The Most Powerful FM StationsThe main factors affecting coverage areas of FM stations are HAAT and ERP. HAAT refers to "height above average terrain" of the transmitting antenna. ERP refers to "effective radiated power," which is the power output of the transmitter multiplied by the power gain of the transmitting antenna multiplied by the efficiency of the transmission line carrying the output of the transmitter to the antenna. This page was last revised on Jan. 14, 2006.
Largest 60 dBu Contours - January 2001The following information was provided by Dave Loudin, who has used the FCC’s contour plotter, which independently calculates the HAAT and resulting VHF propagation along each of 360 radials, to make these calculations. The following data are the median, mean, minimum, and maximum distances to the 60 dBu contour for FM stations whose median distance to the 60 dBu contour is beyond the reference distance of a Class C station (91.8 km).
* The FCC’s TV/FM HAAT calculator and contour plotter needs the antenna’s radiation center above mean sea level. In the current database record, KRTH’s antenna’s RCAMSL is 787 meters, which is at least off by 1,000. Loudin assumed 1787 meters for KRTH; an old FCC database record for KRTH lists the antenna RCAMSL at 1854 meters.
Largest Contours - 1993According to FM Atlas, 15th ed. (1993), the stations with the largest 1 mv/m contours were as follows:
In the above chart, the ERP and HAAT for WIPR-FM ERP were taken from 1999 data, since the ERP and HAAT do not appear in the Atlas.
Greatest ERP - 2000The station which currently has the highest effective radiated power is WBCT, Grand Rapids, Michigan, with an ERP of 320 kW.
Greatest HAAT’s - 1993The stations with the greatest HAAT in the FM Atlas, 15th ed. (1993), were as follows:
Highest authorized ERP’s - 1950In the United States, FM stations are now limited to an effective radiated power of 100,000 watts. However, this limit was not in effect in the early days of FM and some of the older FM stations have been allowed to keep their ERP’s above 100,000 watts. The table below shows the highest ERP’s listed in the 1950 Broadcasting Yearbook, although these figures represent authorizations and probably few if any of the power levels were actually achieved.
Unfortunately, the Yearbook does not list HAATs.
An article in FM and Television, November 1948, reported that WBRC-FM in Birmingham was operating with 546,000 watts. The article reported the transmitter had an output of 50 kW, "but the effective radiation is stepped up to 546 kW by the use of an 8-bay Pylon antenna." The article called the station the "world’s most powerful FM station."
Information on Some Individual StationsKRUZ. At its website, KRUZ Santa Barbara claims the title "The Biggest Station In The Nation" and states, "No other FM station in America covers more area than KRUZ 103.3 FM. Licensed to Santa Barbara at a high power of 105,000 watts and located on a mountain top facing south which sits over 2,980 feet in the Santa Ynez Mountains. KRUZ 103.3 FM can be heard clearly throughout the Tri-Counties of Santa Barbara, Ventura and San Luis Obispo, California and even reaches parts of Los Angeles, San Diego, Orange, Monterey, Kern and San Bernardino Counties." In a message posted on April 21, 1995, Tom King, assistant chief engineer of KRUZ, wrote: "KRUZ-FM is 105kw and on a 4000' mountain above Santa Barbara and has regular listeners 200 miles south of the Mexican border. That is almost 400 miles away. The tropospheric ducting across the ’salt Water Coax' really helps the Southern California stations propagate a long ways. About 200 miles north of the station there is a Billboard announcing sponsored Traffic reports on the station and It is hearable at that sign."
WHOM. WHOM claims the largest coverage area in North America based on the fact that its signal goes over land exclusively. For information on WHOM, see the article earliest FM stations.
WMIT. WMIT Black Mountain, N. C., by virtue of its location at Mount Mitchell, the highest point east of the Mississippi River, had the largest coverage area in the U. S. in a 1942 list of FM stations, which indicated the station had a coverage area of 69,400 square miles.
