insulator to the tower lights if tower lights are required. While many new AM towers do not have base insulators, through the use of a folded unipole, it is still a very popular design and has several technical advantages.

There are two methods for getting 60 Hz AC from zero RF potential to an excited tower:

Tower lighting choke
Austin Ring transformer

LBA Group TC-300 tower lighting choke courtesy LBA Group, Inc

When to use it depends on the tower and the RF potential on the base of the tower. For towers that are under 140 electrical degrees (RF) and carrier power levels up to 100 KW, a lighting choke works well. They are simple and less expensive than an isolation transformer. They can be installed inside the ATU cabinet or placed in their own weatherproof enclosure as required. Tower lighting chokes will add series impedance to the base of the tower and needs to be compensated for by adding capacitance to the circuit. This will become more pronounced at the lower end of the band, where, if one is not careful, RF from the tower can be coupled to the transmitter building’s AC mains, which is very undesirable.

Tower lighting chokes generally consist of three separate windings, one for the beacon, one for the side lights, and one for neutral. Their inductance is typically in the 800-1000 µH at 1 MHz region. They can be stacked to increase their peak voltage handling capacity:

LBA Group tower strobe light choke courtesy LBA Group, Inc

Peak voltage is determined by the base impedance and carrier power + modulation. On any AM station these days, a 150% peak modulation figure should be used (125% modulation allowed by FCC rules plus a 25% safety factor). For example, station B has a base impedance of 50Ω (typical 90° guyed tower) and a carrier power of 50 KW. The peak modulation power will be 600 KW. Thus, the peak voltage will be E_peak = √P_peak x R, or E_peak = √600,000 watts x 50 ohms or 5,477 volts. With higher base impedances, the base current goes down but the base voltage goes up. A typical 140° tower will have a base impedance of 760Ω. Thus the peak base voltage for a 50KW carrier power modulating at 150% will be 21,354 volts. This is the worst case scenario, as few installations are designed that way and every tower impedance is different than the theoretical self impedances given.

For towers over 140 electrical degrees, it is better to use an isolation transformer because of the RF peak voltage/peak current conditions at the base of towers that are electrically tall. The ring transformer design minimizes stray inductance or capacitance at the base of such towers. Austin Insulators (previously Austin Decca) makes a variety of tower base ring isolation transformers. These have varying input and output voltages.

Diagram of typical Austin Ring transformer courtesy Austin Insulators, Inc

I have seen these at many locations over the years. They are rugged and add only a small bit of capacitive reactance to the base of a typical tower. They also completely isolate the building AC mains from the tower. For very high-power installations, Austin has the A-9600, which was designed for the Navy VLF transmitter towers where base peak RF voltages can run 200,000 volts or more:

Austin A-9600 oil filled isolation transformer — Austin A-9600 oil-filled isolation transformer courtesy Austin Insulator, Inc

Voltage drop is another consideration in tower lighting design. Long runs from the transmitter building to the tower should be on heavy gauge wire and at 230 volts if possible. FAA Circular AC 150/5345-43F “Specification for obstruction lighting equipment” advises that the input voltages for incandescent lighting systems vary by not more than ±3%. Additional tower lighting and painting information can be found in FAA Circular AC 70-7460-1K.

Ford begs Broadcasters: “Please install our wonderful HD Radio product.”

In an open letter to broadcasters, the entirety of which can be found here: Ford Exec Writes Open Letter to Broadcasters, Jim Buczkowski, whose official title seems to be “Henry Ford Technical Fellow and Director, Electrical and Electronic Systems Research and Innovation Ford Motor Company,” nearly begs broadcasters to install HD Radio technology at their radio stations.

One thing that seems to be missing from the open letter, is something mildly important called: Disclosure. According to iBiquity’s own website, Ford Motor Company is an investor in the technology. Other investors include:

Clear Channel
CBS Radio
Grotech Capital Group
J.P. Morgan Partners
New Venture Partners
FirstMark Capital
Harris
Texas Instruments
Visteon

Not an inclusive list by any means, but something to keep in mind when reading the letter or the latest iBiquity advertising in various trade magazines.

