One major limitation of both the FUNCube Dongle and the RTL-SDR based receivers is the poor front end filtering.
Connecting a wide band antenna such as a discone to the input often results in a spectrum full of noise which swamps the target signal.
In most cases the noise is actually a result of out of band high-power transmissions such as FM and TV broadcast stations and pagers which overload the receiver and get mixed in to the tuned signal. The effects can be mitigated to a degree by turning down the gain of the receiver but at the expense of sensitivity to the signal you are wanting to receive.
The FUNCube Dongle in my experience seems especially susceptible to the effects of FM broadcast signals when trying to monitor the 2-Meter amateur band. I’m not sure if the current weather conditions were the cause but yesterday evening it seemed the breakthrough was especially bad. So I decided to experiment with a simple coax ‘notch’ filter.
There are two approaches to filtering, one is to use a band-pass filter which passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. The other is to use a band-stop filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. A notch filter is a band-stop filter with a narrow stop band.
As it happens you can make an effective filter using ordinary coaxial cable which will generate a sharp, deep notch which can be used to attenuate the unwanted signal.
A coax stub notch filter can be made if you connect the antenna to the receiver using a T-connector then attach a piece of coax of the appropriate length (the stub) to the remaining connector.
I happen to have a fair bit of 10Base2 Thin-Ethernet cabling (RG-58U) and connectors which were repatriated from a previous employer over a decade ago. The network cards they used came supplied with lovely long high quality 10m patch leads with moulded BNC connectors and a T-Connector and terminator! Even back then the 10Base2 was completely redundant as the network infrastructure used Cat-5 cabling.
So I dug out one of the T-Connectors and a BNC connector and set to work, connecting the antenna and receiver was simple, all I needed was to construct the coax stub.
To calculate the length of the stub you must first calculate the wave length of the desired ‘notch frequency’ for the FM band a good central notch frequency is around 100MHz.
The wave length is found by dividing the speed of light (which is around 300,000km/sec in free space) by frequency
So for a 100MHz signal the wave length is 300/100 = 3 meters.
However the speed of the radio wave in a coax is affected by the material it is made of, this is called the velocity factor. For most solid polyethylene coax (e.g RG-58) this is 66% (a handy table can be found here)
So multiplying 3 x 0.66 = 1.98 meters
The coax stub is 1/4 of this wavelength = 1.98 x 0.25 = 0.495 (49.5 cm)
So cutting a piece of coax of that length, connecting it to the T-Connector and leaving the other end open completes the filter. The filter will have maximum attenuation at 100MHz (assuming the coax is cut accurately) and the attenuation will fall off away from this ‘notch frequency’ (Note the filter will also notch at 300MHz, 500MHz and so on)
Interestingly it seems my RTL based SDR receiver is more affected by pager breakthrough, so I plan to make an alternative stub, adjusting the calculation to use 153MHz as the notch frequency to reduce this. It is possible to add more than one filter at a time using multiple T-Connectors and alternative higher performance designs for coax notch and band-pass filters can be found with a quick Google search.
I made a quick video showing the effectiveness of this simple filter which took about 10 minutes to make!