This may be due to either gain setting or RF Noise.
If it is due to high gain setting, lowering gain setting may improve range/plane count
If it is due to RF noise (strong Cell Phone signals), you will need to add a Filter
What is your gain setting?
If you are using Piaware image 3.5, by default it sets gain to -10 which is AGC (automatic gain control).
Reduce gain in steps of 5 dB to find at what point you get best reception/range
First check what is gain setting:
pi@piaware:~$ sudo systemctl status dump1090-fa
#If default gain settings are used, last line of output will show
piaware dump1090-fa: rtlsdr: enabling tuner AGC
#If you have adjusted gain to a value xx.x, last line of output will show
piaware dump1090-fa: rtlsdr: tuner gain set to xx.x dB
How to reduce gain?
#Set gain to value yy
sudo piaware-config rtlsdr-gain yy
#Restart dump1090-fa for new gain value to take effect
sudo systemctl restart dump1090-fa
#Verify that new gain setting has taken effect
sudo systemctl status dump1090-fa
#Observe for some period the maximum range/ Plane count
Repeat above commands for several values of gain such as 50, 45, 40, 35, 30 … etc.