In an article in FM in August 1941, Gordon Gray wrote:
The breaking of a trail, several weeks ago, along the rocky ridge more than a mile high, between Stepps Gap and Clingmans Peak in North Carolina, marked the beginning of the construction of W41MM. Located in the heart of North Carolina’s mountains, the top of the station’s antenna will reach to a height of 6,885 feet above sea level. This will be more than 200 feet higher than the summit of adjacent Mt. Mitchell, which is the highest point east of the Rocky Mountains. With 50-kW of power, maximum antenna gain and this great altitude, W41MM, on the basis of coverage area, will be possibly the largest frequency modulated broadcasting station in the world. The calculated area within the 50-microvolt contour is approximately 70,000 square miles. Since signals of considerably less intensity than 50 microvolts are useful in rural areas. In all probability, the total service area will exceed that figure. .... If it is possible to carry out the present schedule, the 3-kW driving unit will be available for such operation as may be authorized in the fall and the 50-kW amplifier will be ready for operation early next spring.In Feb. 1942 FM magazine reported, "Gordon Gray will use a 25-watt FM link to carry studio programs to the FM transmitter on Clingman’s Peak. CP has been granted for operation on 337 mc. In addition, a CP has been granted for the use of a 50-watt FM transmitter, to operate on 156.75, 158.5, 159.3 and 161.1 mc. as a spare in case of failure or unsatisfactory operation of the ST link."
According to the 1946 Broadcasting Yearbook, the station went on the air on June 1, 1942. Its original call was W41MM, operating on 44.1 MHz.
According to Norm Gagnon’s database, a November 1943 article indicated that W41MM held the record for the longest STL on the 330 mc band -- a distance of 110 miles from the studios in Winston-Salem to the transmitter atop Clingman’s Peak.
A document released by the FCC on Sept. 12, 1945, indicated that WMIT was authorized a power of 200,000 watts on 97.3 MHz.
In 1948 the station was operating on 106.9 MHz.
The 1950 Yearbook shows an ERP of 75,000 watts.
According to Gagnon’s database, a May 1950 FM article indicated that Gordon Gray had shut down WMIT, saying that the mountain-top installation was very expensive to set up and operate. Gray told the Commission: "I had hoped that we might be able to find a purchaser who would take over the operation, but we have not been successful along that line. ... I still feel that the frequency modulation system is the superior form of sound broadcasting and sincerely hope that some day this feeling, which is shared by quite a few broadcasters, will prove to be the right from a financial as well as a technical standpoint." According to Gagnon, the station returned to the air in August 1951 as WMIT.
Broadcasting Marketbook of August 18, 1952, shows WMIT at Clingman’s Peak with 325,000 watts.
In 1999 the FCC database showed an authorization for 35,000 watts at 948 meters above average terrain and 36,000 watts at 942 meters. The antenna is shown as 2033 meters above mean sea level.
Gordon Gray (1909-1982), owner of WSJS, was publisher of the Winston-Salem Journal and the Twin City Sentinel. He was at various times during his career secretary of the Army, president of the University of North Carolina and national security advisor to President Dwight D. Eisenhower.
Doug Bell, who worked at WMIT in 1951, says that Gordon Gray told him that during World War II German U-boats used the signal of the station to plot their locations and used it as a beam to guide them in to the coast of North Carolina. Bell writes in 2001 that the old structure on the mountain was torn down about three years ago. He says the crew lived on the mountain and that the chief engineer’s wife cooked for them. Electricity was provided by generators. The wall telephone was a crank-type phone. He writes, "We did microwave broadcasts from WSOC radio studios in Charlotte. Muzak was sideband broadcast from the Mountain transmitter..large 15 inch disks about a quarter of an inch thick and, if I remember correctly, lateral recordings." He worked there a second time in 1962. He writes in 2002, "Everything on the Mountain Transmitter site has been replaced in the last 4 years or so - everything. The station is remotely controlled and programmed from Black Mountain, NC."
See also the article "Superpower" Grandfathered FM’s by D. Smith.
