Shown below is my 2200m experimental station, located on Mayne Island, B.C.
The transmitter, in the lower right corner, is my homebuilt version of the "LF Half-Kilowatt" designed by Scott, VE7TIL. My original signal source utilized a 4040 (binary counter) IC as a 2200kHz crystal oscillator driving a 4060 IC in a divide-by-8 configuration to produce a low-level signal at 275kHz. This has since been replaced by a homebrew DDS signal source. The 275kHz signal feeds a 4013 to provide a push-pull divide-by-2 output. The push-pull output on 137.5kHz is fed to an IR2110 push-pull FET driver which in turn drives a pair of inexpensive W34NB20 MOSFETs.
The panel above the transmitter contains two rack-mounted low-voltage power supplies strapped in parallel to run the transmitter, along with a small 12V / 5V supply for the IC's and fans.
The top panel houses my homebrew 500W reflected power meter. It is an LF version of the Drake Power Meter, adapted by LX1PD. Circuit details may be found at GØMRF's website. I have been waiting thirty years to use those big surplus Japanese meters in a project. New meter scales were designed using METER, a freeware program available on the web.
The lower left corner houses the scope which constantly monitors the transmitting antenna tuning.
Both antenna CURRENT and antenna VOLTAGE are displayed. This allows easy tuning for resonance as well as making any matching adjustments with the loading coil very easy. The impedance of the antenna can be observed as well as inductive or capacitive characteristics. It is fascinating to watch changes in the scope pattern while the antenna blows in the wind!
The Scope Match was homebrewed from plans by MØMBU and is shown in both the 'LF Experimenter's Handbook' (G3LDO) and the new 'LF Today' (G3XDV).
The transmitter is connected and matched to the antenna system by a large antenna loading coil. The loading coil was retired from a local ndb transmitter and is extremely rugged.
It is air-wound, on ceramic spacers, with #12 copper wire for a total inductance of 2.8mH. My antenna requires only 2.0mH to resonate at 137kHz so the antenna is tapped down on the coil. A homebrew variometer, between the loading coil and antenna, is used for fine-tuning the system to resonance.
The variometer is a 'variable inductor', with an inner rotating coil connected in series with the outside coil. Rotating the inside coil changes the overall inductance plus/minus 300uH approximately. Plans for the variometer are available at GØMRF's website as well.
The antenna system is a 'Marconi T' 3-wire flatop. One end is attached near the top of a 100' Balsam tree while the other end attaches to my neighbor's Fir tree. Spacing on the top wires is 1m, with an overall length of approximately 30m. The antenna runs parallel to the ocean beach on the eastern shore of Mayne Island, providing an over-water horizon from KL7 to the north around to W6 in the south. At low tide, the effective height of the antenna is ~ 100' or more.
Please see my "CQ CROSSBAND" page if you would be interested in working British Columbia on LF!
