FM Broadcast Filter for Baofeng
Last summer, together with IU5MOI and other friends, we had a trekking day on Mount Capanne (Elba Island). From the top of the mount we did some QRP (low-power radio operations). For this purpose we chose some low-cost handeld radios, a Baofeng UV-5R and a UV-3R, along with a homemade antenna for 144MHz. We started using the UV-5R. After making a QSO (radio contact) with IU5KHP, he informed us that other friends were listening from various areas of the main land, but we couldn’t hear them. Even more strangely, when we swapped the antennas with the default “rubber antenna”, the reception improved significantly, allowing us to hear the responses. Moreover, where the UV-5R had shown itself deaf, the cheaper UV-3R saved the day by not showing any problems even with the external antenna.
IU5KHP’s suggestions and subsequent research 1 revealed that the Baofeng handheld radio is not sensitive enough in the VHF range due to a rather poor RF front-end. In short, the input filter is not very effective against the FM broadcast band, and the amplification stage saturates when using an external antenna. On the other hand, the “rubber antenna” provided with the radios seems to have a filtering effect that avoids this problem.
As I enjoy using this radio with external antennas for experimenting with satellites and QRP operations, I tried to avoid having to buy a more capable device (considering the budget constraints). Therefore, my first experiment was to block at least the 88-108MHz band and verify if the performance improves. On the LNA4ALL website 2, I found a suitable FM trap filter for this purpose, which could also be recycled for use with an SDR (Software-Defined Radio). Here are its parameters:
| Parameter | Value |
|---|---|
| Topology | Third-order Chebyshev |
| Center Frequency | 100 MHz |
| Bandwidth | 55 MHz |
| Ripple | 0.1 dB |
Since I wasn’t interested in going much higher in frequency, I physically implemented the filter using discrete through-hole components and a single-sided recycled PCB (Printed Circuit Board). Also, I used the ready available components to speed up prototyping.
Though it’s not precisely matching the simulation, it gets the job done, both in removing some interference on the Baofeng and allowing me to do simple radio listening with the SDR on bands adjacent to the FM broadcast (CB, 50 MHz, 144 MHz).
