August 1, 2000 marked the end of WMAQ after nearly 75 years of broadcasting with those callletters. 670 is now WSCR, The Score. This author was employed by WMAQ from 6-15-92 to May 21, 1999 as astation engineer with the primary duties being the care of the transmitter site, component leveltroubleshooting of broadcast equipment as well as studio repairs. Many thanksto Scott Childers for providing a link to this website and I am proud to be part of his tributeto WMAQ.If you have any questions about WMAQ's transmitter site, pleaseemail me.
When I started this website, I did nothave any particular audience in mind. Now I have decided that the audience makeup I'mtargeting are people in broadcasting business as well as people with a ham license or similartechnical 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 otherinformation on this site that should suit your interest and curiosity. The contents of thiswebsite are totally the responsibility of the author, so if you see a problem here anywhere, blameme and not WMAQ.
This project was started entirely on my own initiative and was done for severalresons. First, I wanted experience designing and publishing a webpage. Secondly, I wanted topick a topic in which I am passionately interested. Third, maybe I can get someone interestedin broadcast engineering by showing the kinds of things and systems a broadcast engineer workswith. 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 websiteand see what 'the transmitter' actually is. The studio complex and the transmitter complex areseparated by several miles. It is a rarity that anyone from the studio other than engineers, evervisit the transmitter site.
WMAQ began transmitting at 50,000 Watts from this building on September15, 1935. Our first 50 KW transmitter was a Westinghouse 50B, low levelplate modulated and water cooled. The low level plate modulated stage was5 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 byone of the first RCA BTA-50H Ampliphase transmitters. WMAQ was owned by NBCat 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 ContinentalElectronics 317C-3 in the background. The Auxiliary transmitter is a Harris MW-50B and wasmanufactured in 1978 at the Harris plant in Quincy, Illinois. The MW-50B is used on the air onceper 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. Thetransmitter room is air conditioned to about 72 degrees which extends the operational life ofthe smaller electronic equipments, such as STL receivers, remote control equipment, audio processors, etc. The area behind the transmitter, which is accessable through thedouble doors seen between the two transmitters, acts as an air plenum and is not air conditioned. It is not uncommon on a hot summer dayfor this area to reach over 100 degrees!
This is a rear view of WMAQ's auxiliary 50 KW transmitter, the Harris MW-50B. Youcan see the plate transformer, which is about 4 ft x 4ft x 4ft, in the corner on the leftside of the picture. It isa three phase unit with the high voltage rectifier stacks being contained within the transformer enclosure. Therectifiers 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 wellbecause the wall is the same color as the contactor cabinet.
This is WMAQ's main 50 Kilowatt AM transmitter. It is a ContinentalElectronics 317C-3, installed in 1984. The transmitter is about 12 feet wide, 7 feet tall andabout 5 feet deep. A 50 KW transmitter of this vintage cost about $225,000 when new. Broadcasttransmitters have a common useable life as a main transmitter for about 15 years in the Chicagomarket. Beyond this age they become too unreliable and require too much maintenance tokeep them on the air. When a main transmitter is retired, it is usually kept as an auxiliarytransmitter which is then used if the main one fails or is down for repairs. In theory, a transmitter couldbe used indefinitely provided the parts and skilled engineers are available to keep it working.When transmitters get so old that they are not useable even as an auxiliary, they are sold to usedtransmitter dealers and are sold to stations in a smaller market or even overseas. On the usedmarket, a 317C-3 is worth about $10,000 depending on condition.
This is a rear view of the WMAQ main transmitter area. Looking at the photo on the left, the big browncabinet up against the far wall of the room is the plate transformer, platevoltage regulator and plate contactor enclosure The big brown cabinet to theimmediate right is the antenna switching cabinet. The unit next to it, but not visible, isour 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 coaxialpatch panel with the jumpers in place. Remember that the transmitter average power output with a50 KW carrier during 100% modulation is 75 KW! Obviously, the dummy load must be sized tohandle the maximum expected average power. The transmitter is powered by480 volts, three phase. The big coaxial jumpers on the front of the antennaswitch 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, theContinental was on the air at 50 KW and 20.95 Amperes of antenna current and the MW-50Bwas on the dummy load with 18 Amperes of dummy load current at about 16 Kilowatts. Note that theantenna 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 remotecontrol so the WMAQ operators at the studio can read the antenna current on eithertower or the dummy load. It is left as an exercise for the reader to calculate thevalue of the real component of WMAQ's antenna base impedance.
What is the dummyload current if the MW-50 is running 50 KW on the dummy load? Assuming WMAQ's auxiliarytower has a base impedance of 10 Ohms, what should the Aux Tower antenna current be ifit is fed 50 KW? (You will be surprised!) Almost invariably, a broadcast tower has a real andreactive (immaginary) component to its base impedance. For calculating power, the FCC is onlyconcerned 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 toour four inch 50 Ohm transmission line with an impedance matching network at the base of the tower. Itis 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 audiosources 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 canbe accessed with a VU meter to verify that the audio levels are appropriate. Inaddition to this flexibility, an audio oscillator and distortion analyzer can beconnected to the system via the patch panel to verify proper system operation.The patch panel is probably one of the most valuable troubleshooting resources atthe transmitter site.
