Why should I use a BCI filter?
The uBITx has a front end 0-30MHz band pass filter. If you live in an area where there are high power AM Medium Wave transmitters you are likely to suffer from Broadcast Interference (BCI). BCI typically appears worst on the 160m and 80m bands, but as a result of mixer products can appear on all of the HF bands. If you spot what appear to be loud AM signals in amateur bands you are probably suffering from BCI.
ubitx.net Recommended Solution
The simple solution is to place a high pass filter in the uBITx RX line (or between the antenna and the antenna input on your uBITx) with a cut off some distance above the broadcast band (normally somewhere below 3.5MHz). There are many such designs on the internet. A filter design with a cut-off below 1.8MHz is a bit more of a challenge, but see below for suggested values).
VK4PLN BCI Filter Solution
Nick Pullen VK4PLN has inserted a 3 stage BCI filter into the RX line. Nick’s filter is not intended to allow operation on 160m (3.5mhz High Pass Filter)
The design is based on a VK3IL project: http://vk3il.net/projects/broadcast-band-filter/
Nick uses a PCB. You can access his layout here
As this is intended as an RX only BCI filter he uses T30-6 toroids as follows:
- 1.622uF = 21 turns (0.4mm wire, 31cm length)
- 1.461uF = 20 turns (0.4mm wire, 29cm length)
Nick uses 0805 SMD NPO capacitors.
- 2x 560pF
The circuit board is 9mm x 33mm:
Nick cut the trace either side of the new HPF board, exposed a bit of the trace and jumpered onto that. You need to cut the second trace from the PA side (3rd trace down from the 45MHz roofing filter with the crystal) as can be seen in the photo below:
Taken from the top trace to the bottom in the photo, starting from the relay on the left and the PA circuit on top:
1: TX pwr to the PA circuits via R89
2: RX path (between K1 and K3 relays. (CUT AND INSERT HERE)
3: TX pwr to the LPF bank.
4: RX pwr to the receive circuit (R18,R38…)
Nick picked up GND from the 2 unused caps C216 and C210 as can be seen in Nick’s photo below.
Using a QRPMe BCI filter kit with VK4PLN insertion point
Howard K4LXY took care of his BCI issues using using a filter from QRPme and Nick’s insertion point as illustrated in the photo below:
A plug in BCI filter (same filter design as above and same insertion point)
Mike ZL1AXG shows how he used two 4 pin DuPont connectors to make two sockets for a plug-in filter the same size as QRP Labs filters (1.5 x 0.5 inches or 38.1 x 12.7mm).
The plug-in board can be seen below:
A cut-off BCI filter design to allow 1.8MHz (160m) operation
Nik VK4PLN suggests trying the following values for a BCI filter that has a 1.7mHz cut off:
4.207nF – 3.754uH – 1.039nF – 2.341uH – 1.039nF – 3.754uH – 4.207nF
- 3.75uH is 35 turns on a T37-6
- 2,34uH is 28 turns on a T37-6
In practice you will want to make the capacitors (4.207nF and 1.039nF) using 2 or more capacitors in parallel (to get as close as possible to the values Nik indicates).