August 1, 2000 marked the end of WMAQ after nearly 75 years of broadcasting with those call letters. 670 is now WSCR, The Score. This author was employed by WMAQ from 6-15-92 to May 21, 1999 as a station engineer with the primary duties being the care of the transmitter site, component level troubleshooting of broadcast equipment as well as studio repairs. Many thanks to Scott Childers for providing a link to this website and I am proud to be part of his tribute to WMAQ. If you have any questions about WMAQ's transmitter site, please email me.
When I started this website, I did not have any particular audience in mind. Now I have decided that the audience makeup I'm targeting are people in broadcasting business as well as people with a ham license or similar technical background. If you are a techie of some sort, you will find this website interesting. If some of the information on this webpage goes over your head, not to worry because there is plenty of other information on this site that should suit your interest and curiosity. The contents of this website are totally the responsibility of the author, so if you see a problem here anywhere, blame me and not WMAQ.
This project was started entirely on my own initiative and was done for several resons. First, I wanted experience designing and publishing a webpage. Secondly, I wanted to pick a topic in which I am passionately interested. Third, maybe I can get someone interested in broadcast engineering by showing the kinds of things and systems a broadcast engineer works with. Lastly, we train many operators at WMAQ in a year's time. At the station, we talk about 'the transmitter' often. I thought it would be nice if these trainees could visit my website and see what 'the transmitter' actually is. The studio complex and the transmitter complex are separated by several miles. It is a rarity that anyone from the studio other than engineers, ever visit the transmitter site.
WMAQ began transmitting at 50,000 Watts from this building on September 15, 1935. Our first 50 KW transmitter was a Westinghouse 50B, low level plate modulated and water cooled. The low level plate modulated stage was 5 Kilowatts carrier power followed by a 50 KW linear amplifier, 200 KW PEP. The Westinghouse transmitter was used from 1935 until 1960 when it was replaced by one of the first RCA BTA-50H Ampliphase transmitters. WMAQ was owned by NBC at the time, which was owned by RCA. Below is a photo of our old RCA BTA-50H
The WMAQ auxiliary 50 KW transmitter can be seen in the foreground with the Continental Electronics 317C-3 in the background. The Auxiliary transmitter is a Harris MW-50B and was manufactured in 1978 at the Harris plant in Quincy, Illinois. The MW-50B is used on the air once per week so we can be confident that it will be functional in event of failure of our main transmitter. Both transmitters are built into the wall which simplifies the air conditioning requirements. The transmitter room is air conditioned to about 72 degrees which extends the operational life of the smaller electronic equipments, such as STL receivers, remote control equipment, audio processors, etc. The area behind the transmitter, which is accessable through the double doors seen between the two transmitters, acts as an air plenum and is not air conditioned. It is not uncommon on a hot summer day for this area to reach over 100 degrees!
This is a rear view of WMAQ's auxiliary 50 KW transmitter, the Harris MW-50B. You can see the plate transformer, which is about 4 ft x 4ft x 4ft, in the corner on the left side of the picture. It is a three phase unit with the high voltage rectifier stacks being contained within the transformer enclosure. The rectifiers can be accessed by removing the white cover on the front of the transformer. The DC voltage is 25 Kilovolts at about 4.5 amps. The transformer itself is oil cooled. The plate contactor is in the big white cabinet on the far wall between the transmitter, which is on the right, and the plate transformer. It does not show up very well because the wall is the same color as the contactor cabinet.
This is a rear view of the WMAQ main transmitter area. Looking at the photo on the left, the big brown cabinet up against the far wall of the room is the plate transformer, plate voltage regulator and plate contactor enclosure The big brown cabinet to the immediate right is the antenna switching cabinet. The unit next to it, but not visible, is our 75 KW dummy load. The smaller picture on the right shows the 75 KW dummy load, which is forced air cooled. The smaller picture also shows a good view of the coaxial patch panel with the jumpers in place. Remember that the transmitter average power output with a 50 KW carrier during 100% modulation is 75 KW! Obviously, the dummy load must be sized to handle the maximum expected average power. The transmitter is powered by 480 volts, three phase. The big coaxial jumpers on the front of the antenna switch cabinet allow us to bypass a bad antenna transfer switch.
This is the RF meter panel that WMAQ uses to measure the RF current flow to the dummy load, main tower and auxiliary tower. When this picture was taken, the Continental was on the air at 50 KW and 20.95 Amperes of antenna current and the MW-50B was on the dummy load with 18 Amperes of dummy load current at about 16 Kilowatts. Note that the antenna ammeters are measuring the current flow into the base impedance of the tower. The meter signals from this meter panel are fed to our MRC-1 remote control so the WMAQ operators at the studio can read the antenna current on either tower or the dummy load. It is left as an exercise for the reader to calculate the value of the real component of WMAQ's antenna base impedance.
What is the dummy load current if the MW-50 is running 50 KW on the dummy load? Assuming WMAQ's auxiliary tower has a base impedance of 10 Ohms, what should the Aux Tower antenna current be if it is fed 50 KW? (You will be surprised!) Almost invariably, a broadcast tower has a real and reactive (immaginary) component to its base impedance. For calculating power, the FCC is only concerned with the real component of the antenna impedance for making this determination. Do you know why the reactive component is ignored? The base impedance of the antenna is matched to our four inch 50 Ohm transmission line with an impedance matching network at the base of the tower. It is common to place the antenna ammeter at the output of the matching network before the antenna.