This is WMAQ's RCA BTA-5R1 5 Kilowatt transmitter. This is our second stringbackup transmitter and is used only in a dire emergency. Occasionally, thistransmitter is used when tower riggers are doing work on our towers. If we ran with50 KW, the workers would receive severe RF burns whenever they touched any of theirhoisting cables. It is also probable that they would be plagued with drawingarcs anytime they toched anything metal. This transmitter was bought in Januaryof 1960 and has 54,000 hours on it. Most of which were accumulated by running onthe dummy load and not on the air! It is getting increasingly more difficult toget parts for this thing!
These are the control racks for the WMAQ transmitter system. Anything that is notconsidered audio, is in these racks. Here is where the control equipment isinstalled for performing our weekly transmitter switches, the antenna controllerwhich allows any transmitter to be put on any antenna. The remote control systemsare also in these racks. We have a Mosley MRC-1, which is the primary remote controlsystem and a Gentner VRC-2000 which is a dialup backup system. This author has usedthe VRC-2000 to perform a transmitter switch from home! If we have a total poweroutage, which happens, the VRC-2000 is the only system with battery backup. We dialinto it to interrogate the system to determine what has happened. The Studio toTransmitter Link receivers are also in these racks. Our current STL system is a dualmonaural analog system. WMAQ's studio facilities are loctated on the sixth floor of theNBC Tower at 455 North Cityfront Plaza in Chicago.
This is a view of the remote control interface relays. These provideisolation between the transmitters and remote control equipment. The relays areplug-in type, 24 Volts, DC made by Potter and Brumfield. Plug-in relays are outstandingfor mission critical applications because they can be swapped out very quickly inan emergency. Note the outstanding installation arrangement.
This is the test setup used for adjusting and testing the harmonicdistortion and AM stereo performance of the transmitter. The instrumentsare, starting in the upper left and proceeding clockwise, a PotomacInstruments Audio Oscillator and Attenuator set, next is the PotomacInstruments Distortion analyzer, a Motorola AM Stereo Modulation monitor, and finally a Hameg oscilloscope. This equipment is normally rack mounted in another room unlessthere is some tuning or measurements to do. The rack mount arrangement is convenient for day today checks, but for actual tuning, testing and adjustment, it is more convenient to remove theequipment from the rack so it can be closer to the transmitter. It is easier therefore to make anadjustment, check the appropriate meter on the transmitter and then check the results on theseinstruments.
The Oscillator is used to set up sine wave test signals of any givenfrequency from 50 Hertz to 15 Kilohertz. (It will actually cover a widerrange of frequencies than this, but 50 to 15K is all the transmittercan handle) The attenuator set is used to set the audio level going into thetransmitter so as to not overmodulate it. The modulation monitor is used toverify that the test signals are not causing overmodulation. It also acts asa radio receiver by demodulating the AM emvelope and reproducing the originaltest signal. This output then goes to the distortion analyzer.
The distortion analyzer is used to measure harmonic and intermodulationdistortion. The oscilloscope is connected to the filtered output of thedistortion analyzer so we can see the makeup of the distortion products.The distortion analyzer is a filter type analyzer. It filters the test toneout and assumes that what's left over is distortion. Well, the signal couldbe actual distortion products or it could be power supply ripple, 60Hzcomponents from the test setup or ground loops, crosstalk products from thetransmitter 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 usedto measure the critical operating parameters. The power amplifier consists of two 4CX35,000Cpower tetrodes, which cost about $6500 each and last about four to five years. The two 4CX35000C tube anode coolers canbe seen through the viewing window. Attached to the anode assemblies are two grey air chimnies. Maintainingproper 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 manyrebuilds, the tubes have to be thrown away. The built in oscilloscope seen on the left is used toaid in adjusting the RF amplifier, which is a modified Doherty amplifier. The original Dohertyamplifier was designed to be a linear amplifier. Continental Electronics modified this design byapplying the modulating audio to the screen grid of the peak and carrier tubes, thereby reducingthe modulator power requirements compared to that which would be needed for conventional platemodulation. This arrangement is also more efficient than the Doherty linear amplifier. The modifiedDoherty arrangement is called Screen Impedance Modulation. In 1974, Continental built transmitters for RadioBelgrade 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 isconsidered as a series fed tower. This tower is used whenever the 740 foot guyed tower (not pictured) is in need ofmaintenance. This tower was a real life saver for WMAQ back in the fifties when their main tower fellto the ground. WMAQ can switch to this tower at any given time whether or not there is an operator atthe 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 aparallel 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 thepresent time.WMAQ also has a 200 KW generator, which is not pictured, that can run the entire transmitter site at 50 KW. However, only onetransmitter 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 Ampswhen radiating 50,000 Watts carrier power ( I told you that you would be surprised! ). The realcomponent of WMAQ's main antenna impedance is 114 Ohms. The reason that the reactance part of anantenna 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, soplease do not beat up on me if you get better answers on your calculator. If you got all theseanswers correct, you should consider being a broadcast engineer or an electrical engineer!