World’s Most Powerful FM?From: IngoLorenz@aol.com
Date: Sun, 2 Jan 2000 16:17:40 EST
Subject: Most powerful FM transmitters
I'm working in german radio broadcasting and have been following european radio quite closely for 20 years now. To my knowledge, the most powerful fm tx in the world was located in Italy and broadcasting "Radio24", a local station for Zurich in Switzerland.
At the time (about early 80’s) commercial radio broadcasting was not a legal option in Switzerland, so entrepreneur Roger Schawinsky decided to broadcast from Italy over the border into his home country. The fm signal travelled about 120 km’s (roughly 75 miles) through the Alps mountains (nobody was sure where exactly, because there were a number of mountains between the transmitter site on Pizzo Groppera and Zurich). To achieve this, the station engineer installed an american 50 kW Tx and a specially designed antenna with an erp of about 8000 kW (8MW!). The beam was extremely narrow of course and after construction of the 28-ton-antenna it was found out it did not point exactly into the right direction. The engineer simply had not considered the difference between magnetic and geographic noth pole! Nevertheless some time later the heading of the antenna was corrected electronically.
Best wishes from Germany,
WJEF-FM . . . Stereo and SCA with Half-Million-Watts ERP
Grand Rapids Station Uses RCA 50-Kw Transmitter and 12 Bay Antenna for 25,000 Square Mile CoverageThis article appeared in RCA Broadcast News in November 1962. Thanks to John Byrns for providing the article to this website.
by BRUCE M. GLYCADGIS, Chief Engineer
To the people of Western Michigan "FM" means WJEF-FM with true FM quality service with power. For twenty-one hours each day, WJEF-FM radiates its stentorian voice of 500,000 watts (ERP). WJEF-FM began broadcasting with one-half-million watts (ERP) November 15, 1961. It was a great day for FM listeners within the reach of its signal because it meant that all could receive noise-free programming whether they owned small receivers, large receivers, cheap receivers or expensive receivers . . . whether they had or did not have antennas. It brought on a great resurgence in the purchase of FM receivers within the area this signal reaches, especially in areas that had never before heard FM. But, in reality, this was not the beginning but merely the ending of a great struggle to bring powerful quality-FM broadcasting to the listening public of Western Michigan.
The Beginnings of WJEF-FM
The birth of WJEF-FM goes back nearly seventeen years ago to 1946. It was during 1946 that the Fetzer Broadcasting Company obtained its first construction permit for WJEF-FM. This CP was for the, in those days, unheard-of power of 500,000 watts (ERP).
The company was immediately confronted with many complications arising from the fact that equipment capable of producing 500,000 watts (ERP) such as the "RCA 50 kw Transmitter" and "RCA 12-Section High-Gain Antenna" were not readily obtainable. There was also the problem of obtaining a suitable antenna site with adequate ac power available for such a powerful transmitting plant.
In 1950, the Fetzer Broadcasting Company constructed WKZO-TV. It immediately became apparent that the TV tower would also be suitable for an FM antenna at reduced power. On June 25, 1951, WJEF-FM took to the air waves with an effective radiated power of 115,000 watts. WJEF-FM operated from this site for ten years and, in 1961, WKZO-TV moved its transmitter to the new Gun Lake site, located half-way between Grand Rapids and Kalamazoo, Michigan. When the new TV transmitting equipment was placed into operation, it made equipment on hand, such as the "RCA 25AL TV amplifiers", available for the FM operation and to fulfill, after ten years, WJEF-FM’s dream of serving the people of Western Michigan with a one-half-million-watt signal.
During this entire period of time, the income of WJEF-FM was not great enough to pay the power bill, but, the Fetzer Broadcasting Company has always had great faith in the possibilities in FM and felt that it was in the interest of all of the radio public to keep this FM facility on the air for the public’s enjoyment. In order to realize some income from the operation, the Fetzer Broadcasting Company purchased a Muzak franchise for Western Michigan and began the first multiplexed operation in this part of Michigan. The increase in power to one-half-million watts (ERP) produced even greater multiplex potentials which coincided with the initial decision to purchase the Muzak franchise. All this took place during the years that FM stations all over the country were going off-the-air because they felt FM could not exist with TV and AM. WJEF-FM radiates its signal to an area ninety miles in radius from the transmitter site. In addition to its metropolitan-area coverage, it covers a great rural area that would not, otherwise, receive service from a lower-powered transmitter.
