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 transmitter start up:

These units were designed to be switched on and run at 100% duty cycle from 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 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 side band (ISB), which means that either side band 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.

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 1970’s.  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 flash back 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 (6:30am 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:

Method Speed Bits/S
CW 20 WPM 8.3
CW 35 WPM 14.58
Radio Teletype* 75 Baud/100 WPM 41.6
Radio Teletype* 100 Baud/133 WPM 55.41
Async data 300 Baud 300
Async data 1200 Baud 1200
Async data 9600 Baud 9600
Switched 56 (Switchway) 56KB 56,000
DS0 (POTS) 64KB 64,000
ISDN 64KB X2 128,000
DS1 (T-1) 1.54 MB 1,540,000
DS3 (T-3) 45 MB 45,000,000
Ethernet 10BaseT 10,000,000
Ethernet 100BaseT 100,000,000
Ethernet Gigabit 1,000,000,000

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 “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 quite receiver location.  There are many VLF hobbyists that will be tuning in.