Accessories:Purpose Of External Filters
Most of us do live in an area where they are not alone. Many transmitters are around us, not just those that we really want to receive. It is then the task of the receiver to make a selection of the radio spectrum that is actually of interest. This is done with filters at the input. Such filters cut away all unwanted signals from the amplifiers in the receiver. Unfortunately, for the radar signals on 1090 MHz, such filters are either large or do have significant attenuation, usually 3dB (half of the signal is lost).
I decided not to put any filter directly at the input but instead use a proven high IP (Intercept Point) device at the input, which is significantly more robust against out-of-band signals as the low current amplifers used in others. I did not want to have an about 3dB degradation at this point because this value fully adds to the noise figure and herewith decreases the sensitivity of the device. The Radarcape as well as the Mode-S Beast much more should be a device mainly designed for sensitiviy. The story tells that only in a few cases strong out-of-band signals disturb reception, so this decision was correct.
Anyway, in a very few cases those out of band signals are enormeously strong signals and degrade the performance. From my long-term RF experience I know that an external filter can be made so that the system performance does not degrade. These filters are large, based on a mechanical construction and not able to be integrated onto the PCB, as you see in the photos. But the parameters reached with these filters are really outstanding and worth the effort.
There was a receiving station installed very close to a GSM transmitter. The owner noticed that the range was far below what he had expected. After installing the filter, the range and frame rate drastically increased.