Digital AM Transmitter Page

WABC 77 Transmitter Building, Lodi, NJ,
which looks essentially the same on the outside now as it did back in 1966.
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These photos were taken at the WABC transmitter site in Lodi, NJ on February 16, 1997, 31 years after my visit to the site in 1966, which can be seen on my other JIM'S WABC RADIO 77 Transmitter Page. The WABC transmitter site has essentially gone through two generations of technological changes between these two pages.

I owe my thanks to CE, Bill Krause - WA2HDE, who took me on the tour and patiently answered my numerous questions.

The background was created from a photograph of the stack of solid state output units, each of which is capable of delivering approximately 1.5 KW to the total power of the transmitter.
Jim Hawkins - WA2WHV.

The New York Control Room

My friend, Brian Dee, working the board at WABC!

Photo Supplied by Tony (Denicola) Dee

See my article about Brian as it appeared in Radio World.

WABC 770KHZ, New York, NY
Located on NJ Route 17, Lodi, NJ. February 16, 1997
The single, bottom-loaded, omnidirectional tower height is 648 ft.

Photo on right shows much smaller standby tower.


Closeup of Harris Model DX-50 50KW Digital Transmitter.

The first thing I noticed when I entered the transmitter room was that the new transmitters are much quieter than the old vacuum tube transmitters, which used enormous blowers to cool the tubes. The roar from the old vacuum tube transmitters sounded like the engine room of a ship!
These units are extremely reliable with about .01% downtime and need very little attendance.


(1966 and 1997)

Wide view of transmitter room with 50KW XMTR on right, Power Control panels on left and far end and console in foreground as it was in 1966.

Same view of room in 1997, showing two Harris transmitters that have replaced the old GE Unit. An equipment rack is in the middle of the room and the console is gone.

Two Harris Model DX-50 Solid State 50KW Transmitters; one is on the air while one is the standby.

View of the 2 DX-50s on the right and assorted equipment racks on the left.

A full shot of the older Harris Vacuum Tube Standby Transmitter, which is now out of service. Now replaced with Nautel Units.


(1966 and 1997)

Room behind GE 50KW Transmitter in 1966 showing aisle between transmitter and modulation transformer cage. Note large cooling duct in the rear.

Same room area in 1997 showing rear view of HARRIS DX-50s showing how shallow they are. Everything is contained within those cabinets. There are no huge external transformers, chokes or capacitors. You could probably fit both in an average basement!


Within the room behind the WABC transmitters is a 10KW Solid State Harris DX-10 Transmitter, which served as a backup for radio station WINS 1010, while they were reconstructing the towers at the WINS site. Once the work was completed at WINS, this transmitter sat dormant, until moved to the WCBS site on High Island, NY, where it is now. (Updated 1/19/2003)


This flow chart is found on the front of the DX-50 with indicator lights within the symbols to show the status of the various major components.

A/D Converter. This is where the audio signal enters the transmitter and is converted to a digital output, the form in which audio is now stored on CDs. A digital audio signal is a stream of binary numbers, each representing a sampled point along the original audio waveform.

The output of the A/D converter is then fed to the modulation encoder or "integrator" board. You could say this board IS the modulator! This board essentially replaces what would have been a 25+ KW analog high level modulator used in the old vacuum tube transmitters, which is an incredible savings in space and power!

The integrator board controls these stacks of output modules, which make up a total of 64 output modules. There are two sets of these. Rather than a huge tube or pair of tubes delivering the RF output signal in CLASS C, here it is delivered by many modules, each representing a small part of the overall signal. They operate in CLASS D operation, which means that they switch ON and OFF, producing a perfect squarewave.

