A Surprisingly Smooth Assembly of the QRP-Labs Low Pass Filter Kit

April 7, 2024

Nearly every radio project needs a low pass filter at some point, the QRP-Labs LPF kit is a quick and easy build that can knock out harmonics for nearly any low power HF radio. This is a compact, modular kit implementing a 7-element low pass filter which can handle 10 watts safely. Variants are available for bands from 2200m through 1.25m.

An assembled QRP-Labs low pass filter.

The kit was developed by Hans Summers, G0UPL. With Tor F.I Anderson, SM6FWF, contributing the 2m and 1.25m variants. I’m using these filters as the final filtering stage of a 10-band POTA rig I’m building around the HackRF One.

For details on the kit check out QRP-Labs.com.

Pros

Cons

Safety Matters!

Keep a fire extinguisher nearby.

Soldering irons are hot. Everything they touch gets hot.

Have a fire extinguisher nearby!

Wear safety glasses.

Solder splatters. Your eyes are not easily replaceable.

Wear your PPE!

Work in a well ventilated area.

Fumes from heating the enamel coating on magnet wire can be dangerous.

Work in a well ventilated area!

Tools and Materials

Tools and materials used to assemble the filter.

  1. Soldering Station
  2. Multimeter
  3. Helping Hands
  4. Fun-tak
  5. Lead-Free Solder
  6. Tip Tinner
  7. Flux Paste
  8. Brass Tip Cleaner
  9. Solder Sucker
  10. Mini Pliers & Cutters
  11. Jewelers Loupe

Parts Included

Parts supplied with the 60m QRP-Labs LPF kit.

  1. 2x 680pf ceramic capacitors
  2. 2x 1200pf ceramic capacitors
  3. Magnet wire
  4. 2x 5 pin header plug
  5. 3x T37-2 toroids
  6. PCB

Note: These are the parts supplied for the 60m filter kit which I’m building. Part values vary according to the band targeted by the filter.

Assembly

Before you start, check and label your capacitors to figure out which ones are which. Unless you’ve been bitten by a radioactive spider lately, you’ll definitely want to use a jeweler’s loupe for this.

Reading the markings on a ceramic capacitor.
The identified capacitors sorted for assembly.

Like most amateur radio kits you’ll be winding toroids. Unspool and cut the supplied wire into thirds.

When winding it’s important to wind in the correct direction, in this case all the coils are wound counter-clockwise. If you wind in the wrong direction you may get more inductance than you bargained for.

Each pass through the center of the toroid counts as one turn. This 60m band filter requires two 23 turn inductors and one 24 turn inductor. Follow the chart in the kit’s instructions if you’re building for another band.

Don’t worry about spacing yet, keep the turns tight against the toroid and pack them together every few turns.

Pro-tip: If you’re building multiple filters this is a great place to recruit the help of an unsuspecting friend or loved one!

Magnet wire being wound around a toroid.
The three completed toroidal inductors.

Keep a small gap at the bottom of the toroid to reduce capacitive effects, evenly space the turns around the toroid once you’ve finished winding.

Always check your work and keep L2 (more turns) separate!

With all the toroids wound and the capacitors identified it’s time to start soldering.

Start with the capacitors since they are the smallest components. I’m using a bit of fun-tak to keep the capacitors in place while I solder from the bottom.

Capacitors held on the board with fun-tak.
Capacitor leads ready to solder.
Capacitors soldered and trimmed.

The fun-tak will leave a little residue where it was heated by soldering, this isn’t usually a problem and it’s easily removed with a bit of fresh fun-tak once the board has cooled down.

Next, scratch up the leads of your toroid coils with a bit of brass from your tip cleaner. An emery board or a bit of sandpaper works too but I’m cheap and impatient. (I didn’t have any and I wasn’t going out to get any.)

You don’t need a clean strip, it just has to be scratched up enough to get solder through and burn off the rest of the enamel.

Scratching enamel from a toroid lead.
The prepared toroid leads.

Slip the coil ends through the holes and add fun-tak to secure them.

Trim off most of the excess wire and solder the coil ends. It may take a bit more heat or a few extra seconds to get a good joint. The enamel coating needs time to melt and boil off.

Toroidal inductors held in place for soldering.
The soldered inductor leads.

Solder bridges don’t matter between the ends of L1/L2 and L2/L3. These pads are connected on the PCB anyway. Just make sure solder is wicked up the wire and onto the annular ring.

While we’ve got the board upside down, add the headers and fun-tak them into place.

Headers held in place on the filter board.
Header pins ready for soldering.

Flip the board over and solder a single pin of each header, check alignment, then solder the rest of the pins.

Remove the fun-tak and clean off the residue with a bit of fresh tak. Some residue is left on my board but not enough for me to care. If you care about the residue feel free to spend all day cleaning your PCBs.

The completed low pass filter board.
Checking continuity through the assembled filter.

Before we call it a day, check for continuity where we need it and a lack of continuity where we don’t want it.

With this filter design IN should have near-zero resistance to OUT. Both sides have the same GND, and there should not be any connection between IN/OUT and GND.

Checking resistance between filter terminals.
A final continuity check on the filter.

I’m very surprised that I got through this build with no notable mistakes. I usually try to make at least one but I have nine more of these filters to build so there’s still time. Integration into the rig and testing of the filter’s performance is coming in a future post; until then, see you on the bands!

73, AI6YM

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