 |
 |
 |
|
|
|
|
|
|
||||||||||||||||||
|
|
|||||||||||||||||||
|
|
|||||||||||||||||||
|
|||||
|
|
 Previous: Surfin': No Good Deed Goes Unpunished |  Next: ARDF Update: A Year of Firsts in ARDF |
By Rick Lindquist, N1RL
ARRL Senior News Editor
January 15, 2007
An experiment using a spark gap transmitter attempted to demonstrate how early 20th-century "coherer" technology compares with modern receivers.
The WCXLG 375-meter spark gap transmitter. The automotive distributor is visible at the left, while the electric drill that rotates the unit is on the right. Duct tape hold everything in place. |
Tuning the transmit antenna to STA specification at 797 kHz. |
The Italian coherer in the field and ready for testing. |
WI7B's Italian Navy coherer consists of tapped graphite and stainless steel rods enclosed in an acrylic body. Another coherer used a carbon rod instead of graphite, as Marconi and others did in 1901. |
All is ready: The transmitter is on the right, the receiver on the left. A hard copy of the WC9XLG STA is stapled to the antenna support post. |
Jim Follansbee, NY7T, takes spectral measurements of the spark gap signal and a nearby broadcast carrier. |
A team of radio amateurs in Eastern Washington is calling its recently concluded experiments in early spark-gap and receiving device technology "a partial success." A team consisting of Jim Follansbee, NY7T, Pam Follansbee, WM7R, and Ken Beck, WI7B, wrapped up its experimental efforts with WC9XLG (eXperimental Longwave spark Gap) at the end of September. Beck obtained a Special Temporary Authority (STA) from the FCC in early 2006 to operate a low-power spark gap transmitter on 375 meters (approximately 800 kHz). The goal was to compare early coherer technology -- the sort of receiving apparatus Marconi used in the early 1900s -- with modern receiver technology.
"The team clearly received reception of the spark gap signal on 800 kHz over a distance of 30 meters -- the distance separating the transmit and receive antennas -- using both a metal-filing coherer and Italian Navy coherer," Beck said. "However, Italian Navy coherer auto-coherence was not achieved."
Beck says that in its most usual form, a coherer consists of metal filings or particles lying in a small air gap between two metal plugs fitted tightly into a glass tube. "One plug is connected to the receiving antenna, the other to Earth ground," he explains. When the coherer detects an RF pulse, the filings move into a low-resistance state.
"Auto-coherence," the ability of the particles in the tube to return to their latent, high-resistance state, is necessary to receive rapidly modulated waveforms such as voice, Beck explains. "What this meant was that we were unable to complete our comparative sensitivity measurements between this auto-coherer and a modern receiver for the reception of AM signals."
Early wireless experimenters used a coherer specifically to receive spark gap (damped wave) emissions, and some coherers employed a mechanical "tapper" to "de-cohere" the metallic particles after receiving an RF pulse.
Beck says he believes the WC9XLG experiment marked the first time an experimental spark gap transmitter was operated legally above 200 meters in the US since 1912. Authority for Amateur Radio operators to use spark gap transmitters ended in 1928, and, Beck says, some spark gap transmitters continued to operate in maritime service until the mid-1950s.
The WC9XLG transmitter was a rotary-type spark gap built around a single-point V-8 automobile engine distributor and spark coil. Beck says it was technically similar to the one Marconi used in Poldhu, Cornwall, to span the Atlantic in 1901. A multi-speed electric drill powered the distributor, and all eight wires were tied together to form one pole of the spark gap.
"To limit the possibility of interference to broadcast stations, WC9XLG was authorized to use horizontal quarter-wave Marconi antennas placed two or three meters above ground for both transmission and reception and limited to 1 W ERP," Beck notes.
According to Beck, Marconi had refined the metal-filings -- or "Branly" -- coherer after many careful experiments. The Italian Navy -- or "Bose" -- coherer was a further development. It involved adding a small drop of mercury or carbon particles to the metal filings and used one plug composed of carbon.
"The phenomenon involves the breakdown of the thin oxide layers on the surfaces of the metal to form a good metal-to-metal contact," Beck said. "In essence, the coherer is the first solid state rectifier."
The WC9XLG team conducted a series of bench tests to compare the sensitivity of different metal/metal-oxide compositions and grain sizes to medium frequency RF energy. It used promising coherer prototypes to receive nearby AM broadcast stations, judging each prototype selectivity with various matching antennas configurations.
Research in the 20th century has concluded that the Italian Navy coherer would detect (ie, rectify) AM broadcast signals, but very inefficiently. Beck says no recent research had characterized coherer reception of damped waves versus continuous waves, or of modern digital modes for communication.
"Thus, one important aspect of the research program on the Italian Navy coherer was to gain an understanding of the differences between its receptions of damped waves as opposed to continuous, modulated waves," Beck said. Based on its preliminary results, he says, the team is planning to apply for a second STA to continue its experiments this year.
The WC9XLG transmitting antenna in the field. |
Ken Beck, WI7B, visits Cabot Tower on Signal Hill in St Johns, Newfoundland, where Marconi received a transatlantic signal for the first time. |
| Previous: Surfin': No Good Deed Goes Unpunished | Next: ARDF Update: A Year of Firsts in ARDF |
