Filtering is one of those things that you either don’t need it at all or you need a lot of it. Here is a brief rundown on why some people need some band pass filtering.
The max range of your system depends on the Signal to Noise ratio. The more signal (frequency we want) to noise (any frequencies you don’t want) the futher away your system can see planes.
The FlightAware band pass filter has a 1.5dB insertion loss at 1090MHz and 2dB insertion loss at 978MHz. Other frequencies will have a 30dB of rejection or more. You are going to lose a bit of the 1090MHz signal (1.5dB loss) but it will absolutely remove any other frequency on the cable (30dB rejection). If you are close enough to other radio systems (cell phones are @ 700MHz - 900MHz or 1400MHz+) the filter will remove these unwanted noise signals and improve your signal to noise ratio. This should increase your range.
There are two types of noise sources that you probably have to worry about:
- Cellphone tower or close to an electrical motor (elevators motors, AC units, and heavy machine shops are extremely noisy).
- You are using an amplifier. Amplifiers are wide band and amplify everything (Amplifiers have their range written on the case and this usually includes cell phone frequencies.). You can actually lower your signal to noise ratio because of the wide band nature will amplify the noise as well as the signal. If you are seeing a drop in range after using an amplifier then you need extra filtering. If you are seeing only a marginal improvement in range with an amplifier you might gain a bit more range with more filtering.
All receivers already have a some filtering built into them. Some receivers have more filtering than others. The two receivers mostly used are the RTL dongles and the FlightFeeders.
RTL dongle has a wide open front end. This means that the receiver is actually listening to all the frequency at once and then filtered down inside the RTL chip to whatever frequency it is tuned on. The internal RTL filter is very good for the price but we have found having an external band pass filter usually increases the range under most cases. If you are using an amplifier you will see a bigger improvement. In very quiet location you will not see any improvement.
The FlightFeeders are using two different receiver boards. One receiver has more internal filtering than the other. In most cases you will see a drop in range because the internal filters are already doing their job and you are adding in the 1.5dB insertion loss of the external filter. Most likely you need to be in a noisy location for the external filter to help.
So the question is does my system need the extra filtering?
I can’t really say because of the variables involved. It most cases the RTL dongle will get a marginal improvement (10% more range is probably). More range increased in noisy locations. Less range increased in quiet locations.
How do you know if the filtering is working?
Check the range before the filter and after.
Where do I put in the filter?
It doesn’t matter where you install the filter. It can be close to the antenna or close to the receiver and it will do the same filtering.
Which direction do I put the filter?
Their is no front or back to the filter. It can be placed in any direction.
I am using an amplifier where do I put the filter?
Low noise amplifiers have less than 2dB insertion loss. These amplifier should go before the filter to maintain the low noise of the system.
Most normal amplifiers have a ~6dB insertion loss. Adding the filter before or after the amplifier doesn’t change the noise characterisitcs of the system.
The filter doesn’t seem to do anything or lowers my range?
Congratulations, you found your location is quiet. Filtering nothing and you still have nothing.
This is a filter and not an amplifier.