Category: history

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
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.

Radio Shack catalog archive

This is a trip down memory lane.  Someone has taken the time to preserve and document Radio Shack, its founding, history, and all of the catalogs printed from 1939 to 2005. The website archive is Radio Shack Catalogs.

I remember reading these very catalogs cover to cover when they came out in the mid-1970s.  At that time, this stuff looked expensive, and in relative dollar terms compared to today, it was.  We had one of these computers in our “Math Lab” in 9th grade:

Radio Shack catalog, TRS-80
Radio Shack catalog, TRS-80

In fact, when I found one of these computers stashed away in the corner of a transmitter site, I had a flashback of Mr. B scowling as yet another student made a mistake plotting x/y coordinates on the backboard.

Tandy TRS-80 Model 4D computer
Tandy TRS-80 Model 4D computer

Ahhh, memories.  Enjoy!

SAQ Grimeton

Historic VLF (Very Low Frequency) station SAQ Grimeton will be on the air to celebrate United Nations Day on October 24th at 10:30 UTC (6:30 am EST) on 17.2 KHz CW.  This station was established in 1922 and is the last radio station in the world employing an Alexanderson transmitter.  More information at their website.

This is a great 2011 video of a tour of the station, including transmitter start-up:

It is great to see that old gear come to life and transmit a message.  The electric motor/generator sounds like a jet engine spooling up.

Before solid-state or even hollow-state rectifiers, motor generators were used to create the DC voltages needed to transmit high-power radio signals.  This method was used by high-powered Naval shore stations through WWII and beyond.

By way of comparison, an average CW operator can send and receive Morse code at about 20-25 words per minute.  A good CW operator, about 30 words per minute and a Russian CW operator, somewhere near 50 words per minute.  This was the main wireless data transmission method until Radio Teletype came into widespread use in the 1950s.   Here is a comparison of data speeds through the years:

MethodSpeedBits/S
CW20 WPM8.3
CW35 WPM14.58
Radio Teletype*75 Baud/100 WPM41.6
Radio Teletype*100 Baud/133 WPM55.41
Async data300 Baud300
Async data1200 Baud1200
Async data9600 Baud9600
Switched 56 (Switchway)56KB56,000
DS0 (POTS)64KB64,000
ISDN64KB X2128,000
DS1 (T-1)1.54 MB1,540,000
DS3 (T-3)45 MB45,000,000
Ethernet10BaseT10,000,000
Ethernet100BaseT100,000,000
EthernetGigabit1,000,000,000

The Morse Code (CW) and Radio Teletype data rates are not a direct comparison, as most radio teletype systems use 5-bit Baudot code instead of 8-bit ASCII.  Morse code varied in length from one to five bits, if one thinks of each dot or dash as a data bit.  Back in the day, before the “Netcentric” mindset, we used mainly radio teletype to communicate from ship to shore.  A premium was placed on brief, concise, operational communications.  Everything else was sent via the mail.

It is quite amazing to see the increase in data speed, which directly correlates to information exchanged (or the ability to exchange information) in the last 90 years.

To receive SAQ Grimeton, one needs a VLF receiver or converter capable of receiving 17.2 KHz and a very quiet receiver location.  There are many VLF hobbyists that will be tuning in.