Gap Transmitter Signals for Marconi Centennial
12 December 1901 to 12 December 2001
Spark gap transmitter signals in today's
radio spectrum could be considered analogous to horse and buggies
on our motorways, but during commute rush hour they may actually
out perform rival cars with their impatient drivers. There are
legal equivalents as well. However, exceptions for the banned
Spark Gap Transmitter apparently are being made for the Marconi
Centennial where under controlled conditions they may be used.
One such transmitter has been constructed, tested by David Wilson
VE3BBN and given the green light in Canada. The background sound
is a test signal heard and recorded by Dave VA3ORB 150 miles away
3 November 2001. Click here for more picture detail.
If you are over 90 years of age you may have heard spark gap transmitter signals long ago. If not, have a good listen as you probably won't get another chance to hear such signals for another 100 years!
The following Ontares Net members in Canada reported hearing this signal:
VA3JNA Joe Owensound 130 miles
VA3BBD Percy Owensound " "
VE3SUT Dave Elliot Lake 270 miles
VE3BDB Bob Orillia 100 miles
VE3OF Harry Fenelon Falls 100 miles
VE3MIO Maureen Manitoulin Island 250 miles
VE3EAV Hugh Echo Bay 450
VE3LJG Lloyd Saulte Ste Marie 450 miles
VA3ORP Dave Kingston 150 miles
Marconi Day Schedule for Spark Gap Transmissions by David VE3BBN
This is the official notice for the spark event: Transmissions will be between 2100 and 2200 hours local Canadian Eastern time.on 12 December 2001 (between 0200 and 0300 Zulu on 13 December 2001) Note that Europe will already be into the next day 13 December 2001.
The freq. is 3.550
mhz bandwidth approx. 20 khz power out approx. 15watts.
Transmission will begin on the hour for approx. 15 seconds and rest for
45 seconds to begin on the next minute mark.
The message will be "MARCONI S" repeated each minute for the hour.
Receivers should be operated in the AM mode for best bandwidth. and
If the signal is heard please QSL to VE3BBN@RAC.CA with the following :
Your call, rst, distance from Niagara falls Ontario as the crow flies.
Click here for initial Marconi Centennial Transmitting results:
SPARK GAP TRANSMITTER DETAILS
A schematic of the spark gap transmitter is shown on the right. Keying is accomplished by switching the ac voltage to the primary of the power transformer on and off with relay contacts. The secondary output voltage of 12kV powers the transmitter by charging up C1. The rotary spark gap discharges C1 into the primary coil of the RF transformer. During this discharge when the spark gap is firing, the capacitor and primary coil oscillate at their natural frequency (approximately 3.56MHz. This burst of oscillation is loosely coupled into the high Q secondary circuit which is also tuned to 3.56MHz. Output RF is taken from a tap one turn off the bottom of the secondary coil and fed to a Windom antenna.
The rotary gap can fire on both positive and negative polarities of the 12kV applied voltage. The ratary gap fires at a minimum of 120 times a second giving a minimum of 120 bursts of RF a second. Actually it is firing at about 6 to 8 time each 60Hz cycle or about 360 to 480 times a second. As such the transmitter output consists of about 360 to 480 bursts of 3.56MHz averaging about 15 Watts.
C1 is a home brew sandwich type constructed capacitor immersed in high grade transformer oil. This arrangement provides a highly stable capacitor. Pressure is applied to the top of the capacitor with a turn screw for fine tuning the primary circuit.
Protection against high voltage spikes (in excess of 18kV) is provided by the safety gap. The rotary gap consists of 16 rotating electrodes against 2 stationary electrodes. The spark gap acts as a voltage controlled switch: open when not firing, and short (~2 ohms) when firing. The power transformer is a Neon Sign transformer rated at 12kV 60mA. The coefficient of coupling is approximate 0.95. Thus, when the secondary winding is short circuited by the rotary spark gap, the maximum currwnt is limited to 60mA.
More pictures: Click on images to enlarge. (back space to return to this page)