A 375 Meter / 800 KHz Spark Transmitter On The Air

September, 2006

Submitted by Ken Beck, WI7B

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 Eastern Washington State.

WC9XLG was the callsign of an Special Temporary Authority (STA) granted by the FCC OET for the use of a spark gap transmitter on 800 KHz (800 AM broadcast band).

"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, Cornwall in 1901 to bridge the Atlantic.

Figure 1 - WC9XLG 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 antenna system. The transmitter on the right, the receiver on the left.

Figure 4 - Checking the transmitter frequency.

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.

Little 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 UK. However, these have never been completely documented. What is known (as reported by V.J. Phillips) is the Grisdale attempted to use the Italian Navy coherer as a diode envelope detector for the reception of AM signals and to compare its action with that of a germanium point contact diode. His conclusion was that it would indeed rectify, but very inefficiently. However, up to now recent research had been conducted that characterizes 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.

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.


This article has been posted on this web site with the permission of WI7B

The entire contents of this web site are Copyright 2006 - 2012 by Ralph M. Hartwell II, all rights reserved.