The 0.1 Pound Station

I'm glad we had the chance to work each other during the last Sprint. Thanks for your kind words. I can appreciate your desire to get the weight down. I can well remember a point last year when I scratched my head in "wonderment" when one station posted a weight of 0.3 Lbs. There I was, running my Sierra with a 3 Lb. gell cell and a heavy brass paddle.

For me, the key was setting goals for small weight reduction each successive month. Taking the top of the Sierra was the first step. That top weighs the equivalent of a pack of eight AAA's! The battery weight is a major issue. Switching to AAA's will make a big difference. Rigs with very efficient finals may allow the use of a 9V lithium battery. I laughed the first time I picked one of those babies up at Radio Shack. The lithium batteries are much lighter than the pack of AAA alkaline's. Combining the AAA's with a small xcvr like a SST or a SW-20+ will get you near 0.5 lbs if you go with lightweight paddles. By dispensing with the outboard paddles and keyer, I was able to shed more weight. Using a microswitch is not the fastest way to work a contest, but it is certainly doable--and quite light. My SW-20+ needs about 10V to keep the rx happy, so I put a AAA alkaline cell in series with a 9V lithium battery. I can run 2 hours + at the 150 mW level with that power plant. If you feel that you MUST run at the 1W level, stick with the AAA alkalines. For 2 Watts, AA alkalines will probably get the job done. For 5W, join the Tubby Club-HI!

The SW-20+ was performing well at 0.3 lbs. (I replaced the standard size controls with pc mounted trimpots to get to this weight). VFO tuning was done with a plastic tuning tool. The volume control pot was removed and the input rx bandpass xfmr was adjusted as a volume control using the same tuning tool. Crude, but it works and saves weight…The rx still worked like a champ with these changes--no case, no outboard controls, no external paddle or keyer, small battery--Boy, the Sprints started getting a lot more interesting, and the Q's per pound ratio shot on up there. I simply place the circuit board on a piece of plastic sheeting to keep the sand off the bottom of the board.

Having gotten this far, I began dreaming of a way to pull out all the stops and see how far the weight reduction process could be taken. The SW-20+ is a wonderful rig, but it has a relatively heavy double sided circuit board, and the design of the tx PA would not allow any reduction in battery voltage. The battery question had to be resolved, too. I searched the website of Duracell and other mfg's to determine if there were any really small batteries that could supply sufficient current to allow operation at the 50-150 mW output level for a 2 hour time period.

A revelation took place when I saw a tiny lithium photo battery, type PX28L. It is readily available many places--even Lowe's. This little critter weighs only 9 grams, yet is rated at 250mA continuous at 6V and 3A surge current. Knowing that a suitable battery was out there, I began studying various QRP sources for a really barebones RX/TX design that would work at the 5-6V level. I ended up going with a minimum component DC receiver using a NE602 and a LM386. The TX consists of a 74HC240 IC, which runs quite efficiently on a 5-6V power supply, producing from 100-200mW output…A VXO was built to provide stable frequency control for the rx and tx.

At this point, I must confess that I am not an engineer. Some engineers would roll their eyes at my little "Firefly 20." All I can say is that it gets the job done. The rx bandpass is barndoor wide. You hear everybody in the contest. Heck, you get to hear 'em twice, and Radio Canada Internationale at the same time. What more could a guy ask for, as he's trying to zero beat some weak station amongst the hordes? :-)

I have never laid out and etched a real pcb. For this project, I used a miniDremel tool with a tapered grinding bit to make the traces. This is not a slick way to do it, but it works VFB. I like Manhattan style construction using glued on pads, but for this project it made more sense to grind unwanted stuff off than to glue extra pads onto the board. No surface mount components were used, and sometimes 2 parts were used in parallel when one part of the "right" value (which wasn't in my junkbox) would have sufficed.

BNC connectors weigh a TON, relatively speaking. I used plastic miniclips to attach power to the battery and to the antenna. These little connectors have spring loaded brass contacts. I bought mine at R/S. Stay away from heavy metal connectors if you can.

I used single sided pcb stock for the little Firefly. The final dimensions were about 2-1/2" X 3" I was really nervous when I put it on the balance for the first time. I have access to calibrated balances at a gov't facility for which I do contract work. The weight came in at 0.09 Lbs (including battery, earphone, microswitch key). Stunned, I walked into another lab and asked permission to use their balance. Same result, down to the thousandth of a gram!

Here is the top side of the board:

And here is the bottom:

The next thought was, "hmmmmmm, would it be possible to include the antenna and still be able to stay in the 0.1 Lb range?" A physics textbook revealed that a 10M length of #28 copper wire would weigh around 7 grams--perfect for a 20M halfwave vertical. It took a couple days to come up with a suitably featherweight L/C matching network that would match the antenna to the rig. The final result was the complete station, including xcvr, battery, key, earphone, and antenna system (excluding the physical supporting mast) weighing in at 0.143 lbs. A few months ago, I would have told myself that such a thing absolutely impossible, but you never know until you try. :-)

A number of fellow Sprinters have asked for details on the antenna. Some have had good success with verticals, while others report dismal performance. So much depends on the environment. I am fortunate in that the beach is not too far away, and I can take advantage of the significant low angle gain provided by locating my station right at the salt water line. 100mW into a vertical is plenty of power under fair to moderate condx at such a location. That same vertical at my home 100 miles inland from the sea does not even begin to compare with my dipoles.

My point is that if you want to run a really low weight station, you will sooner or later have to go QRPp. To be successful at QRPp levels, go with the best antenna you can find, and you will be amazed at how well your fellow QRP'ers will hear you. I often find myself grinning when I call a guy who is just above the noise level and he comes back without missing a beat. You won't run stations or generate pileups while running a featherweight station, but you will make 10-25 contacts if you have the confidence to pull it off. Fellow Spartan Sprinters have really honed their listening skills, which makes our QRPp shenanigans possible in the first place.

20M is THE band. If you have to choose one place to park your featherweight rig, you'll hear more stations and work more stations on this band.

I'd be glad to help in any way if you have any more questions. Have fun with the Sprints. A lot of us agree that the SP's are our favorite operating activity. The timing allows us to have fun while not taking too much time away from domestic and work related responsibilities. I'll be listening for N7SR next Sprint.

72, Paul AA4XX nr Raleigh, NC

aa4xx@ipass.net