AM Transmitters – Page 7 – Engineering Radio

The CCA AM1000D

Still in use as the main transmitter after 42 years at WCKL 560 KHz, Catskill, NY.

CCA AM1000D transmitter, WCKL Catskill, NY

The last seven years or so, it has not had much use, the station being caught in some strange LMA with Clear Channel, then sold to the Black United Fund of NY something or another. They basically had it dark, turning it on for a few days each year to as not to lose their license. Finally, they LMA’d it to Family Broadcasting (not to be confused with Family Radio). There are rumors of a sale, but it remains to be seen.

They have been broadcasting an eclectic, free-form programming style which appears to be the work of mostly volunteers.

The station was first licensed in 1970, thus this is the original transmitter:

CCA AM 1000D name plate, WCKL Catskill, NY — CCA AM 1000D nameplate, WCKL Catskill, NY

Towers are 446 feet tall, which works out to 90 degrees at 560 KHz.

The station is licensed to Catskill, but the transmitter site is located in Hudson, across the river. With the current ownership situation in flux, I would characterize the operation as “tenuous.”

The transmitter itself is a pretty simple high level modulation tube type transmitter. It uses 4-400 tubes, like the RCA-BT1AR transmitters, and is built around a similar design, which makes sense as they were designed and built by former RCA engineers. One of the CCA principles, Bernie Wise, still makes Energy Onix transmitters about 10 miles away in Valatie, NY.

Parts are fairly generic and still available. Things like the modulation transformer may be harder to come by, however, Goodrich Electronics, Harbach Electronics, Energy Onix and others will be able to steer one in the right direction. I’d put up a schematic if I could find one.

I find these older tube-type transmitters often sing with modulation, especially the higher frequencies. That sound and the soft sound of the blower moving air is the sound of radio, at least to me.

Implementing MCDL (Modulation Dependent Carrier Level)

Since the FCC waved some of its rules regarding carrier power and carrier shift on the AM broadcast band, AM stations are now able to implement MCDL or DCC (Dynamic Carrier Control) technology to save money on their electric bills. This technology has the potential to save tens of thousands of dollars for higher-powered AM stations (high power=greater than 10 KW carrier level).

On a standard AM broadcasting station, the carrier represents two-thirds of the energy being transmitted, with the modulation index containing the other one-third. The carrier contains no information; it is simply there on the center frequency at the power level authorized by the station’s license. Thus, if the carrier can be reduced without affecting the quality of the broadcast reception, it will reduce to the overall power consumption of the transmitter. In areas where electric costs are high, the savings can be substantial.

There are various ways to accomplish this. The first is called Dynamic Carrier Control (DCC), where the carrier voltage is reduced during moderate modulation levels (between 20-50%) and restored during peaks. This reduces the output power during average modulation, restoring most of it during quiet periods and peaks. The next is Dynamic Amplitude Modulation (DAM), which is similar to DCC. The most savings will be noted with less heavily processed programming such as talk radio because the higher the average modulation density is, the less the MDCL circuit reduces the carrier voltage level. The little graph in the diagram shows the reduction in the carrier voltage vs. modulation levels.

Nautel DAM block diagram, courtesy of Nautel, Ltd.

Finally, Amplitude Modulation Companding (AMC) reduces the voltage in both the carrier and modulation product during peaks. This results in better savings for higher-density modulation indexes. It is also the most transparent of the three schemes, as the carrier is restored to full power during periods of low or no modulation levels. During peak modulation, the reduction does not drop the power level below the un-modulated carrier level. The little graph in the diagram shows the reduction in the carrier voltage vs. modulation levels.

Nautel AMC block diagram, courtesy of Nautel, Ltd

Nautel has done extensive work on MDCL and includes several algorithms in their NX series transmitters. For older Nautel transmitter models such as ND, XL, XR, and the J-1000, there is an outboard exciter, which is in a one-rack unit chassis. Older transmitters may need a simple field modification to create a DC-coupled audio input. The cost for the upgrade is approximately $5,000 USD, however, check with the regional Nautel sales rep.

Once the system has been installed, there are several things to be aware of:

Modulation monitors may not work properly, especially older units, which will show significant carrier shifts and have carrier alarms. Belar AMMA-2 modulation monitor is specifically built to work with MDCL transmitters.
When making field strength readings, the MDCL circuitry must be turned off to get accurate readings.
For stations running IBOC, the amount of carrier power reduction may need to be experimented with, as the effect of the carrier reduction may cause the transmitter to exceed the NRSC mask.

Currently, only Nautel and Harris are selling MDCL transmitters. I spent several minutes poking around the Harris website and looking through their product brochures for the DX series transmitters and no mention of DCC o MDCL was found. I’d be happy to include any information from Harris if it were made available.

The Nautel AMPFET 1

The Nautel AMPFET series transmitters date from the early ’80s through early ’90s. They were Nautel’s first attempt at MF Broadcast transmitters and were quite successful. This particular transmitter was installed in early 1990 at WBEC in Pittsfield, MA:

Nautel AMPFET 1 AM broadcast transmitter

I believe Nautel got started making MW transmitters for Marine Radio stations, Aeronautical and Marine radio beacons, and similar equipment. Their early equipment is very rugged and designed for rough/continuous service. The early solid-state broadcast transmitters like the AMPFET were not hot pluggable but who cares, they almost never break. The design is simple, and efficient and it sounds good on the air.

Early transmitters were housed in racks that were much shorter. In later versions, the racks became larger to standardize the transmitter size with comparable units of the day. Inside this cabinet, there is a lot of empty space.

The design is modular, RF modules and power supplies can be removed from the transmitter for repair, unlike the Harris AM transmitter products of the same or later periods.

There later AM transmitter versions built on the AMPFET experience.

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.