Hi Folks,I have been tinkering with various aspects of 630 m operation, with no serious aspirations, rather in the spirit of exploration and practical learning.
On the one hand, RF amplifiers for a modest (<10W) of power. The point being to apply some long half remembered theory, and put it into practice - all with junkbox parts. One area I always seem to come back to is RF transformers, and once again this is true. Exploration settled on use of some medium power NPN transistors (BD139) configured as a push-pull pair. The P-P transformer is wound on a pair of 3/4" long ferrite sleeves (high permeability material e.g. type 73 or similar) configured like a "binocular" core. An initial calculation for winding, based on required inductance proved to be excess, and better results were obtained with about 4 turns for each primary section, and 8 on the output side. The OP stage is stable into 50 Ohms, with some feedback implemented by 1.8 Ohm resistors in the transistor emitter legs. To feed this arrangement, a BC317 emitter follower drives a phase splitter transformer wound on a green torroid salvaged from a switch mode PSU. These again are high permeability types. The secondary is center-tapped, and other ends of each winding feed the bases of the OP transistors. So far "development" sees about 2 W with a 10V supply and 3-4 V drive from the signal generator. Most of this is about practical learning by re-inventing the wheel.
In parallel, a fascination with loop antennas continues. I already had the frame for such a device, wound with thin magnet wire and tuned by a receiver type variable cap. A change to 18 awg dog fence wire resulted in an dramatic increase in "q", which tracked the measured reduction in winding resistance by a factor of 10. A single turn coupling winding feeds the loop. Initial connection to the amplifier output resulted in bewilderment, no apparent tuning and sometimes instability... The difficult part is that the electronics bench here at BLM towers doesn't have good analysis tools.
Eventually I used a trick to get some characteristics. The amplifier output was connected to a 50 ohm dummy load via my SWR/power meter reading power. I also watched the draw from the bench power supply to the amplifier. The loop antenna was then connected and tuned to resonance, and the change inin parallel with the dummy load and the power draw again noted. From the increase in power taken by the added load (which was very small compared to the dummy load power) I could calculate an estimate for it's impedance. This turned out to be about 8000 Ohms! A 16:1 impedance transformer (more salvaged PSU parts) was wound and the antenna-amplifier system tested. As hoped, the SWR meter indicated a pretty good match (1.1 : 1) with 1-2W output, so the impedance estimate was close. Power is limited by the capacitor voltage rating - something anticipated at the outset.
Going further, it is interesting to note the bandwidth of the antenna arrangement. Tuning is remarkably sharp - it is very hard to see the dip in SWR. A sweep across resonance, with the center frequency at 476 kHz suggested the limits for a 2:1 SWR were +/- 2kHz, an approximate "q" of 240.
Last thoughts - I'd like to configure some manner of keying the system and exploring whether the signal has any reach at all - I may be able to enlist my local ham group, or nearby online receivers for this. Most of all, it is fun to play around and see what works.
I welcome thoughts and ideas, and would also be happy to write all this up as some manner of scrapbook notes article for the LowDown if there is any interest.