Take your pick on the title as they are both accurate. What the titles do not imply is just how important this little project is.
For some time now I have been talking about wanting to go camping and operating 630m portable. I thought a good start would be car camping at Cedar Hill State Park, just 3 miles away as the crow flies and within the 10 km limit of fixed-portable operation as specified in my grant. I had visions of a 100-watt amp with CW waveform shaping built in, a tall wire Marconi suspended from the trees with a laptop to run digital modes and an SDR to receive with. A nice sized RX loop would be good too. Well, OK, that sounds like a nice idea but requires a lot of work to make everything talk and a lot of prep to make sure I am not heading home because I forgot important, mission critical parts.
I’ve never been considered a minimalist but do follow and respect the QRP community quite a bit. I’ve personally known a few hardcore, minimalist, outdoorsmen that take every ounce of bulk into consideration. I tend to hike heavy and suffer for it later. In this particular instance, the challenge of a small, very portable, battery-powered 630-meter station sounded like an interesting proposition. Now, in the interest of full disclosure, this is not my design, I simply packaged it for my own use in Altoid tins. I have GW3UEP to thank for his well thought out design.
Here we have the QRP QTX (R) with additional filtering that was excluded from the original GW3UEP design. In fact you will notice the small LPF network that was stripped and modified from the Hans Summers 2200m LPF kit that I acquired and never used since my grant does not include 2200m. I’ve added an additional pass-through and switching option to accommodate a receiver. Switching is a manual process. This 1W PA is driven by the the GW3UEP VFO/buffered divider (L) and develops 12V P-P at 475 kHz. The actual tuning range of the small variable cap put my CW signal right at 474.5 kHz where I am often operating CW these days. The whole system runs off of a 13.8V power pack but the 12v regulator seems happy with 13V. The output wave form is very clean after adding the LPF. Its really quite amazing just how harmonic-rich the small class-D amplifier actually was.
Not pictured is a small reflectometer of GW3UEP design that generates forward and reflected DC voltages that are read with a small, very lightweight and certainly disposable Harbor Freight VOM. In the field the idea would be to look at both the forward and reverse voltages while making antenna adjustments, probably involving the moving of taps on a loading coil. The idea is simple – maximize the forward power while minimizing the reverse power.
As SWR is really a nebulous metric, particularly at these frequencies, and losses are low for even the smallest interconnects (like the RG174 I intend on using), best case values for the measured voltages in the field are fine. This particular PA is not particularly sensitive to open circuit conditions and oddball impedances. The goal is simply to find the highest forward voltage which will correspond to the lowest reflected voltage.
And finally, what we have all been waiting for: the antenna. This is still a work in progress but the current thinking with the interest of weight in mind is to use a non-conducting hiking pole which is wound with a tapped loading coil. RF can be fed at one end and some length of antenna wire can be clip-leaded to the top. Its unclear just how much voltage might be developed on a short antenna at 1W TPO but lets just assume for safety reasons that its a few hundred volts. That is really dependent on the final wire length, ground conditions and radial system presented.
Hopefully I can make some progress on this antenna situation very shortly and provide an update here or from the field. Stay Tuned!