Sold as a kit; assembly is required. Please consult the table below for the component layout. Capacitors are 1206 (or 1210) size SMD. This size is reasonable for hand soldering. All supplied magnet wire is solder heat stripable. Improved assembly and technical documentation is coming soon.

T50 Size

*This is my best guess for regular use. This could change drastically (+ or -) in different conditions. The main limitation is core heating. All supplied 1206 and 1210 capacitors are rated for at least 500V. PCB traces are 3mm 1oz copper.

**<1 dB only below 51 MHz as this is optimized for CW and SSB operation. Up to 3 dB attenuation at 54 MHz.

Features

Designed with compact size, low weight, and ease of deployment in mind, these filters are ideal for portable amateur radio operations, including SOTA, and POTA.

• Many of these filters use G3RJV‘s recommended component values (Table 1) which are tried and true.
• Attenuation measurements are from NanoVNA (see picture gallery).
  • The shape of the frequency response has been verified with an oscilloscope. However, the attenuation values from the oscilloscope are less accurate due to losses in the measurement setup. I would trust the VNA’s values more.
• Additional testing and characterization are in progress. More detailed test results will be added as they become available.
• The filters work well with the QRP Labs 50 W PA. The PA’s built-in filter must be bypassed when using these external filters. Automatic filter bypass module for this amplifier will be available in October.
• I am also working on a QMX modification that will use these filters to provide 160m–6m operation with only four external filters.
  • 2200m and 630m operation should be possible as well. This is in the works.

Connector Note

The compact connectors used on these filters were selected specifically to keep the overall size small and allow for quick deployment. They are not as mechanically durable as larger, more conventional connectors. This connector style has become increasingly popular in recent years, and I believe it provides a reasonable balance between size, convenience, and durability for portable operation.

Important Information

• Kit is sold unassembled. Assembly is required.
• Component layout is provided in the table.
• Documentation is currently being improved and expanded.
• Components may vary from images due to availability.
• No returns on assembled kits.
• Shipping is limited to the lower 48 United States.
• Contains lead.
• Do NOT touch during transmit. High voltages exposed.

These filters are designed with portable operation in mind and prioritize small size, light weight, and quick deployment over ruggedness. They are a great option for operators looking to add external harmonic filtering to a compact or portable station.

Designed and sold by AD8C

The Fine Print
Use at Your Own Risk: This is an independent project and is provided as-is. This kit is intended for educational, evaluation, or hobbyist assembly. By purchasing and assembling this board, you accept all risks. The creator/seller accepts no liability from the use or misuse of this product.

Trademark Notice: QMX is a registered trademark of QRP Labs. This project is entirely independent and is not affiliated with, authorized, or endorsed by QRP Labs.