WKZO-TV Moves to Gun Lake
During 1960, Fetzer management decided that it was time for a modernization program. This resulted in the conclusion that WKZO-TV should locate its transmitter and antenna at a point midway between Kalamazoo and Grand Rapids so as to serve both communities equally well. Since the planned antenna tower for the new site would be 1100 feet high, it would make an excellent location for WJEF-FM’s antenna.
To keep WKZO-TV’s signal on the air during the move from Kalamazoo to Gun Lake, management purchased a new TV transmitter for the new site thereby releasing the two 25-kilowatt TV-power amplifiers at Kalamazoo. These were Type TT-25AL, amplifiers and, since they operated on Channel 3, it was a relatively simple task to raise their operating frequency from 61 and 66 megacycles to 93.7 mc . . . WJEF-FM’s assigned frequency . . . to diplex the amplifiers and raise the 10-kw output of the new RCA Type BTF-10D driver transmitter to 50 kilowatts of FM power.
The Type TT-25AL amplifiers use the well-known cluster of seven Type 5762 triodes operating in parallel in a grounded-grid configuration. This cluster design, owing to the physics of VHF frequencies and the mechanics involved, lends itself excellently to frequency conversions.
The RCA BTF-50B transmitter design utilizes two TT-25AL amplifiers diplexed and driven by the BTF-10D. The modification of our units, as made in the field by RCA engineers resulted in the first BTF-50B transmitter. The performance of the system has been very satisfactory.
Side-Mounted Antenna System
WJEF-FM uses a 12-section, gapped-ring antenna (BFA-12A) attached to one leg of the TV tower at the 800-foot level. Center-fed, this array provides a power gain of 12.5 and raises the 50-kw transmitter output to 500 kw ERP.
With the antenna system mounted on one leg of the triangular cross-section tower, it provides substantially circular coverage over a 25,000 square-mile area. This includes Kalamazoo and Battle Creek on the South, Grand Rapids on the north and Lansing on the east, plus dozens of smaller municipalities at all points of the compass. For 90 miles in any direction, FM receivers can receive WJEF-FM.
Gun Lake Facility
The transmitters of both stations, TV and FM, are housed in a modern, well- engineered building of prestressed concrete, clad in aluminum and buff-colored brick. The 1100-foot antenna tower stands immediately outside the north wall to practically eliminate outdoor-horizontal runs of transmission lines to the antennas.
Locating the tower so close to the building presents a falling-ice problem during the Michigan winter. However, this is overcome through a specially-designed roof on the structure that withstands the potential damage of the falling ice.
A novel arrangement in building design places all of the electronic gear of the transmitters on grade level with the power transformers and blower equipment arranged in a basement room. This set up makes the transmitter room extremely quiet since the hum of transformers and the sounds of moving air stay confined to the lower level. Further, this arrangement reduces, considerably, blower vibration in addition to increasing the accessibility to the equipment.
Massive Plenum Chamber
Each of the blowers operating in the basement performs without an individual air filter. Instead, all of the air in the room is filtered prior to its entry into the room.
Outdoor air, entering the side of the building, is first filtered via spun-glass filters. This air then passes through electrostatic filters to axial fans, which "pressurize" the basement room and thus make it a massive plenum chamber. Since the blowers operate in a dust-free atmosphere, there is no need to provide each blower with an air-filtering device.
The blowers, mounted on concrete piers, push the filtered air upwards through the basement ceiling and into the bases of the transmitter equipment. Ducting, at the top of the transmitter cabinets, is thermostatically controlled so that the warmed air from the equipment goes directly outdoors in summer or, indoors in winter to thus heat the transmitter room.
