Experimental radio history was achieved on September
30th when a team consisting of NY7T (Jim), WM7R
(Pam), and WI7B (Ken), ended experimental efforts with WC9XLG (eXperimental
Longwave spark Gap), a 375 meters spark gap station in
“Team Marconi 2006” utilized the transmitter to
compare the sensitivity of 19th Century coherer technology to modern reception
techniques. The spark gap transmitter they constructed was technically similar
to that used by Marconi and Fleming at Poldhu,
Figure 1 WCXLG 375 meter (800 KHz) spark gap transmitter
WC9XLG (Figure 1) was a rotary-type spark gap based on a single-point V-8 engine distributor and spark coil. The distributor was driven by a multi-speed electric drill with all 8 distribution wires tied together to form one pole of the spark gap (Figure 2). In order to limit the possibility of interference to broadcast stations, WC9XLG was authorized to use horizontal quarter wave Marconi antennas placed 2-3 meters above ground for both transmission and reception (Figure 3) and limited to 1 W ERP.
Figure 2 testing for a contiguous spark with an optimized (and legal) bandwidth
Figure 3 The tuned antennas system. The transmitter on the right, the receiver on the left.
Figure 4. Tuning the transmit antenna to STA specification at 797 kHz.
This is the first experimental spark gap transmitter authorized by the FCC on 375 meters, and the first legally operated since 1912.
In the application for the STA, WI7B states the intention for the request as part of an effort to duplicate Marconi’s 1901 trans-Atlantic feat by using:
“unsophisticated receivers, including a replica of the famous Marconi “Italian Navy” or “Bose” coherer to receive AM broadcasts. To design a sufficiently sensitive Italian Navy coherer it is necessary to optimize the design and construction using pulsed, dampened waveforms. The coherer was utilized in the early days of radio to specifically receive spark gap (dampened wave) excitations. By using a low ERP spark gap transmitter (<1 watt) of emission type X0N, I will be able to optimize the design and construction of a coherer that will be usable to intercept and receive AM broadcasts.”
The coherer was utilized in the early days of radio to specifically receive spark gap (dampened wave) excitations. This was the normal method of receiving radio signals in 1901. In its most usual form, the coherer consists of a mass of metal filings 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. On reception of an RF pulse, the filings coherer moves to a low resistance state. Marconi refined the filings coherer as a result of many careful experiments. The Italian Navy coherer is a further development involving adding a small drop of mercury or carbon particles to the metal filings and using 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. In essence, the coherer is the first solid state rectifier. A good technical description of the electronic properties of the Italian Navy coherer is provide by V.J. Phillips (The “Italian Navy Coherer” Affair: A Turn-of-the-Century Scandal”, PROCEEDINGS OF THE IEEE, VOL. 86, NO. 1, JANUARY 1998).
To design a sufficiently sensitive Italian Navy coherer (which uses a mercury drop as the auto-cohering material) it was necessary to optimize the design and construction using 800-820 KHz RF radiation. In one series of bench tests, the sine wave output of an RF generator was used in a closed circuit (no RF emissions from the circuit) to analyze the sensitivity of different metal/metal oxide compositions and grain sizes to medium frequency RF energy. Promising prototype coherers were employed to receive nearby AM broadcast stations and judged for selectivity with various matching antennas configurations.
Figure 5. Spectral measurements of spark gap signal and nearby broadcast carrier
research has been done into coherer reception of AM radio signals. Some
experiments were carried out in 1974 by G. L. Grisdale at the Marconi
laboratories in the
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.
WC9XLG proved a partial success. The team clearly received reception of the spark gap signal on 800 KHz over a distances of 30 meters (the distance separating the transmit and receive antennas) using both a metal filing coherer and Italian Navy coherer, although auto-coherence was not achieved. Based on their preliminary results, the team is already planning for a second STA to continue experimental studies next year.