Gain an edge on crowded bands, or, NEScaf(e) anyone?

New England QRP Club's NEScaf audio filter, built, buttoned up, and ready to run.

I don’t care what Punxsutawney Phil says. It’s winter! With one snow storm after another rolling in — even at latitudes where flurries are rare –  there’s plenty of time to:

1. Operate.

2. Break out the soldering iron and whip up a mean mess o’ accessories for the station.

In this case, one project at hand was the New England QRP Club’s spectacular audio filter, which the club christened NEScaf. Check out that link for the full scoop on the kit. This project was ultimately destined for another ham who wanted one but acknowledged that his kit-building days sat squarely in his rear-view mirror.

Warning, Will Robinson: Digression ahead!

For those of you who may be new to this ham-radio stuff, an audio filter is one sweet piece of gear to have, especially if you operate QRP and your radio is not graced with the built-in filters or digital signal-processing tools found in many a higher-power/higher-price radio.

If you’re a Morse-code buff, as I am, you’ve noticed that when you listen to another station, signals at higher or lower frequencies on the band have a different pitch in your speaker or headphones than the station you’ve tuned to.

An audio filter basically stomps down the volume of signals with pitches other than the one you’ve set the filter to emphasize — usually a pitch matching the “spot” tone that your radio produces to ensure your transmitting frequency is the same as the frequency used by the station you’re trying to contact.

This emphasized pitch is the filter’s center frequency. In practice, this “peak” actually may cover a few hundred hertz before the volume of an adjacent signal drops dramatically. You can adjust the width of this peak, allowing you to tailor the amount of filtering you need to the relative presence or absence of stations crammed near the frequency of the station you’ve set your QSL-card sights on.

Very knowledgeable hams will be happy to lay out the superiority of digital signal processing and filtering. I have no doubt they are right. But I had one of those radios once. Frankly, I found an audio filter simpler and more straightforward to use.

And no, I’m not a Luddite. I run Linux on my laptop, have Twitter and Facebook accounts, and bake bread.

And so, on to the NEScaf kit:

Here's what you get for your $30-ish -- neatly packed bags of parts. Pen and case not included.

Hams who have built this kit have housed it in everything from an Altoids tin to the insides of home-brew receivers or transceivers. I happen to like a case I found at an electronics store not far from home — the You-Do-It Electronics Center, or, as some of us say, You Blew It Electronics. Watch your polarity!

The case is cast aluminum, pretty rugged housing for portable operations.  And it’s just the right size to sit nicely on top of my Yaesu FT-817 (or atop an IC-703, for that matter) or the small outboard antenna tuner next to the radio. If you’re interested, the case is a Hammond 1590BBBK.

From the foregoing paragraph, you can perhaps sense that while I’m building this for someone else, I’ve got another stashed away for me!

And now for some solder smoke:

1. NEScaf board with the power-supply stage installed.

An experienced kit-builder can knock this puppy off in a day without breaking a sweat. I’ve built one of these before (and foolishly included it with a radio I sold). I tackled this latest project while on a week’s vacation recently — one where the to-do list of non-ham activities was long and included much quality time with a snow blower and snow shovel. So I built the kit in stages. And yes, those little tan capacitors really are the size of well-fed ticks!

2. NEScaf board with the rest of the components on board.

I put the pedal to the metal, or, I should say, the iron to the solder for construction increment No. 2. I had promised to finish the filter by date sort of certain, and I was falling behind schedule.

3. The case for the NEScaf filter, with controls and jacks mounted.

Though the case has thick walls, it’s not hard to work. The kit comes with a garden-variety green light-emitting diode as an “it’s on, oh Great Kahuna” power indicator. But I had a nicely pimped out LED in the junk box, so I used it instead.

When I’m working with cases, especially cases as nicely finished as this one, I put a old hand towel under it to minimize scratches during construction. Once this filter hits the field, it probably will get more than its fair share of scrapes and scratches, so let’s at least give it a clean start!

4. NEScaf board and battery mounted and final wiring complete.

Oh man, the end is in sight!

But wait! This just in from the Why Didn’t I Think of This Earlier Department:

Dear WDIToTED: I’m trying to install a battery clip, which uses a very short screw, lock washer, and nut, and I’m trying to secure small nylon nuts to circuit-board mounting screws very near the end of the case. I have big hands and no nut driver short enough or narrow enough to fit the small spaces involved. All of the hardware is 4-40. What to do?

Dear WTD: Grab one of the longest 4-40 screws in your junk box, feed the screw half way into the nut, then press this makeshift nut driver against the end of the actual mounting screw. Gently turn the mounting screw with a screwdriver until both screw ends share the nut. The nut will have just enough purchase on the mounting screw to allow you to remove the “helper” screw and use you finger to hold the nut in place while you finished the tightening process.

Rube Goldberg? Yep. But it solved what was becoming a frustrating problem as I tried to mount tiny things in tight spaces with XL-glove hands.

Once I licked that problem and finished connecting the circuit board to the controls, jacks, LED, and switch, I took the filter up to my radio, powered it up, set the center frequency for 600Hz and the amp section for no gain (less shock to the ear drums when you wear headphones and power up the filter). All was right with the world. QRM be gone, or at least mostly gone!

Now, if you’ll excuse me, I think I’ll head back to the basement la-BOR-atory and start in on my own filter.

Hmmm…I do have that memory-keyer kit to tackle as well.

Aw, heck, let it snow!