This is one of the output modules containing 8 output transistors. The module is capable of delivering approximately 1.5 KW output. The input and output is a squarewave whose fundamental frequency is that of transmitter's radio frequency. In the case of WABC, it is 770KHZ. Note the Quarter next to the unit for size comparison. Each has a pair of green lights to show the modulation occurring and a red trouble indication light.


by Jim Hawkins - WA2WHV


Amplitude modulation is a process which uses the audio signal to modulate or vary the actual power of the radio frequency signal called the carrier. When no modulation is applied, a steady radio frequency signal is transmitted, creating a "quiet" spot on your radio dial. When analyzed, the AM signal is quite complex. It can be shown to be made up of 3 basic components: a steady carrier, an upper sideband signal and a lower sideband signal, which I will not explain here. AM is susceptible to static interference from lightning, motors, light dimmers, fluorescent lights, etc. When I am listening to an AM program at home and hear a steady, static "buzz", I find that it is usually one of the light dimmers in one of the bathrooms. I also remember the buzz of the electric shavers on an AM station playing while sitting in the barber's chair and hearing a POP when the shaver was turned on or off. These sources of interference are, themselves, forms of AM signals, which is why they easily infect your listening pleasure on AM with undesirable sounds.

In contrast, FM (Frequency Modulation), discovered by Edwin Armstrong, is a method where the carrier frequency is varied or modulated rather than the amplitude of the carrier. It is almost free of the aforementioned types of interference and is now the most popular means of broadcasting music programming. AM remains the choice for "TALK/NEWS" radio formats. AM broadcasting signals also have a longer range than FM, which is more because of the frequency at which they are transmitted than the method of modulation.


Digital Amplitude Modulation was invented by Harris Senior Scientist Hilmer I. Swanson. In older vacuum tube AM broadcast transmitters, the carrier was modulated by using an analog modulator, which was essentially the output of a powerful audio amplifier superimposed on the supply voltage to the transmitter. For reasons I won't explain here, this type of modulation required an audio signal which was 1/2 the power of the transmitter. That is, for a 50,000 watt transmitter, you needed a 25,000 watt audio amplifier! That is a slightly simplified explanation, but it suffices for this comparison. These modulators also required large modulation transformers to couple the modulation output tubes to the power supply system.

In the Harris solid state digital transmitters, the audio signal enters the transmitter through an A/D converter, which converts the signal to a digital audio form, much like you would find on a CD or a "WAV" file on your computer. The digital audio is a stream of binary numbers, which represent the amplitude of points sampled at particular time intervals along the audio input signal.

The digital audio output of the A/D converter is fed to the "integrator" board, which contains logic which uses the digital input information to turn on (and off) the appropriate RF modules. There are four module step sizes: 100 Watt, 300 Watt, 500 Watt and 1000 Watt. The integrator module logic figures out which modules to switch on or off during the course of modulation. For example, as the audio waveform value goes up, a 100 watt module turns on. Then it is turned off and replaced by a 300 watt module, then a 500 watt module, then finally a 1000 watt module. As the audio waveform value continues on the upward slope, the 100 watt module turns back on, but this time, leaving the 1000 watt module turned on. Then the low power switching sequence continues as before, but this time, adding to the first 1000 watt module. As the audio signal value continues to climb, 1000 watt modules are added together and the smaller modules are swapped to handle the smaller steps. So, the output power step sequence might be 100, 300, 500, 1000, 1100, 1300, 1500, 2000, 2100, 2300, 2500, 3000 and so on, until it reaches the positive peak of the modulation signal. The same thing happens as the audio values swing toward the negative peak, but this time, the small module steps are swapped to smaller steps, and the 1000 watt modules are continually shut off as the modulation signal moves toward the negative peak. The lowest possible output is 100 watts, where only one 100 watt module is turned on. This occurs only at maximum modulation percentages. This is a form of pulse modulation, but is not like the Pulse-Step modulation used in ASEA BROWN BOVERI shortwave transmitters, which employ a combination of this additive stepping and pulse width modulation. As an additional bonus, if half of an output module fails, the integrator detects it and switches to a working module. So, a single module failure results in no reduction of power. In fact, ten modules could fail without it being noticed until the engineer actually goes to the transmitter and notices the failure lights turned on.

The multi-ton, multi-KW modulator is thus replaced by a few small printed circuit boards!

There are other types of digital modulation such as PWM (pulse width modulation) and PPM (pulse position modulation), which I will not discuss here.


