history – Page 8 – Engineering Radio

Restoration work on an RCA transmitter

I read through this article about the ongoing restoration work of an RCA SSB T-3 transmitter and found it interesting. The RCA T-3 transmitter is a 20 KW SSB/ISB HF (2-28 MHz) unit designed for point-to-point telephony service. Because SSB requires class A or AB low distortion amplifiers, this is a large unit, even for its age and power levels.

From the looks of the before pictures, this transmitter was in sorry shape.

Here is a brief video of the transmitter start-up:

These units were designed to be switched on and run at 100% duty cycle for most of their operating lives. That is some heavy iron there. This particular unit was made in 1959. More here and video part 2:

Anyway, before geosynchronous satellites, HF point-to-point transmitters were used to make long-distance phone call connections and send data and pictures back and forth over long distances. Out in Hicksville, Long Island, Press Wireless ran a data and fax system that used HF for long-haul data transmission. Much of the WWII reporting from Europe and the Pacific Theaters was carried over this system.

Text would be printed out on a mechanical teletype machine at something like 60 words per minute, which was considered fast for the time:

Tuning across the band, one can often hear Radio Teletype (RTTY or RATT) which uses a 5-bit Baudot code, 170 Hz shift with 2125 HZ representing a Mark or 1 bit and 2295 Hz representing a Space or 0 bit, which is a bit different from the Bell 103 modem specifications. This is what it sounds like at 75 Baud:

So slow you can almost copy that by hand.

The RCA H (SSB T-3) unit above was independent sideband (ISB), which means that either sideband or both could be modulated independently of the other, thus two channels of information could be transmitted. SSB bandwidth is about 2.7 KHz, which is good for telephone-grade audio or low-speed data.

I sort of wish I was living in California again, I’d lend a hand.

Call Sign Trivia

I found this youtube video about Pittsburgh, Pennsylvania radio stations:

That’s cool and all that, but it brings up the question about the K/W calls which were misplaced during the early days of broadcasting. Originally, call letters were assigned to ships and coastal radio stations in the following way:

Three-letter call signs were for coastal (land) stations. K letter calls were for shore stations in the west and W letter calls were for shore stations east. Ships were assigned four-letter calls, W calls signs were issued to ships homeported on the west coast and K calls for ships homeported on the east coast. There was a period of time when a few K call letters were issued to east coast broadcasting stations, no one is quite sure why. Prior to 1923, the K/W boundary was not the Mississippi River, but the eastern border of the states of Montana, Wyoming, Colorado, and New Mexico. Thus, there are many more misplaced W call signs than K call signs.

Of course, KDKA and KQV come to mind. Philadelphia has KYW. What other misplaced call signs are there, e.g. W’s west of the Mississippi and K’s to the east? Of course, one can google it and get an answer, however, there is one that is pretty obscure.

WKNR 1310; gone but not forgotten

Reflecting the state of the economy in Detroit, WDTW went silent on January 1st. Less than two weeks later, the towers come down:

Thanks, Chris R for the video link.

The license has been donated by Clear Channel to MMTC (Minority Media and Telecommunications Council), but not the land or towers. It remains to be seen whether the station will return to the air, however, given the costs involved and the economic conditions in Detroit, that is unlikely.

The station signed on in 1946, moving to 1310 KHz with full-time operation in 1948. Back in the day, it was a flame-throwing top 40 station and is purported to be the source of the “Paul is dead” rumors that surrounded the Beatles in the late 60’s. Much more history at Keener13.com.

Take pictures of your favorite AM stations now because tomorrow, they and all their history may be gone.

The Continental D323C medium wave transmitter

I found a 1981 Continental Electronics equipment catalog at an old transmitter site. These finds are great if one is interested in history and looking at the way things used to be done. This particular transmitter is a 2,000 KW (2,000,000 watt) medium wave unit:

Continental Electronics D323C, Circa 1981

I believe most units like this were destined for use by government broadcasters in either the middle east or Western Europe. I know there were several 1,000 KW medium wave stations in West Germany at one time. The Continental transmitter is basically two 1,000 KW units (323C) combined. They used a modified version of Doherty modulation, that is called “Screen and Impedance,” which accurately describes how it works. More information from the Continental Catalog can be found here: Continental D323C. The tubes (or valves depending on where you are located) used in the D323C were 4CW25000A tetrodes as modulators and IPA the final was a pair of X2159, which is an impressive tube.

EIMAC X-2159 water cooled power tetrode — EIMAC X-2159 water-cooled power tetrode

The tube sat anode up. The filament, grid, and screen connections are underneath. Cooling water was pumped through the two connections on the top at about 130 gallons per minute depending on the plate dissipation. With a 30° C rise, that equals about 96,000 BTU per minute. The D323C had a dissipation of 400,000 watts for the carrier tube and 240,000 watts for the peak tube (640 KW total) under 100% modulation. That equals about 2 million BTU per hour. Notice the lifting hook, this tube weight in at 175 pounds. Tube date sheet here.

Continental no longer makes medium wave transmitters, their closest high powered broadcast product now is the 418/419 and 420 HF (shortwave) transmitters. The 420D does a wimpy 500 KW using a solid-state modulator section.

I remember in the early 1990’s when I was at the Harris plant in Quincy, they were working on a 1,000 KW solid state DX series AM transmitter for Saudi Arabia. It had to be liquid cooled, which added another layer of complexity to an already complex system.

I don’t know if there is much call for 2 MW medium wave transmitters anymore as there are more efficient ways to reach remote populations and I can’t even imagine what the electric bill would be like.