The VE3TO Radio Beacon Transmitter
|The VE3TO/B beacon has ended operations in the 30 m band. Since beacon operations began in September 2006, 30 m band propagation conditions have improved considerably and Amateur Radio activity has increased greatly. Band openings are now almost continuous with long periods of reliable propagation; thus, the beacon no longer serves a useful purpose. Operations ended as of 00:08 UTC 31 March 2012.|
The VE3TO beacon transmitter is a hybrid, consisting of both modern solid state and old vacuum tube technology. Basically, the transmitter consists of a K1EL "K-ID" keyer chip keying a Technical Materiel Corporation (T.M.C.) GPE-1A HF exciter which feeds a DL-QRP-PA 5 watt broadband power amplifier.
The K-ID beacon keyer from K1EL provides 7 different user-specified messages programmed into the chip. In use, three switches are used to select the desired message, using 4-2-1 octal arithmetic. Two extra transistors allow the K-ID keyer to key the +40 V keying line in the T.M.C. exciter.
The T.M.C. HF Exciter
The HF exciter, made by Technical Materiel Corporation in the 1960s, is designed for continuous-duty commercial and military operation. It provides switch-selection of any one of 3 crystals and continuous adjustment of the drive level, up to 200 mW from the 6CL6 output stage, ample drive for the DL-QRP-PA final amplifier. I have owned this exciter for several years but until now had not found a suitable use for it. It is nicely built and evidently very reliable in operation. Just the thing for a beacon operation!
The beacon frequency is controlled by a piezoelectric crystal in the T.M.C. exciter. The crystal was obtained from Vincent Jakomin of IQ Electronic Design in Chessington, Surrey, England. You can reach Vincent at the e-mail address at right. The exciter has a well-built oven for the crystals but the oven heater has been disabled in order to be able to use crystals intended for operation at 25° Celsius. Due to the excellent construction of the oven, it has a large thermal mass, thus ensuring an extra degree of frequency stability, no matter what the operating temperature.
The Power Amplifier
The famous DL-QRP-PA broadband HF power amplifier was developed by DL2AVH and is now sold as a "semi-kit" by the Reader Service department of FunkAmateur magazine in Germany for about 40 Euros. The "semi" refers to the fact that all the surface mounted components (SMT) are pre-installed at the factory for reliability and convenience of the kit builder. The amplifier provides at least 5 watts of undistorted output. I use a 1 mA meter with a small series resistance, connected to the "Ts6" test point on the PA board to monitor the collector current of the push-pull power transistors. My heat sink is just a large U-shaped piece of 3 mm thick aluminum and a small computer-type fan blows fresh air over it to keep things cool.
A neat feature that I have added is an LM35DZ sensor chip to monitor the temperature of the heat sink. The LM35DZ is a simple (on the outside) 3-terminal device in a TO-92 case and provides an output voltage directly and linearly proportional to the sensed temperature and is available from BG Micro in Garland, Texas. With a 3-1/2 digit multimeter one can read the sensed temperature to one decimal place (for example. 0.262 VDC equals 26.2 degrees C). At 5 watts into a matched load, there is little danger of overloading this power amplifier, but having the temperature sensor is nice insurance. In my set-up, the sensor is mounted a few millimetres from the PA output transistors and the temperature there runs about 5-6 Celsius degrees above ambient temperature. I think the LM35 sensor should find wider use in Amateur applications!
Being tube technology, the exciter runs from 120 VAC using its internal power supply connected to a small computer-type uninterruptable power supply (UPS). The keyer and power amplifier run from an Alinco DM-330MV power supply set for 13.8 VDC (overkill capacity here), also connected to the UPS. The UPS provides AC power during any of the short power interruptions which are not uncommon at this semi-rural location. The keyer circuit receives +5 VDC from a simple regulator driven by the 13.8 V. The same +5 V is used for the LM35DZ temperature sensor on the PA heatsink.
A low-pass filter, borrowed from another project, and an Oak Hills Research WM-2 QRP directional wattmeter complete the transmitter. RG-213 coax is used to feed the antenna.
At present, the antenna is a λ/4-wave vertical, made of telescoping aluminum tubing and mounted at ground level with 16 ground radials of 9.1 m length. (I know, that is longer than tradition calls for, but the wires were that length and I didn't bother cutting them shorter, knowing that the extra would not hurt.) I had started with just 4 radials but could not get the SWR below about 1.4:1; the addition of 12 more radials was a considerable improvement.