Some online calculators and open source software made wideband loop helper design easy. Using a simulator also helps you make adjustments to use readily available components.
Ok, so we are doing a wideband loop helper design to include switchable attenuators and bandstop filters. Attenuators are by nature wideband and matched to 50 ohms. You can easily design these as a Pi network of three resistors. A number of online attenuator calculators make your job easy.
Just enter your desired attenuation and the calculator provides resistance values. Or, you can enter your resistance values and see what attenuation you can achieve. I started with a basic design and then tested my resistors to find the best match. In my case, I made a small adjustment based on availability, and ended up with an 18 dB attenuator.
You can see the attenuators on the left above, made with two 68 Ω and one 180 Ω resistors. SWR of this combination is about 1.2 which is good enough. Since the wideband loop helper design is for receiving only, one-eight watt resistors sufficed.
Bandpass filters are a bit harder to design. You can find a very useful LC filter calculator at the RF Tools web site. For my first effort, I chose a 3 pole Butterworth filter design, which requires three tuned circuits (inductor – capacitor combinations). After measuring my components in the workshop, I chose the best combinator to provide me with the deepest null around the strongest local stations, from 660 to 1060 kHz.
LTSpice came in handy to simulate how the circuit might perform, as shown on the right above. Using real world components and simple construction practices, I was not expecting to achieve the simulated >100 dB null. But I figured I could at least achieve 30-40 dB results. Real components usually contain some series resistance which tends to make nulls broader and shallower.
Wideband Loop Helper Design Circuit Board
Designing my printed circuit board in KiCAD was straightforward. All of my components were standard sizes and footprints. To accommodate my isolation milling learning curve, I made the traces and pads fairly wide at 2 millimeters.
You can see the 3D view of the PCB component side in the above left picture. I used simple header pins to connect wires from the attenuator and filter sections to the DPDT switches. Later, the switches would just be soldered to the center pins of the BNC connectors. More header pins were provided to attach the ground plane and BNC shields.
When I mill the PCB, I will leave most of the lower side as a ground plane for all the common connections. Since my target frequency range is under 30 MHz, this rudimentary design should work. Of course a much tighter design with better component tolerances and SMD circuit board would be better, and I might try that later.