SignalSurge

This is an open source (check licensing here) Bandpass filter for VHF and UHF amateur radio bands, with a selectable LNA and a sequencer. Projected using Eagle CAD 9.

Why?

I live in a metropolitan area and very RF-polluted. One of these days, only FFS I have added a FM bandstop filter and I found that, contrary to my belief, my reception was way better! That means:

1. The front-end radio filtering is less than great (even being an expensive FT-991A)
2. Strong out of amateur band signals (and there's plenty of them) will desensitize your radio
3. By filtering out unwanted signals, you give more dynamic range to your radio, improving your reception.
4. See for yourself here.

That said, the objective of this project is to remove as much as possible signals outside 2m and 70 cm bands (like: FM broadcasts, air band, commercial VHF & UHF, and everything else inbetween), ensuring that the precious front-end gain on the radio built-in amplifier focus on amateur band signals only.

I could buy a 300 EUR filter, but... man, that's expensive. So why not expend EUR3,000 man hour-worth of my time and parts to build one?

Features

• Two selectable amateur band (2m and 70 cm) bandpass filter weeds off signals, ensuring that the transceiver front-end focus its gain figures on inband signals only
• High performance and selectable low noise 15 dB amplifier after the bandpass filter, adding some oomph to weak signals
• The amplifier is shut down when the radio is transmitting by powering off its 5V rail (by cutting the regulator "Enable" line)
• Specialty RF relay allowing 50W in VHF/UHF frequencies to flow to the antenna with low loss, while keeping unparalleled isolation to the downstream components (north of 60 dB)
• Relay default state (NC) is to bypass radio directly to antenna, allowing the board to be safely powered off
• TX Inhibit functionality to prevent the radio to transmit while the relay isn't positioned
• Static bleeding and small surge protection when switched to BPF/AMP
• Co-Planar Waveguide (CPW) design on RF lines for impedance control and good RF performance

Library

Eagle custom component library here

Bill of Materials (CSV format, DigiKey format) here. Notice that there's a full RF Inductor and Capacitor kit specified in the BOM - and that is costly. If you have the suitable RF L/C components, fine! Otherwise (like me), buy a proper one.

Schematic:

Board layout (Rev. A)

Why the large exposed pads around the BPF/LNA?

The objective is to test an isolation layer, by covering the components with Kapton tape and covering it with copper tape, to mitigate external interferences.

BPF performance

Check the test results page.

BPF theoretical performance

Here's the calculated theoretical filter performance, for VHF and UHF bands:

Performance notes

Notice that the theoretical performance involved ideal components, no parasitics, infinite Q, etc components. The filter will be refactored with lower value components to bring the top of the curve to the target area.

Amplifier performance (untested, theoretical)

Here's the theoretical BFP460 gain figures as per the application notes:

KNOWN ISSUES

Ensure to check the Issues backlog

RELAY LOGIC

BPF and Relay control

Truth table (NOR)

RADIO_TX	FORCE_BYPASS	Signal Level	Behavior
0	0	1	VIA BPF
0	1	0	BYPASS
1	0	0	BYPASS
1	1	0	BYPASS

RX/Via BPF states:

Circuit	Connected to	Power
Relays	NO	+
Isolation	NO	+

TX/Bypass states:

Circuit	Connected to	Power
Relays	NC	-
Isolation	NC	-

LNA

Truth Table (AND)

BPF_ON	LNA_ON	Signal Level	Behavior
0	0	0	OFF
0	1	0	OFF
1	0	0	OFF
1	1	1	ON

LNA On

Circuit	Connected to	Power
V.REG.	EN	+
LNA SW	NO	+

LNA Off

Circuit	Connected to	Power
V.REG.	EN	-
LNA SW	NC	-

JOURNEY

• Mar 17 - Tested relay logic (it is sound!), voltage regulators. And also fried the first board by shorting the 13.8V line on the 3V line before the fuse.
• Mar 16 - Tweaked the filter to bring the peak 2m BPF performance to the center of the band (146 MHz). Done in trial and error.
• Mar 11 - The boards arrived and assembled the Rev. A board - the BPF works! A bit out of frequency, but it works. More testing and component tweaking in the make.
• Feb 19 - All parts inventoried and accounted for. Placed the board order
• Feb 07 - Bought the needed parts!
• Jan/2025 - Reached the "minimally lovable project" stage.

CHANGELOG

• Rev. A: Initial release (Testing it)

ACKNOWLEDGEMENTS

I'd like to thank the following contributors:

• The fine r/rfelectronics community for helping me square out a RX mute/isolation (tip: it was right under my nose) issue and how to select the large signal switch and a critical review of the bandpass filters
• The r/AskElectronics community for the sanity checking on using a PNP transistor to switch a ground signal.
• The mighty and fine Gordon Hudson (AD5GG) for: guiding me on component layouts, deciding for a proper impedance control, his primer on CPWs (coplanar waveguides) and assorted awesomeness.