FlightAware Discussions

My new setup

Hi all,
I have been using a rtl sdr v3 with a paperclip stuck in the antenna port as the antenna, today I swopped the paperclip out for a FA 1090 antenna, also added in the rtl sdr triple filtered LNA, a fair improvement even with the antenna at ground level and 45 degree polarisation.
Set up is rpi 3B+ directly into the sdr dongle, directly into the LNA then currently 10m of LMR400 coax to the antenna, coax length will be reduced on final install. I have noted that using bias t the dongle gets warm as does the LNA to a lesser extent so both will have external heatsinks added to their cases, the pi has a small fan and heatsink.
Weekend will see it installed above roof height, about 9-10 metres, coax run through attic and down into airing cupboard at top of stairs for easy access.
My other main interest is HF QRSS but rx is impossible here due to high noise floor so very happy to have piaware running alongside my qrss tx with no issues.
73
Andy
M0RON

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I know this has been suggested here, but it’s a dumb thing to do as the chance of destroying the SMA socket is high. Even if the centre contact “looks” ok, you may have deformed it enough that you will never have a reliable contact.

The RTL-SDR blog filter/amp has a very high output as it expects to be mounted near the mast head. It is recommended to have an attenuator between the amp and dongle for best results. Reducing the receiver gain is an alternative which may/may not give the same results.

The paper clip is highly unlikely to damage the sma connector, the paper clip was quite a bit smaller than the hole in the centre connector, it had to be held in place with bluetack otherwise it just fell out. Sma connectors are good rf connectors but really need to be tightened using a torque wrench, once set, forget.
I am looking at reducing the gain on the rtlsdr dongle, at present it is reporting 67% strong signals, it is running on factory settings. As I have LMR400 coax between the LNA and antenna the LNA is effectively next to the antenna as loss in the coax is very little, less than 1db using present length of coax which is going to be shortened anyway. I expect that the LNA is not necassary, receiving to +250 miles, but as I live in a heavily polluted RF environment plus my own amateur radio activities any filtering is a good idea. So my end set up will probably be reduced gain in dongle and if LNA is retained attenuation, or more likely a second receiver on a mag mount antenna for local traffic.
Andy

The SMA pin diameter is 0.91mm nom. suggesting you either have a very small paper clip or you’ve destroyed the socket.

LMR400 is very good cable, but it’s not that good. 10m of LMR400 is not “less than 1dB”.

I think you missed the point about the amp gain. RTLSDR Blog recommend some attenuation between the amp and the dongle to avoid saturation.

I have measured the paperclip and it is 0.81mm so no damage to socket will result from it’s use.
My mistake on the LMR400 attenuation, from the datasheet it is 12.8 dB /100m, as I’m using 10m attenuation will be 1.28dB although it will be less than that once coax is shortened by at least half so around 0.6dB at final set up.
No mistakes on amplification, the LNA was bought specifically for it’s filters, I live next to a large school which chucks out loads of RF pollution throughout the day and also a fair amount at night. Gain was altered to 27 on the sdr rtl v3 which gives a % of strong signals of 6.24%, I am seeing all of the small local traffic along with planes out beyond 250m, I’ll take that for now. Antenna is to be raised a further 3.5m at which point it will be in it’s final position and gain will be reassessed and altered if necassary.
73
Andy

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