Each output module is fed with a square wave from the driver section and outputs a square wave. The square wave output from each module is fed through a coil wrapped around a toroid. A pipe runs through the center of all the toroids, acting as a secondary transformer winding for all the toroids, which picks up the combined output of all energized toroids. The toroid filters most of the square wave harmonic components out, leaving an almost pure sine wave which represents the radio signal. There are other filtering networks before it gets to the output network in the transmitter, so by the time it gets to the output, the signal is a pure sine wave.

You can think of the modules as pistons in an engine, each putting out bursts of power, and the toroid coil as a flywheel which smooths the oscillation. In fact, if just one burst of power were applied to the toroid, it would continue to "ring" momentarily just as a fly wheel would continue to spin if you gave it one push. Without continued pulses of energy, the energy would eventually spin down due to losses. In the case of a flywheel, the losses are due to friction. In case of the toroid, the losses are due to resistance in the conductors. A toroid transformer is a donut shaped piece of iron, with coils of wire wrapped around it.

When there is no modulation (silence), 48 modules will be turned on simultaneously to generate approximately 55 KW (5 KW is lost on the way to the antenna). To modulate the transmitter, modules are turned on and off. As you turn more modules on, you have more RF carrier and when you turn more off, you have less RF carrier.

The digital technique used in these transmitters is extremely efficient (90%) as opposed to about 64% with the old high level plate modulated vacuum tube transmitters. That is, the older transmitters might use 78,000 watts to obtain a 50,000 watt output signal where the modern, solid state transmitters might use 55,000 watts to obtain a 50,000 watt output signal. That's a 23,000 watt savings and quite a difference in the electric bill! The voltage applied to the output units is 240V and current runs about 300 AMPS. The power supply is fully contained within the cabinet and basically consists of a transformer which steps 440 VAC down to 240 VAC with some big diodes for rectification.

From power line to output, the efficiency is approximately 78% for these transmitters.


In the older transmitter, huge blowers were used to cool the tubes. The air heated by the tubes was typically around 130 degrees F. With the new transmitters, relatively small air conditioners are used for cooling. Upon putting my hand in front of the output air vent, the air was barely warm.

See the Harris DX SERIES page for more information.

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This text is © Copyrighted 1997 by James P. Hawkins and may not be republished on the web or elsewhere, without the written consent of the author.

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Equipment racks containing, Audio processors, Stereo Processor (not used), Satellite receivers, etc.

Orban 9100B AM Optimod Audio Processor Unit. This unit conditions the audio signal to sound clearer and stronger at the receiving end. It also compensates for hi and low frequency rolloff on AM receivers. Click on the links for more info on this interesting device.

Satellite feed dishes.

Air cooling metering and control panel.


Antenna coax switch.

Maze of coax (hardline).

View of inductors and capacitors inside tuning house located at the base of the antenna tower. Note: two Jennings vacuum tuning capacitors (with the control knobs on them) which allow tuning for both sidebands. The dark colored inductor coil in the rear is part of a resonant circuit to filter the WINS signals if the emergency transmitter and antenna are so used. Due to the strong magnetic fields in the coils, the actual modulation signal could be heard, making the coils act as speakers as a side effect. Press to hear what coils sounded like when they

View of inductors and capacitors inside tuning house leading out to the base of the tower, visible through the window at the far wall.

Huge insulator supporting the entire weight of the 648 ft. tower in addition to wind forces moving the tower. Notice "Johnny Balls" which provide a gap large enough to prevent arcing from the normal 15,000 volt radio signal, but close enough to cause lightning hits to be diverted to ground, protecting the transmitting equipment.

Wider view showing base of tower, fed from tuning house through lucite window. I was told that the antenna impedance is 121 OHMS.

Looking up from base of antenna.


Installed by FEMA (Federal Emergency Management Agency)

Two emergency generators. On the left is a 250KW generator. On the right is a 150KW generator.

Full shot of 250KW generator. There's enough fuel and power to run everything in the joint for a month, which is a FEMA requirement!

There are three, 5000 gallon diesel fuel tanks underground to supply the fuel to these generator engines.

Accessed times since February 17, 1997.


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Contact: Jim Hawkins