This loop system, using entirely-outdoor air, reduces the BTU requirement of the air-conditioning equipment in summer and, delivers fresh, warmed air in winter.
Another advantage in prefiltering the air before it enters the basement room is the elimination of a settled-dust problem in the basement as well as providing a dust-less atmosphere in the transmitter room. This of course, simplifies maintenance.
When the station went to the half-megawatt ERP, management decided that the station should be an entity in itself and, for that reason, should generate its own programming.
The staff spent many months in surveying and devising the programming that is now a part of everyday operation. WJEF-FM programs its educational and informational shows throughout the day instead of the early hours of the morning. The surveys also pointed out that the listener desires large blocks of time set aside for each musical category. As a result, WJEF-FM maintains a considerable disc- and tape-recorded library which provides an excellent range of literature in each musical category.
Discs and tapes supply the high-fidelity program material. The discs are played on BQ-51 Turntables and tapes on RT-21 and RT-7 tape machines. The RT- 21 is a reel-to-reel tape transport while the RT-7 operates with endless-loop cartridges.
RT-21 Stereo Equipped
To play commercial stereo tapes, the RT-21 Tape Recorder is equipped with a four track playback head in addition to the separate erase, record and play heads for two-track stereo tapes. This is a special feature of the RT-21 in that it permits the machine to play virtually all quarter-inch-tape recordings; full-track, half-track-stereo and four-track stereo tapes.
BQ-51 Turntables Use Lightweight Tone Arm
In the center of a horseshoe arrangement is a BC-6B Audio Consolette while the turntables are at either elbow of the announcer. Each turntable contains its own preamp and attenuator pad. The knob for the pad is located conveniently next to the turntable controls. Each RCA Lightweight Tone Arm supports a stereo phono cartridge famous for its quality reproduction. For Cueing purposes, BA-8 Cue Amplifier with self-contained speaker is used.
Cartridge Tape Simplifies Cueing
The self-cueing feature of the RCA RT-7 Cartridge-Tape Machine serves ideally for spot announcements. This, of course, permits greater use of the turntables for programs material. The cartridge-tape facility, being monophonic only, serves only in non-stereo announcements; the stereo commercials being played on the RT-21 or either of the two turntables.
Muzak Programming on Long-Play Tape
The SCA subchannel programming comes from two long-play tape machines located in the main control center adjacent to Studio 1. These two machines operate almost completely unattended in that the tapes come from Muzak pre-programmed for such operation. The signal from these machines travels via multiplexed microwave to the transmitter site. Here it modulates a 67-kc sub-carrier of a Type BTX-1A Multiplex Generator which, in turn, modulates the BTE-10B Exciter in the FM transmitter.
FM Stereo Programming
A portion of each day’s programming is transmitted in multiplex stereo (FM Stereo) using an RCA BTS-1A Stereo Subcarrier Generator mounted at the top portion of the center cabinet in the BTF-10D FM transmitter.
Since introducing stereo programming, WJEF-FM has built up a separate stereo record library (see Fig. 9).
The primary signal-coverage area serves more than a million families. More than 50 per cent of these families have indicated, via mail, that WJEF-FM was the first FM station they had ever heard. In many cases, the primary reason for buying an FM receiver was word-of-mouth praise for WJEF-FM programming by a neighbor or a friend. It’s important to note that most of these new listeners are located in rural areas and, quite possibly, would have no FM service were it not for the half megawatt signal of WJEF-FM.
Fulfils A Dream
Fetzer Broadcasting management believes that high power and high fidelity go hand in hand. The two converted RCA TV amplifiers provide 50 kilowatts of transmitter power for WJEF-FM. The combination of this transmitter power and the new BFA-12A Antenna produces 500 kilowatts (ERP) of power. The availability of this equipment made it possible to fulfil a dream of serving the people of western Michigan with a one-half-million-watt signal.
More on WJEF-FMXen Scott of ABC News transcribed the following from the FCC microfiche files on Aug. 22, 1994, and provided it to this website.
