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Building a Direct Conversion Receiver: Part 3

Building QRP Labs Band Pass Filters: 3 Tips you should know!

QRP Labs Band Pass Filters are these wonderful little kits that largely take all the question of building a filter. They use a proven design, the kit includes the inductors and wires, and the build instructions are top notch

One thing I love about the QRP Labs Band Pass (and Low Pass) filter kits is that they are great for home brew radios too. If you purchase a PCB header kit, you can make your own sockets for the QRP Labs filter, making them useful in any of your own radio projects. 

I have used the QRP Labs Band Pass Filters in my BITX40, U3S, and now in the DC40 that I’m building. For the DC40, I am using a 40m band pass filter from QRP Labs. I built it over a year ago but it didn’t perform right,  so I shelved it for another time. Well, that time came, and I needed to get it working correctly. Thanks to Randell Curtis (W5ZJ), I have a new trick to share along with a couple of my own tips.

Also, of note: These tips apply to any filter circuit you might use for your radios. I use QRP Labs filters, but the principals apply much more broadly than that.

Tip 1: Follow Directions!

I know, I know- a basic tip. But really, read the directions thoroughly before even opening the kit. Pay particular attention to the directions for winding the toroids for 40 meters and down. The inductors on 30 meters and up are sparsely wound on the toroid, and so you can adjust the windings on the toroid to change its inductance after you build it. The bands below 40m are wound too tightly on the toroid to be able to move the windings, so to change the inductance you have to change the number of turns on the toroid. The instructions account for this. 

If you follow the directions, it will be easy to add/remove winds after installation. If you don’t, you might end up trying to add/remove turns from under the secondary winding, which is impossible. Then you have to pull it all apart and do it again. Twice. Like I said, the 40m filter didn’t work at first! 

The only possible modification to the directions that I am going to try the next time I need to wind a 40m or lower band filter is to leave extra wire for the 4 extra turns that Hans calls for, but only wind two of them and leave the other two unwound. I ended up not needing them, but you won’t know that until you tune the filter.

Tip 2: Tweeze, Zoom, Enhance.

Zoom, Enhance isn’t just for cheesy TV crime dramas anymore. It’s for winding toroids. Some builders have a hard time with toroidal inductors. “I hate winding toroids!” they say. Really, they do look kind of complicated- but it just looks that way. Toroids aren’t hard, but if you’ve got big hands and fat fingers like I do, putting 40 turns on a tiny ring only 0.37″ across can be quite a challenge. Tools to the rescue! 

When I wind a toroid, my first dozen or more turns are done by hand. I don’t use a fixture to hold the toroid. Fold the wire in half and start the first turn from the middle. This makes wire management easier. The instructions actually recommend that, so nothing new there. The problem is that sometimes it’s hard to get the turns not to overlap, or the wire gets short and is hard to manage. Tweezers are your friend! I robbed a pair from my wife, and they have an extremely fine tip. They’re invaluable for delicate work. I think they were intended to pull eyebrow hairs, but they work on electronics just fine. 

The next tool you will want is a good camera on your phone or tablet. I use my Samsung Galaxy Tab A. When I lose count of how many turns I’ve made, I set the zoom level to about 2x, and then take a photo of the inductor. Then I zoom in on that photo until the inductor is just about full screen. Then I start counting. This makes it so much easier. See the picture below. This is a T37-6 0.37″ toroid, but this method makes it trivial to count the number of turns.

How many turns do you count?

Tip 3: Tuna Filter

You’ve got your filter built, and now it’s time to tune it for its intended frequency range. The QRP Labs band pass filter directions are very very good, and if you have just a radio, you can tune the filter sufficiently to get good performance. But, if you own an antenna analyzer such as an MFJ-259 or 269, then you’re in for a treat! I’m going to show you a trick I learned from Randell Curtis on the BITX40 and uBITX Facebook group. 

On the side of the filter marked “Out/GND” connect a 50 ohm resistor, and connect the IN to the center of your favorite RF connector (I used an SO-239) and the GND to the outer conductor as seen below.

Those are two 100ohm resistors in parallel. I didn’t have a 50ohm anywhere!

Connect your antenna analyzer, and tune put the tuner in the middle of the intended band. For 40m that would be 7150khz. Sweep across the band and find out how the filter performs overall. You’ll tune the filter just like you’d tune an antenna.

A band pass filter will have a low SWR at its target frequencies, just like an antenna does. This is because the filter (in this case two LC circuits) resonates at the target frequency, and so it presents a low impedance to the target frequency ranges, letting them through. To all other frequencies it presents a very high impedance, effectively blocking it. 

I swept from 6 to 8 mhz and found that my filter worked best at 6.6mhz- too low! If you use your imagination you can “see” the plot just as you would on a spectrum analyzer. Too low a center frequency means the inductors have too many windings, so for the 40m filter I removed turns until I got in the intended range and then fine tuned them with the capacitors. It takes just a moment to read that sentence, but it took me way too long to get it working, mostly because my filter was built wrong and I overheated the trimmers, so they had to be replaced too. 

Check the video below to see a sweep of the filter after spending some quality time tuning it up. Notice the very low SWR across the 7-7.3MHz range, and how the SWR rises sharply before and after it. This indicates a good tune. 

I hope these tips have helped you! Leave your own tips below, and lets keep the knowledge flowing.  73 de W7RLF

2 comments

4 pings

  1. If you don’t have an antenna analyzer, you can also use a dip meter. connect the 50 ohm resistor to the filter output, and a jumper lead between the input and ground. Wind the jumper lead around the coil of the dip meter (try 2 turns to start). Then use the dip meter to find the resonant frequency. You can set the dip meter to the middle of the band, and then tune the two trimmer caps to get the filter right on. Check the dip meter frequency by listening for it on your receiver. My home made dip meter has an output that I can connect to a frequency counter, even better!

    • Golodkin on December 17, 2018 at 3:08 PM
    • Reply

    Would this work with an MR100?: https://i.imgur.com/kQVO3KC.jpg

    Would measuring the toroids passively with an LC meter (there are some that say that passive inductance measurements are inaccurate — I dunno) locate the best/worst of the three?

  1. […] Building QRP Labs Band Pass Filters: 3 Tips you should know! […]

  2. […] Yes, you could buy the discrete components and build a band pass filter yourself, but you won’t come out ahead. The QRP Labs kit build instructions are top notch so be sure to go there and follow them carefully. Also check out our page “Building QRP Labs Band Pass Filters: 3 Tips you should know!” […]

  3. […] (and wrote a giant series on it: Part 1, Part 2, Part 3 and Part 4. And somewhat unofficially Here!) and at the same time, Hackaday.com posted their Circuit Sculpture contest. Would it be possible […]

  4. […] the rest of the tips we’ve collected, check out our post “Building QRP Labs Band Pass Filters: 3 Tips you should know!” […]

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