Back to the letter; the cliff notes version is this:

Through the use of HD Radio, AM/FM broadcasters can now embrace the digital age
Drivers now have many choices for in-car entertainment, including satellite radio (Sirius XM) and online services (Pandora, et.al) that offer “Crystal Clear” audio
Through Satellite radio, MP3 players, and IP streaming services, drivers now have extra features like Title, song, and artist; Song tagging; iTunes; album art, etc which they have become accustomed to
Installing HD Radio will be a big upgrade and make AM/FM stations on par with those “digital age” services

For the first part, there is not a single broadcaster in the country that is not already aware of HD Radio. Every radio station manager and owner knows that it exists, but most people in the general public do not. Radio stations are hesitant to install HD Radio equipment because it is expensive, has a questionable return on investment, is unimpressive, and technically dubious.

Making the comparison to Satellite Radio and or IP streaming services, which all require subscriptions or data plans, is a bit of a stretch. Someone who will pay a fee for in-car entertainment is usually a tech geek. As the subscription rates for Sirius XM show, that works out to about seven percent of the US population (~20 million subscribers/~300 million people). It is a bit harder to nail down those who listen to streaming products like Pandora, iHeartRadio, or other webstreams in their cars, but I’d estimate not more than ten percent do.

While 3G and 4G wireless services are great, it still does not have the same coverage as standard and FM broadcasting stations. The last time I tried to listen to Pandora in my vehicle, it kept dropping out and was not easy to deal with. With TuneIn radio, I had the same experience during urban, suburban, and rural driving. Thus, the “Crystal Clear” reception is also a bit of a misnomer.

Further, fooling around with iPods, iPhones, TuneIn, Pandora, etc while driving is not the best idea. Even on vehicles with built-in IP connectivity or satellite radio, looking for song titles and other information while driving is not recommended. Thus, the value-added services of HD Radio are of questionable at best in a moving vehicle.

I hate to keep beating a dead horse, but for as long as the iBiquity crew continues to spout disingenuous bull sh!t about their failed technology, I’ll keep posting about it.

Modulation Dependent Carrier Level

Coming to an AM transmitter near you. The FCC announced that starting immediately, stations can employ Modulation Dependent Carrier Level or MDCL technology on AM transmitters. According to Public Notice DA 11-1535 (.pdf):

Use of MDCL technologies requires a waiver of Section 73.1560(a) of the Commission’s Rules, which sets upper and lower limits for an AM station’s operating power. We hereby establish procedures for AM broadcasters to seek a rule waiver in order to use energy-saving MDCL technologies.

Several transmitter manufacturers offer some version of MDCL in their newer models with the ability to update some older models. Harris Corporation offers something called “Amplitude Modulation Companding” (AMC) and “Adaptive Carrier Control” (ACC). While Nautel includes an option called “Dynamic Carrier Control” (DCC) on all NX series transmitters with the ability to upgrade some older transmitters. Continental offers Controlled Carrier Level Modulation (CCM) on later-model shortwave transmitters by installing SSM modulator, which can be retrofitted.

Nautel’s Dynamic Carrier Control (.ppt) (.pdf available here) reduces the carrier level during moderate modulation periods. The effect of this is to increase the perceived loudness at the receiver. During higher modulation periods, the carrier is increased to prevent distortion. The net effect is between 3 – 6 dB carrier reduction. During quite periods, the carrier is returned to full power to reduce noise.

Nautel AM Dynamic Carrier Control wave forms — Nautel Presentation on Dynamic AM carrier control

The potential savings are from 20-40%, which for a 10 or 50 KW station, would represent a significant reduction in the power bill. For a 50 kW station running an older transmitter, the savings would fall in the $37,000 to $56,000 per year range. In some cases, smaller stations may be able to get rid of a demand meter, which would also represent significant savings. The threshold for demand meters around here is 5,000 KWh per month.

The FCC further notes that:

The reduction in AM signal power at certain modulation levels inevitably exacts some penalty upon audio quality. Depending on the content of the audio program, MDCL algorithms may introduce some audio distortion or may decrease the signal-to-noise ratio in the receiver. In addition, MDCL algorithms may erode coverage slightly at the fringes of the AM station’s protected service area. Both the long experience of transmitter manufacturers and broadcasters abroad, and the initial reports from experimental operations in Alaska however, indicate that such adverse effects are generally imperceptible.

This would be especially true for higher-powered stations that stand to save the most money.