GOING TO EXTREMES:
Something to Write Home About

To do well in the ARS "Spartan Sprint," the rig used must have a very high "performance to weight" ratio. Very simple rigs are OK for casual use, but in a contest like the Spartan Sprint, they usually aren't very effective.

The performance to weight ratio got very high when the "AT Sprint" rig was introduced about a year ago. This rig fits into an Altoids tin and as such weighs just 2.7 oz. Combining microprocessor control and DDS VFO, an efficient 4-5 watt transmitter and a sensitive, selective super heterodyne receiver, this little rig has performance to spare. In the hands of a good contest op with a decent antenna, this rig is hard to beat.

Original AT Sprint

A new version of the AT Sprint, the AT Sprint II, will soon be released. This new rig has the same high performance of the original, but draws half as much current on receive (25 ma vs 50 ma). However, it is somewhat larger in order to make it more build-able, to include an internal battery pack and to take advantage of a commercially available plastic box. This makes it a little heavier than the original, but at 7.5 oz., including the six internal "AAA" batteries, it’s still pretty competitive.

AT Sprint II

Enter the "Ultra-Light" version

It occurred to me a few days before the March Spartan Sprint to build a really, really, ultra-light rig, based on the new AT Sprint II design. Beating the 2.7 oz. weight of the original AT Sprint rig wasn't going to be easy though. The goal was to make a rig with a total station weight of 1/4 pound or less.

We were going to have to go to extremes. Every means possible to reduce weight would need to be employed.
The first thing to do was simplify the Mark II design some, so that it would fit on a smaller, Atloids tin sized board, yet keep the over all level of performance high. This wasn't too hard to do. Eliminate the slide switches for power and receive attenuation. Eliminate the headphone, paddle and power jacks and replace with SIP pin headers. Eliminate an op-amp in the receiver's audio chain. Use an audio AGC chip to eliminate the need for a volume control. Use all 0805 sized passive components and pack the parts closer together.

When we still didn't have quite enough room, we started placing parts on the bottom of the board. Instead of being mufti-band, the Ultra-light is monoband. Instead of T37 cores in the low pass filter, T27 cores were used. Using 031 mil circuit board material instead of the more common 062 mil board saved an additional 1/3 oz.

Okay, so we got the basic board weight down as low as it can be - 0.9 oz. Just over 1/2 the weight of the original AT Sprint board. We were off to a good start.

Now, what to do about the box? We’ve got to have a box. An Altoids tin is 1.2 oz. Way too heavy. We are, after all, going to extremes! Turns out a Sardine tin weighs just 1/2 oz. and is just the right size. We now have a rig which weighs only 1.4 ounces, almost a 50 percent reduction from the original AT Sprint !

Circuit board mounted in Sardine tin

The "Ultra Light" AT Sprint, showing Spaddle, battery and headphone piece

What about batteries?

This is the make it or break it area. I wandered over to the local department store to look over the battery selection. Wow, Lithium photo flash batteries look interesting, but they are wicked expensive.

Hummmm. How about these giant button cells? They looked promising. Bought three of the CR2450 button cells. At 550 maH, and less than an ounce for the three of them, they seemed like they'd do the trick.

Then I found their specifications. 5 ma max. continues current. 15 ma max. pulse current. Not too good. As the specs implied, they had a very high internal resistance and died under transmit current load.

I decided to give the good old 9 volt alkaline battery a shot. It worked! 2 watts out with a not so fresh battery found on the bench. We'll give these a shot. At 1.7 oz., our total station weight is now up to 3.1 oz.

Final touches

A weight of 3.1 oz. is outstanding, but we still need to add paddle and headphones to complete the station. And we only have 0.9 oz. left in our weight budget to stay at or under a quarter pound.

AA1MY's "Spaddle" design is about the lightest paddle you'll find. It's a vertically mounted, single lever paddle made from 031 mil double sided copper clad circuit board.

I made one which mounts to the top of the rig's box which added just 0.2 oz. to the package. Impressive. And it works!
All we're left with now is the headphones. I had a "hands free" cell phone mic and earphone set up which I found on a "anything for a dollar" rack in the local supermarket. I don't have a cell phone, but for a buck to get a mic and earphone, what ham could resist?

With a little rework, I had a single ear-bud headphone with a SIP plug on the end to connect to the rig. Why use two ear buds when one will do? Weight? Just another 0.2 oz. Now our total station weight is 3.6 oz. Our goal for the ultimate ultra-light station is meet.

Proof of the pudding

This is all well and good, but does it work? Probably not for me. My lousy antenna, sitting at the bottom of a valley in the mountains, hardly gets out past the door yard at night.

We got to put this gem into the hands of a pro, someone who can do it justice. Time to call on my buddy Seab, AA1MY. I contact Seab and ask if he'd like to win the Spartan Sprint that night, hands down. "You bet," he says, what you got? "You'll see" I say. "Be there in a bit."

On the way over to Seab's, I stop and buy a package of Duracell M3 Ultra 9V batteries. "These should do the trick", I say to myself. Half hour later I get to Seab's and he feeds me dinner. After dinner, we go hook up the little rig and I give him a run down on how it works.

With the old 9V battery I pulled off the bench, we give out a quick CQ before the sprint and get an immediate reply from Alan, N3BJ down in VA, with a 5X8 signal report. "Guess it works", says Seab.

The clock ticks down, the Sprint starts. We juice up the rig by connecting the fresh Duracell. Power out, nearly 3 watts. Seab starts working stations like it's field day, one right after the other.

Lots of mid-west stations, a few east coasters. Band goes long, start working into the west coast. Just before the end of the sprint, Seab pulls in a KH6. Grins ear to ear.

The final shot, sprint ends. Smoke clears: 56 contacts, 224 point per pound. Got to be an all time record. The 9 volt Duracell still hanging in there, power out down to about 2 watts. We're both impressed.

Getting late, got to run home. Seab asks if he can keep the rig for a while. "Sure, why not?" I say, and wander off home.
In the following days, Seab uses this little gem to work DX in Eastern Europe, Nambia, Somoa and Australia. Oh, did I mention this is a 40 meter rig? Some of the credit has to go to Seab's ladder line feed, 176 foot EDZ, up about 50 feet. And his good operating skills. And good band conditions. Maybe a little luck thrown in for good measure.

Can we go even lower in weight?

Is this rig the limit to how light we can go? Maybe not. I was given a 9V lithium battery to try, which takes another 1/2 oz. off the station weight. Though, I haven't yet actually tried it to see how well it works under load. 10 mil and even 5 mil double sided copper clad board is available to make the circuit on. The 5 mil might be a little too thin, but using 10 mil is tempting. Maybe make the box to enclose the board out of Balsa wood. It just might be possable to make a station which weighs about 2.5 ounces - talk about going to extremes!

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Steve Weber, KD1JV, is a renowned QRP designer, builder, operator and outdoorsman living in Gorham in the White Mounatins of New Hampshire.