Chuck Gustafson posts on WJEF/WBCT 1993-1994May 15, 1993. WJEF-FM (changed to WJFM and now WBCT) in Grand Rapids, MI used to run 500.0 KW Horizontal from 800 feet. I'm not kidding 500 KW. Power dropped to 470 KW when they added 64 KW of vertical. Now the station (WBCT) runs 320 KW Circular from a Harris 50 KW transmitter (Dual 25’s) at 800 feet AAT.
May 18, 1993. Of interest is the fact that on 1/6/53 the FCC issued a modified CP (I don't have the original one that was modified) that specified operation with 550 Kilowatts at 810 feet AAT. Apparently this was never built as the CP was modified again to specify operation with 115 KW at 420 feet. Subsequently with the availability of two channel 3 25kw TV amplifiers (anyone remember those old round beasts with all the 5762 tubes?), WJEF applied for 500.00 Kilowatts ERP at 800 feet AAT. That application was granted on 8/12/60 on BPH-3049.
May 21, 1993. According to the VP of Engineering at the time, and later owner, The FCC was doing all it could to promote FM at the time. The owner of the station got a call from a friend of his who was (here it gets fuzzy) either on the Commission or high on the staff asking that they do all they can to push FM. At the time, there were no rules like today concerning separation, just contours not crossing with no power limits. The station was operating with 115 KW on the side of a TV tower. CBS was after the co-owned TV station to cover the ADI better so plans were made to move the TV. With the TV’s 25 KW amplifiers available an application was filed to move the FM and increase its power using the TV amplifiers (modified) as FM Final Amplifiers. On 3/7/62 a License was issued covering the CP authorizing 500 KW @ 800 HAAT with a 50 KW RCA BTF-50B transmitter and a RCA 12 bay horizontal antenna. Interesting fact: Later they added some vertical and some beam tilt with a antenna that consisted of 16 bays, 4 horizontal, 8 circular and 4 more horizontal. This gave about 470 KW horizontal and 64 kw vertical. In 1980, the station was updated with a Harris FM-50-K (dual FM-25K’s) and a ERI "Superpower" antenna. ERP is now 320 KW V/H. It almost stayed at 500 KW as they were looking at buying a 80 KW transmitter (dual 20’s plus dual 20’s).
June 3, 1993. Since my last post concerning the WJFM megapower, I have gleamed some further information from old timers who worked there. The original CP was issued in 1941 for 500KW at about 500 HAAT. The CP was continued thru WW II and eventually was built with 115KW at 450 feet at the site of a co-owned TV. When CBS told the TV station to move its site to better cover the ADI or lose the affiliation, the TV station put up a 100KW @ 1000 HAAT operation and moved the FM along. The original 500KW CP was reinstated and the old TV amplifiers were used as final finals in a dual configuration. About 1980, before my short affiliation with the company, they knew the FM transmitter and antenna needed replacing. They looked around for the most powerful FM stations in the country and found that at 320KW they would still be the most powerful. They toyed with dual 20’s paralleled to get a 80KW TPO (I've seen the bids), but it was just going to be too expensive and complicated. Remember, this was late 70’s technology. Eventually they settled on the new Harris FM25K dualed to get 50 KW TPO. It’s interesting, all the manuals have preliminary stamped on them. The antenna is an ERI 12-bay super power antenna fed with 6 1/2 inch line.
Dec. 17, 1993. Let me correct a slight error: When WJEF/WJFM ran with horizontal polarity only, they were 500KW at 800 feet. When they went to vertical/horizontal polarity they dropped to 464 KW horizontal, 68 KW vertical New antenna (circular) in 1984 dropped the ERP to 320 KW.
Aug. 25, 1994.
