After a couple weeks of messing around with DIY antennas, I’m a bit smarter and a bit more confused.
I built a total of 5 different models of antenna and compared their performance (via daily “aircraft reported” totals). All of these are basically @abcd567’s designs from various past posts
v1 = Quick spider - no soldering built out of a scrap of RG-6 Coax I had laying around.
v2 = Similar 1/4 wave spider design but made on a N-type panel jack, with 4 crimped on legs for the ground plane, using 14AWG house wire for the various conductors
v2.1 = v2 with the ground plane legs soldered on
v2.2 = v2.1 with 6 legs instead of 4
v3 = V-stub with 6 legs for the ground plane (soldered)
v1 was built by dimensions only, and then deployed with my first RPi on an ADBS-Exchange Blue dongle.
The rest were tuned via a NanoVNA to drive 50 Ohm Impedance and minimum SWR at 1090 MHz (usually ~1.2). I then deployed them one at a time on a second RPi/ADSB-Ex setup - basically identical to v1 - and ran them in comparison to the v1 installation.
I am in a fairly good location (I think) for plane spotting - with an unobstructed from SW to NW of the Pacific Ocean - near the coast between KSAN and KLAX.
With the v1 running, I typically get ~3000 aircraft detected/day, with a big dead time overnight (0:00 - 6:00), and some daily variation I attribute to the private pilots (roughly +/- 200) depending on the day of the week and the weather. Average max range (from graphs1090) is about 220 nm.
When I put up v2, I was expecting to see an improvement in range and aircraft count; however I really didn’t. I occasionally got a plane a bit farther away, but since it was nearly always heading closer (likely coming to LAX) the v1 station would see it eventually. In the end I would routinely get ~30 more aircraft a day (1% improvement), but that was it.
v2.1 and v2.2 gave basically identical performance - increase the ground plane leg count and soldering them on didn’t help (not that I really expected it would).
Going to v3 (V-stub) was a bit of a disappointment, in that it did improve both average max range (up to 240nm) and plane count - but only ~90 more planes than v1; so a measurable improvement, but not the significant step up I was hoping for.
Watching the performance, what I think is probably happening, is that most commercial aircraft I am seeing are generally coming towards me - there are very few that skirt the edge of my viewing area, so extending from 220nm to 240nm doesn’t buy me a lot. I suspect the difference is coming from private aviation up the coast - small planes flying out of the regional airports around LAX and north, heading out over the ocean, but low enough their signal isn’t very clear.
At this point I feel glad I didn’t sink $50 into a 5-7dB antenna to discover no appreciable improvement - for roughly the same price I got a NanoVNA and made myself smarter about the whole antenna process - which is a win even without more planes to count.
I am curious to see if I can prove the difference is private aviation, but to do that I think I need to get a dumped list of “all the aircraft I’ve identified”. Looking in the discussion archives it doesn’t seem that FA can provide that - although there were suggestions other sites might.
I’d be interested to know if anyone has a good suggestion for an easy way to list out the specific aircraft ID’d, so I can do a compare and see which ones are the “extras”.
I expect I’ll stick with the V-stub (90 more planes is better than nothing), unless someone has a suggestion for something else to try to improve range. I’m just a bit too far south to get a good look at direct transmissions from SFO/SJC planes, but maybe with the right equipment and conditions I could see over the horizon and pick a few of them up.