The WREK 425 Watt RF Power Amplifier Story

October 2000

The WREK 425 Watt RF power amplifier, also known as the "Goat-Mitter" was designed by Geoffrey N. Mendenhall (dubbed the Goatman by WREK announcer, Ed Esserman) and constructed entirely with hand tools by Geoff and the WREK staff in August of 1968. The power amplifier was built on a 19" rack panel that fit above the FM exciter within the BFE-10C transmitter where the stereo generator would normally fit.

Geoff obtained a used, but functioning, EIMAC 4CX300A, VHF transmitting tube from a television station where he worked to help earn his Georgia Tech tuition. The tube was free and seemed the ideal choice to increase the WREK transmitter power from 10 Watts to 425 Watts. The only problem was that the FCC normally did not allow radio stations to use home built transmitter equipment. Since the Georgia Tech engineering experiment station had the necessary measurement equipment, it was decided that this barrier could be overcome by obtaining FCC type acceptance on this one-of-a-kind transmitter. The transmitter was designed, built, and then tested the same way that a transmitter manufacturer would obtain FCC type acceptance for a commercial transmitter product. The WREK transmitter was granted the FCC type acceptance identifier GNM69, which were the initials and EE class of Geoff Mendenhall.

Many of the parts for the WREK power amplifier were obtained from the J.S. Betts Electronic Surplus Parts Company in Fairburn, Georgia where Julian Betts and Bob Armstrong hand built 10KW AM transmitters for customers in South America. This company later evolved into the 3rd reincarnation of CCA electronics under the direction of Ron Baker and Steve McElroy.

The original instruction manual was written by Glenn Sirkis. The FCC type acceptance data and hand written project description were completed by Geoff Mendenhall, who held an FCC First Class Radio Telephone License. This instruction manual can be viewed as a series of GIF graphic files located on the WREK archive website (GNM69 Manual).

The 450 Watt WREK transmitter was known as the "Big 600" because the combination of 450 Watts transmitter output power with an antenna system power gain of 1.41 yielded an effective radiate power of 600 Watts. Within a few months, the original 2 bay Andrew "V" antenna donated by WTOP in Washington, DC was replaced by a Jampro, 8 bay antenna donated by WKLS in Atlanta. The Jampro JA8B antenna bays and power divider were retuned from the WKLS frequency to 91.1MHz by Mendenhall and the WREK engineers. Mendenhall's senior EE antenna course project was the construction and pattern measurement of a scale model of the new 8 bay WREK antenna. The spacing of the 8 elements was modeled and adjusted to minimize downward RF radiation into the EE building laboratories. The gain of the 8 bay antenna combined with the 425 Watt transmitter produced a new effective radiated power of 3400 Watts. WREK was now really reaching into the Atlanta suburbs and other area college campuses. The 450 Watt transmitter was successfully operated for more than 8000 hours before being replaced by a used Gates Radio FM-7.5B, 7.5KW transmitter in 1969.

The Gates FM-7.5B was operated at a little over 5000 Watts output power into the Jampro antenna producing and effective radiated power of 40,000 Watts. Although the transmitter was capable of 7,500 Watts output, the RG-8 foam coaxial interbay cable that was used to rebuild the Jampro antenna was already running beyond it's ratings and would have failed if the transmitter power had been increased. With 40,000 Watts effective radiated power, WREK could now be heard as far away as Lake Hartwell in South Carolina.

The hand built, 425 Watt WREK transmitter was later used by WUVT at Virginia Tech in about 1974 where it ran until 1980.

The 2 bay Andrew "V" antenna that WREK originally signed on the air with was retuned by Mendenhall to 102.1 MHz and sold to KQCA, a small FM station in Canton, Missouri. Ironically, in 1973, Geoff Mendenhall wound up working for Gates Radio/Harris in Quincy, Illinois just across the river from Canton, Missouri. Many years later, Glenn Sirkis and Geoff visited KQCA to photograph the original WREK "V" antenna at its new home in Missouri. Those photos can be found on the WREK archive website.

Geoff Mendenhall was the Chief Engineer of WREK in 1968 and become the General Manager of WREK in 1969. He graduated from Georgia Tech with a BEE degree and has spent virtually his entire career working in the broadcast equipment manufacturing industry. Geoff was on the design team for the Harris MS-15 FM exciter in the mid 1970's. He later designed the FX-30 / FX-50 FM exciters and a complete line of high power FM broadcast transmitters for Broadcast Electronics. More than 10,000 FM broadcast exciters in operation around the world utilize technology developed by Geoff. He learned that WREK was in need of a new transmitter in the late 1980's and in his position as VP Engineering at BE, he arranged for WREK to obtain a new BE transmitter at low cost. The BE FM-10A transmitter currently in use by WREK was designed by Mendenhall and his team at Broadcast Electronics.

In the early 1980's, a newly hired EE (John T.M. Lyles) working in Geoff's engineering department, was telling a story to the other engineers at coffee break. He was talking about a home made FM transmitter that he maintained at WUVT, the Virginia Tech Student FM Station. He described a transmitter very familiar to Geoff, that WUVT had gotten from WREK.  (  John's opinion of the workmanship and reliability of this transmitter were not flattering. Geoff didn't say a word, but when John Lyles come into work the next morning, the instruction manual to the original 425 Watt WREK transmitter was on John's desk. He couldn't believe his eyes! Later, we all had a good laugh about how small the world is!

Geoff returned to Harris Broadcast Communications in 1993 where he is now Vice President of Advanced Product Development, managing the development of next generation digital television and digital radio transmission products. Geoff holds four US Patents for innovations to broadcast equipment. He received the 1999 National Association of Broadcasters Radio Engineering Achievement Award for his significant contributions to the improvement of FM broadcast transmitter technology over a period of 25 years. Geoff was active in the W4AQL, Georgia Tech amateur radio club during his WREK years in the late 1960's and still enjoys the hobby with the call sign W8GNM.

Glenn Sirkis was General Manager of WREK in 1970 and 1971. He received his B.S. Industrial Management degree from Georgia Tech in 1974 and when on to become one of the founders of Hayes Microcomputer Products, the originator of the Hayes Smartmodem, personal computer modem, first introduced in 1981. He is now CEO of Stradis Inc. a developer of video compression technology.