========================================================================= Date: 08-25-94 (14:41) Number: 9172 of 9186 (Refer# NONE) To: XEN SCOTT From: CHUCK GUSTAFSON Subj: Re: WJFM-FM History Read: NO Status: PUBLIC MESSAGE (Echo) Conf: BROADCAST (380) Read Type: GENERAL (+) ------------------------------------------------------------------------- XS> WJFM (FM) XS> 8/8/60 Granted a C.P. to move transmitter site, increase ERP to XS> 500kw, increase HAAT to 800 ft., install new transmitter XS> (RCA BTF-50B) and new antenna (RCA BFA-12A). This xmtr was two of those "old" round VHF RCA TV amplifiers (25 kw ea) run in parallel. (The old WKZO-TV finals made unnecessary when a TT-10B was installed along with a 12 bay Super turnstile) XS> 1/6/70 Granted a C.P. for 93.7mc, ERP 470kw (H), 64kw (V). XS> Change antenna (RCA BFAC-10/6, 10 horiz. & 6 vert. sects.), XS> HAAT 800 ft. Made a lot of money when I sold that miserable excuse for an antenna for junk! XS> 2/27/79 Granted a C.P. to make changes in antenna (change antenna XS> to Harris FMH-12BC-6, 12 sections H&V), change transmitter XS> (change transmitter to Harris 40/40K), increase ERP to XS> 500kw (H&V) and change TPO. Harris never could get the 40/40 combination to work. XS> 11/6/79 Granted a C.P. to change ERP to 320kw with HAAT of XS> 780 ft. License to cover the C.P. granted 7/21/80. Changed to a Harris FMK-50 (dual 25’s pushed to 26.5 kw at factory) No change to this date except for: Changed exciter to BE 50 watt unit (audio improvement was dramatic) This also got rid of insane exciter muting arrangement that always gave trouble. Changed 5x2 (parallel solid state IPA) for each transmitter to one module (per transmitter) from a Harris 1 KW SS FM Xmtr. Greatest mod for the FM25K series EVER!
546,000 Watts on FM
WBRC-FM Birmingham, World’s Most Powerful FM Station, Has 200-Mile Primary Service RadiusThis article appeared in FM and Television, November 1948.
Alabama not only has 14 FM stations on the air, but it has the most powerful of all FM stations. It is WBRC-FM, an RCA installation just completed on Red Mountain, near Birmingham. The transmitting radius of 200 miles gives primary service coverage over an area of about 125,000 square miles. This station is unique in another respect. It is owned, together with WBRC and WBRC-TV now under construction, by a woman, Mrs. Eloise Smith Hanna.
The transmitter has an output of 50 kw., but the effective radiation is stepped up to 546 kw. by the use of an 8-bay Pylon antenna.
Except for the modulator and driver sections, the design of this equipment departs radically from conventional transmitters, the 1, 3 and the 10-kw RF amplifier employ grounded-grid RF circuits with concentric-line construction. All three stages have 7C24 tubes.
The 50-kw. stage is comprised of two 5592 tubes and there is enough space to walk through to the area at the rear. Electrically and mechanically, each of the two final sections form an integral part of the grounded-grid circuit. This construction eliminates neutralization and RF radiation that would affect the RF circuits in adjacent units. The base of the enclosure forms a plenum chamber for cooling air, and contains the control wiring and high-voltage bus. The plate line is tuned by shorting bars carrying contact fingers which move vertically along the center conductor. Motor driven lead screws actuate the shorting bars, with the control located on the front panel. Input tuning is accomplished by two flat-plate air capacitors, One motor-driven and other operated manually. For output coupling, motor driven loops are used, with series capacitors for reactance tuning. The amplifiers feed equal load impedances, also individually motor-controlled, providing easy load balancing and smooth adjustment of output power. At the top right are the transmission-line monitor and harmonic attenuator. The latter, used to insure maximum suppression of harmonic radiation, consists of a pre-tuned low-pass filter capable of 38 db attenuation.
In case of any appreciable change in signal intensity, the transmission-line monitor actuates relays which shut down the transmitter. A reclosing mechanism then puts the transmitter on the air again. If the fault persists, the process is repeated, locking out after the third unsuccessful attempt.
While this installation was being made, a similar transmitter was under construction for WTMJ-FM Milwaukee, with 349-kw. effective radiation. These are the first FM transmitters of such high power rating to go on the air.