@zenom:
I use Flightaware Antenna and it improved my reception considerably. I am pretty sure Flight Aware antenna will improve reception in your case also.
The LMR400 has an attenuation of 4.5 dB/100 ft at 1090 Mhz. Your 50 ft LMR400 will have an attenuation of 2.2 dB. The FlightAware Antenna has a gain of 5 dBi, and the built-in RF pre-amplifier of FlightAware ProStick has a gain of 19 dB. This will more than compensate the attenuation of 50 ft of LMR400, and you may even have to reduce gain setting to prevent overload of the front end/tuner chip.
Meanwhile, try this tweak with your mag mount:
How to improve performance of whip antenna of DVB-T
You can also try indoor, an easy to make antenna (Cantenna), made of a 12 oz/66 mm dia drink/beer can and a F-female to female barrel connector:
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