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longjohn119

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    longjohn119 got a reaction from chicken in [POTM] dAISy - A Simple AIS Receiver   
    Initial test results
    dAISy Hat with an FT232 RS232 convert - Half wave vertical dipole at approx 30 ft fed with 80 ft 75 ohm RG-6 quad shield with balun at feed point (The mismatch is largely irrelevant in modern active front end **receivers**, transmitters are a different ballgame altogether)
    Noise, no Antenna = -126 dbm
    Noise with antenna = -123 dbm 
    Results from 800+ messages:
    Minimum received signals = -117 dbm
    Maximum received signal = --67 dbm
    Average received signal = -91 dbm
     
    NOTES:
    (1) The more than acceptable 3db increase in noise from the antenna is mainly due to using quality coax and a vertical dipole with a proper balun which is a balanced antenna and tends to work more with the magnetic portion of the electromagnetic signal. A 1/4 wave vertical ground plane antenna would likely add another 3 - 6 db to the noise levels because it is an unbalanced antenna that works mainly with the electro (voltage) part of the electromagnetic signal and thus is more sensitive to RFI noise. Another engineering term for a vertical ground plane antenna is a Voltage Probe Antenna. However on most vessels a 1/4 to 5/8 wave vertical will be used since they are shorter (1/4) and/or are end fed rather than in the middle making them much easier to mount and feed on most smaller vessels.
    (2) While this could benefit from a 10 db preamp with input and output filters and scaling one of the 2 meter amatuer band designs would be relatively simple, trying to get there with only 5 volts is tough as most are designed for 12-14V. Plus you are going to need to use fairly large air coil or a slug tuned ferrite coils (like in IF circuits) which have a higher Q and thus lower loss. filters using SMT inductors while convenient and small are inherently low Q and lossy and that is why everyone is getting poor results putting a preamp with SMT filters in front of these. No preamp will increase the SNR of a receiver only increase it as they will amplify both the noise and the signal present on the input in equal amounts. The only way to lower noise is by decreasing the input bandwidth and filtering out the noise before it reaches the input to the preamp. 
    (3) My receive range roughly tripled over a RTL dongle and doubled compared to a HackRF SDR. This is most likely due to the poor dynamic range and SNR of the 8 bit ADCs even in the otherwise well designed HackRF
    Now it's off to mate this with my boat's GPS and the output to a RS-422 converter so I can add it and the GPS to my Lowrance Elite 7 Ti NMEA inputs and I can give it on on the water test this weekend .......
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