Here is part of what will eventually be one of a number of web pages on ADS-B related topics.
You may have run into one or more of my posts talking about filters. Why are they needed and what will they do? Let’s look at some real world data.
I have a discone antenna on the roof, with good feedline (9913) to my office downstairs. We have a good view of the Silicon Valley.
This is the spectrum from 50 MHz to 2.0 GHz, along the horizontal axis. The vertical axis is signal intensity, at 10 dB intervals. The numbered diamonds are frequencies of interest
<1> is 1090 MHz, the frequency we’re interested in. Any ADS-B signals are down in the weeds.
<2> is 2451 MHz, a nearby 2.4 GHz Wi-Fi access point
<3> is 884 MHz, a DTV station
<4> is 748 MHz, another DTV station
<5> (no diamond) is around 1900 MHz, a local cell site
The spikes below <4> are other DTV stations, business band things, and the FM broadcast band.
Those DTV stations are around 30dB stronger than any signal we want at 1090 MHz – 30dB is a factor of 1000!
From this it’s clear that we have a LOT of signals that are hundreds to thousands of times stronger than the ADS-B ones we want. These strong signals are going to swamp our little SDR!
Pretty much everything below 750 MHz is gone. The spike at <4>, 748 MHz, has been cut by 25 dB – a 320 times reduction. Our spike at <3>, 884 MHz, has been cut by 6 dB, which is pretty good.
Note that our cell site at <5> is untouched – this is a high pass filter after all.
The addition of a high pass filter has cleaned up the signal we need to work with tremendously. This will be all a lot of sites need to improve ADS-B reception. Another advantage to adding a high pass filter is that the filter will provide some protection for the front end of the SDR.
This frequency sweep is from 50 MHz to 1200 MHz, not 2.9 GHz like the top two; the response from 1200 to 2900 MHz isn’t that interesting.
This SAW has a measured insertion loss of 4.72 dB at the center frequency, measured at 1089 MHz. The measured 3 dB bandwidth is about 26 MHz. Signals below 1100 MHz are attenuated by around 46 – gone. (This plot was generated by a HP 85630A scalar test set which works with the HP8594E spectrum analyzer.)
The combination I use – a SHP-1000 high pass filter connected to the discone antenna. This filters out most of the garbage, and more important, has a very low insertion loss, so we’re not attenuating the signals we’re interested in by much. The output of the high pass filter goes to a low noise amplifier (LNA). The LNA is followed by a SAW – and the LNA compensates for the insertion loss of the SAW. This provides a signal for the SDR to process that only has energy around 1090 MHz.
And by the way, if you have a SDR, you have a spectrum analyzer – you can find out just what your antenna is delivering, with and without a filter.