Building an RF Probe for Fun and Measurements

BITX40 PTT Enhancement for Digital or CW

MultiBand BITX40 Conversion

MultiBand BITX40 Introduction

In Several Older Posts I’ve touched on ways that you can make a multiband BITX40 radio. While those methods worked, they were experimental. The Dual Band option will likely work fine, but I found it overly complicated for the purpose. In this post, I’m going to show you a way that you can use your BITX40 on almost any band without compromising any filtering. That means 80m, 60m, 40m, 30m and 20m. Other bands are likely possible, but I have not tested them.

Step Zero: Overview

We’re going to remove the Low Pass Filter and Band Pass Filter while saving all the parts to re-build them on perforated prototyping board, and augment them with Low Pass Filter and Band Pass Filter kits from QRP-Labs, which are inexpensive and simple to build. This will allow you to plug in any LPF/BPF combination and get on the air on that band, provided you have a sufficient antenna for it.

A Word About Power Output

Before we get too far along, you need to know something: The BITX40 PA’s power drops off past about 13mhz. By the time it gets to 14mhz, it puts out about 1w of RF. Testing has shown that the power loss is all happening in the PA. I am still working on understanding the reason for the power drop and coming up with a remedy for it that is easy to duplicate. But for now, if you proceed with this modification, please know that power output on 20m is limited to 1w. This is plenty for WSPR and will get you contacts on any of the Digital Modes.

Step One: Demolition

I recommend reading through all this if you decide to proceed, since these steps should be considered irreversible.

First we’ll remove the Low Pass Filter. That’s L6 and L7, and C160, C161, C162, and C163. HFSigs used super glue to hold these inductors down, and so be careful when desoldering them. The fumes can be quite overwhelming! For desoldering the capacitors, you don’t need anything special- just use your soldering iron to heat one side, and then quickly heat the solder on the other side, while pushing it off its solder pad. You don’t need a fancy soldering iron, but it helps to have adjustable temperature so you can turn the heat up. I use this inexpensive soldering iron I found on Amazon and wrote about here.

For the Band Pass Filter, there are two options: You can bypass it by cutting the traces as shown on the Dual Band post and either manually or automatically switch between them, or you can remove C1 through C7 and L1, L2, and L3 and prepare for the next step. Be careful with the removal of L1-L3, as the toroids can break. Be gentle with them as we intend to reuse them later.

TIP: Write C1 through C7 and C160-C163 on a piece of paper, and tape the capacitors to the paper in their respective positions. This way you can re-use them later.

Lastly, remove the Antenna connector and the PA Power connector. The holes will have solder in them, but we’ll need to re-use those holes. I recommend a solder sucker, but if you don’t have one, then heat the solder until it is molten, and gently tap the board against your workbench. The solder will come out of the hole and form into a little ball on your workbench. Be careful doing this, you don’t want to damage anything!

Step Two: Power and Antenna

If you’re never going to use more than 12V to power the PA, then simple run a wire jumper under the board to the 12V connector on the opposite side next to the PTT connector. Otherwise, you’ll need to figure out another way to get power to the PA.

The antenna connector can be carefully re-soldered to a bit of wire, and that wire can be soldered into the holes it the connector originally went to. You can run this above or below the board, it doesn’t matter.

It should look something like this, minus the scars from previous surgeries:

Step Three: Filter Module Sockets

In this step we build filter module sockets. I made my sockets compatible with the QRP Labs filters. For the Low Pass Filter I robbed the 4 pin sockets from another QRP Labs kit and used perforated PCB board to mount them. In the picture below you can see the underside of the board:LPF Socket

The top side of the board will be used for grounding. I used pre-tinned wire, but any solid wire works fine for this build as long as it is bare and not enameled. You can see the wire bent up at the corner. This goes in the 12V PA power ground hole and grounds the filter. The right bottom corner will connect to C155 with a short wire first soldered to the LPF mount. Test fit first before soldering.

The bottom right corner is the signal ouput, and will have a wire going to the hole where L7 used to be, right next to the mounting hole on the corner of the PCB. I also recommend using tape to insulate the bottom of the LPF mount from the BITX40 PCB and use hot glue to affix the board at the edge of the BITX40 PCB. Here is how mine turned out:

You can use the same arrangement for the Band Pass Filter, but I found it easier to use a connector from the floppy drive cable from an old PC. I used my DVM to trace the wire continuities and made the same connections to as we did on the LPF: Signal input/output and ground. If you follow the trace from C41 until it gets to C6, and solder one side of the filter there. Solder the other side to the where L1 is closes to the PTT connector, and solder ground to the ground side of any of the capacitors C1, C3, C5 or C7. If you are unsure which side is ground, check with your DVM for continuity to ground elsewhere on the board. Here’s how that turned out on my build:

Low Pass Filter Socket

Step Four: Filters

For the last step, there are two different routes to go. If you saved the LPF and BPF components, now you can use perforated board to re-build them into pluggable modules. I’ve done so, and here’s how my Low Pass Filter turned out:

You can see the ground bus that all the capacitors connect to, and the connections for the signal side. It’s very basic and just follows the schematic. Below are the 40m, 80m, and 30m Band Pass Filters. The 40m filter is the original BITX40 Band Pass Filter and the others are from QRP Labs.


I highly recommend the QRP Labs filters. They’re easy to build and pretty easy to adjust, too. You can likely get away with using 80m, 60m, and 40m BPF’s with the 40m LPF, and then 30m and 20m will need their own LPF’s. Likewise you can likely use a 15M LPF for 15m and 17m too.

Step Five: Have fun!

You can see the multiband bitx40 in action on our Youtube channel here:

Now that you’ve built a multiband bitx40 you can go have some fun. Check out our post on a simple 40/20/15 Dipole antenna for under $20, and also our post on how to build an L-Match tuner so you can use an End Fed antenna on 80m or 60m, or even other bands if a dipole won’t work for you. Let us know how it goes in the comments below! 73!

Wait- That’s it?

Okay, there’s one major thing missing in this blog post- the VFO! The Raduino will have to be re-programmed to allow you to tune other bands. There are multiple projects for expanding the Raduino programming, and I suspect that Allard’s BITX40 Raduino Sketch will work fine for this. You can also consult the BITX20 group and/or the QRP Bitx40v3 40m radio kit page on Facebook where many of us hang out and discuss such modifications.

My rig is using the excellent QRP Labs VFO and it allows me to simply tune up and down the band without any modifications. The offset does need to be set to +12mhz for bands 20m and higher. If you want a multiband BITX40  with no software mods, I highly recommend it.

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