First off, I am not an antenna engineer, but I have used a lot of different antenna configurations and designs for various purposes. Coming from an audio background, some of the most frustrating RF projects can be wireless microphones - in some countries limited to 25mW output and needing working ranges up to 500' (never a great plan with any hardware) Questions after playing with my new Pi and ADSBX install; I am seeing that my peak signal is running up to -1dBFS on a regular basis My Mean Signal is showing around -10dBFS My noise plot is running between -25 and -30. I live in the LA area - Fullerton specifically - and am clearly in a busy environment in terms of RF. I am running a roughly 45' piece of Belden 1694A (a 3gig certified 75 ohm RG6) to a ProStick+ The antenna is a modified Larson 900mhz antenna but is really just a 1/4w on a ground disk. Seeing roughly 150NM range now. First question for folks - have you played with the diameter of the center element with your "spider" and "can" antennas? I started with an 1/16" copper wire (which I trimmed down 0.1" at a time until and noted where the max was) I replaced that with an 1/8" copper tube with the end capped with solder I finally tested the same length with a piece of 3/8" brass rod, threaded in the bottom to directly mount to the stud of the antenna base. What I think I saw was that the peak sensitivity widened with the increasing diameter (making exact length less critical and more tolerant of rain) and the overall sensitivity seemed to improve with the larger diameter (a relatively simple improvement) Has anyone else gone through this either theoretically or practically? NEXT - If my peak signals are at full scale, what are the benefits of antenna gain vs receiver gain? My experience says gain at the antenna is free and "quiet" but electronic gain is expensive and noisy - BUT is this a case where the optimal coverage angles and pattern outweigh noise? I have been looking at the j-pole, double j-pole 1/4 wave and co-linear options but have seen the co-linears be very finicky to get to actually perform - Excellent when just right but worse than a 1/4 wave if they aren't. j-pole and its double cousin seem to have nice low radiation angles but do they fall off too much overhead? Lastly, as I intend to mount my Pi much closer to the antenna than it is now, how do I turn down the gain in the most beneficial place in the chain - or does the software do that automatically? If it doesn't, I will clearly be clipping the signal on local planes once the 45' of RG6 becomes 6' of 1/2" hard line on the tower. Thanks! Mark
I drive my reciever wity daytime noise floor about -18...-20 db. Range is about 200 nm. Consider, Antenna gain is best gain. Second best is low cable loss (short, good cable). Then adjust the stick in the midfield of amplification and use hardware amplifier e.g. moonraker which you can fine adjust by turning a real knob...at your location i think a filter either before or after the hardware amp will be fine. As the moonraker has BNC connectors and the most filters and the dongle are SMA connected using the filter between amp and dongle will need less adapters... Maybe you can place the dongle "half-way", use shorter RF antenna cable and a 30ft active USB Cable...
