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Relay Switched Medium Wave Filter

relay switched medium wave filter

Here is my latest design for adding a relay switched medium wave filter to my receiving station. Designed with diversity reception in mind.

Previously, I wrote about the wonderful little Distill MW Bandstop Filter. You can buy this filter online for around $15. It’s a simple shielded notch filter that reduces local broadcaster signal strength by more than 30 dB.

If your SDR receiver lacks good filtering, this unit gets rid of most intermodulation and harmonics arising from strong local AM stations. It’s a cost-effective alternative to building your own filter. But, how to mount it?

I designed a double sided PCB board for mounting and switching two of these relays. With a cheap DPDT relay, you can bypass the filter or route signals through it.

I chose the 12V HK19F relay, but the 5V version would work just as well. Above, you can see two of my PCB mounted in a box. My Distill filter is attached through SMA connectors including some right angle elbows. Each board requires two PCB mount SMA connectors for the filter. The BNC connectors for signal channels are just soldered to the board.

Filter performance is almost identical to the medium wave notch filter built into the SDRplay radios. Some of my homebrew filters provide better attenuation, but for the price and convenience, my Distill filters are pretty good.

Relay Switched Medium Wave Filter Project Cost

Amazingly cheap. For a single channel, my hardware cost was under $20 for the filter, connectors and relay. I got the printed circuit boards fabricated at PCBGoGo for $5 (for five boards) plus $13 shipping.

So, the whole project came in around $50 for a dual channel system. Not counting a bit of 3D printing for the project box. Good stuff.

If you are interested, I can provide you the Gerber files and you can order your own PCB. All of the parts are readily available.


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