This is the audio patch bay that is used for diagnostics and testing. All transmitter site audio sources and destinations appear on the patch bay. In the event of a system failure, an alternative audio path can be set up to feed program audio from another source to any desired transmitter. Proper audio levels are critical in a radio station. All critical audio points can be accessed with a VU meter to verify that the audio levels are appropriate. In addition to this flexibility, an audio oscillator and distortion analyzer can be connected to the system via the patch panel to verify proper system operation. The patch panel is probably one of the most valuable troubleshooting resources at the transmitter site.
This is WMAQ's RCA BTA-5R1 5 Kilowatt transmitter. This is our second string backup transmitter and is used only in a dire emergency. Occasionally, this transmitter is used when tower riggers are doing work on our towers. If we ran with 50 KW, the workers would receive severe RF burns whenever they touched any of their hoisting cables. It is also probable that they would be plagued with drawing arcs anytime they toched anything metal. This transmitter was bought in January of 1960 and has 54,000 hours on it. Most of which were accumulated by running on the dummy load and not on the air! It is getting increasingly more difficult to get parts for this thing!
This is a view of the remote control interface relays. These provide isolation between the transmitters and remote control equipment. The relays are plug-in type, 24 Volts, DC made by Potter and Brumfield. Plug-in relays are outstanding for mission critical applications because they can be swapped out very quickly in an emergency. Note the outstanding installation arrangement.
This is the test setup used for adjusting and testing the harmonic distortion and AM stereo performance of the transmitter. The instruments are, starting in the upper left and proceeding clockwise, a Potomac Instruments Audio Oscillator and Attenuator set, next is the Potomac Instruments Distortion analyzer, a Motorola AM Stereo Modulation monitor, and finally a Hameg oscilloscope. This equipment is normally rack mounted in another room unless there is some tuning or measurements to do. The rack mount arrangement is convenient for day to day checks, but for actual tuning, testing and adjustment, it is more convenient to remove the equipment from the rack so it can be closer to the transmitter. It is easier therefore to make an adjustment, check the appropriate meter on the transmitter and then check the results on these instruments.
The Oscillator is used to set up sine wave test signals of any given frequency from 50 Hertz to 15 Kilohertz. (It will actually cover a wider range of frequencies than this, but 50 to 15K is all the transmitter can handle) The attenuator set is used to set the audio level going into the transmitter so as to not overmodulate it. The modulation monitor is used to verify that the test signals are not causing overmodulation. It also acts as a radio receiver by demodulating the AM emvelope and reproducing the original test signal. This output then goes to the distortion analyzer.
The distortion analyzer is used to measure harmonic and intermodulation distortion. The oscilloscope is connected to the filtered output of the distortion analyzer so we can see the makeup of the distortion products. The distortion analyzer is a filter type analyzer. It filters the test tone out and assumes that what's left over is distortion. Well, the signal could be actual distortion products or it could be power supply ripple, 60Hz components from the test setup or ground loops, crosstalk products from the transmitter that is on the air and many other possibilities. It is a complete waste of time to make distortion measurements without an oscilloscope!
This is a closeup of the 317C-3 RF power amplifier showing some of the panel meters used to measure the critical operating parameters. The power amplifier consists of two 4CX35,000C power tetrodes, which cost about $6500 each and last about four to five years. The two 4CX35000C tube anode coolers can be seen through the viewing window. Attached to the anode assemblies are two grey air chimnies. Maintaining proper operating parameters is important to the longivity of the tubes. At the end of their life, they are sent to a tube rebuilder that replaces all internal consumable parts. After so many rebuilds, the tubes have to be thrown away. The built in oscilloscope seen on the left is used to aid in adjusting the RF amplifier, which is a modified Doherty amplifier. The original Doherty amplifier was designed to be a linear amplifier. Continental Electronics modified this design by applying the modulating audio to the screen grid of the peak and carrier tubes, thereby reducing the modulator power requirements compared to that which would be needed for conventional plate modulation. This arrangement is also more efficient than the Doherty linear amplifier. The modified Doherty arrangement is called Screen Impedance Modulation. In 1974, Continental built transmitters for Radio Belgrade and other organizations using this design up to 2000 KW carrier power!
This is WMAQ's auxiliary self-supported tower. It is about 250 feet tall and sits on four big insulators. Thus, it is considered as a series fed tower. This tower is used whenever the 740 foot guyed tower (not pictured) is in need of maintenance. This tower was a real life saver for WMAQ back in the fifties when their main tower fell to the ground. WMAQ can switch to this tower at any given time whether or not there is an operator at the site or not. It can be switched to the air at any time by using the remote control system.
WMAQ has two 240 Volt, three phase 75 KW generators, one of which is here pictured. At one time, these were operated in a parallel arrangement, thereby giving 150 KW of available power. This proved troublesome and unreliable, so they are not operated together anymore. The 75 KW generator can run the lighting, small electronics, HVAC system and the BTA5R1 5KW transmitter. Unfortunately, WMAQ cannot operate at 50 KW while running on these generators. The generators were given to WMAQ in May 1946 by the US War Department. The generators are not useable at the present time. WMAQ also has a 200 KW generator, which is not pictured, that can run the entire transmitter site at 50 KW. However, only one transmitter can be operating while the station is on generator.
Here are the answers to the homework questions! The dummy load current is 31.6 Amps when the transmitter is putting out 50,000 Watts. The auxiliary tower antenna current is about 69.4 Amps when radiating 50,000 Watts carrier power ( I told you that you would be surprised! ). The real component of WMAQ's main antenna impedance is 114 Ohms. The reason that the reactance part of an antenna base impedance is ignored for purposes of calculating power is that reactance, by its nature, is incapable of dissipating or radiationg power. If a transmitting antenna could not radiate power, there could not be radio waves! By the way, these answers were calculated using a slide rule, so please do not beat up on me if you get better answers on your calculator. If you got all these answers correct, you should consider being a broadcast engineer or an electrical engineer!