Nice filter for 1090 MHz ADS-B site

Over a year ago I decided that I needed some good filters for ADS-B reception on mountaintops. Not that I had an immediate need, I didn’t have anything on a mountaintop, but I suppose I had a little extra cash and felt excited to be able to imagine a good ADS-B receive site atop an Arizona mountaintop (or hilltop) location. So, I drafted up what I thought would be a reasonable spec and went into Alibaba to find a filter manufacturer to build one. I ended up with 5 filters, all exactly to my spec, and for a bargain price (well, relatively speaking).

IMG_4253

Above is the finished ADS-B receiver assembly, complete with Raspberry Pi, RTL-SDR 1ppm TCXO SDR dongle, a eBay-purchased LNA, an eBay-purchased 12 vdc to 5 vdc DC-DC converter, and some coaxial cabling (also from eBay). The black square in the center of the image is the 1090 MHz filter, and it’s a quite good one.

Img_4290_small

It’s a straightforward cavity filter, a little aluminum brick with fine performance.

Img_4283_small

Solid out of band rejection, and I suspect around -100 dB ultimate rejection. The SA just doesn’t have the range to see it.

Img_4289_small

While the signal of interest is only a MHz wide, I wanted a filter that was wider so that temperature and mechanical variation would never haunt me, and I wanted a low bandpass loss (the above shows less than 1 dB loss) across the band.

Behind the filter is a run-of-the-mill eBay wideband LNA with a 1 dB NF, and somewhere around +30 dBm IP3. The RPi is running the most current version of FlightAware’s PiAware, rev 3.0.4, and supports just about any off-the-shelf USB SDR dongle.

After setting it up, it looked like I needed to reduce the overall gain a bit, so I discovered how to go into dump1090 and change the gain from “automatic” (really not, I think it’s just max) to 42 dB. That gave me best range and most received a/c.

The antenna for the site is a FlightAware fiberglass stick, about 12′ above the ground, mounted on the side of the tower.

HeyWhatsThat_30aug16Coverage seems to be pretty close to the model generated by HeyWhatsThat.com (above). The blue line is the 40,000′ contour, while the orange line is the FL300 contour.

24 hours or so of actual flight logs produces the following plot, which is more or less pretty similar to the HeyWhatsThat plot.

ADS-B_Measured_30aug16

The primary notch in the pattern, in the SE, is the higher part of the ridge on which the radio site sits. It ends up blocking any coverage of flights in and out of Tucson, over 110 miles away, until said flights get to FL300 or so.

It will be interesting to see how the coverage shapes out over the next few weeks – I hope that it will get up near the top of all the local receive sites in performance.

ADS-B Update

Installed an ADS-B receiver up on White Tank Mountains over the weekend. Coverage seems pretty darned good, and I lose a/c only when they’ve gone about 1.5° below the theoretical horizon.

Below is a screenshot of the display. The a/c out beyond the 200 nm range ring are all at FL300 or greater, but at least according to heywhatsthat.com, they’re all significantly below the local horizon from White Tank.

RPi_WT_29Aug16_1027

Also seems that I have about a 10 dB dynamic range on the receiver. Signals are never stronger than about -1.6 dBFS and I always lose the a/c when the signal drops below about -12 dBFS. Using HeyWhatsThat.com, it’s very apparent when it drops out.

Here’s AAL110 headed NE from LA. I lost it as it was about due east from Cedar City UT.

AAL110_29aug16_1055

The RSSI is below the minimum required for this particular station to decode successfully. Not that the a/c is over 220 nm distant, but it’s at FL410. So why did it suddenly fade out? HeyWhatsThat gives an indication.

AAL110_29aug16_1055_HWT_profile

From this plot, it’s obvious that the a/c, even at 41k feet altitude, is well below the local horizon and the actual path is probably ducting or multiple knife edges. Either way, it seems to explain for me why I can’t hear farther than this even with the receiver atop a significant “hill” in the Phoenix area.

ADS-B Receiver for mountain-top comms site

Monitoring aircraft via ADS-B is a terrific hobby and super easy to do. I get to have a display here that shows sometimes hundreds of aircraft (both commercial and general aviation) out to about 200 miles from the house.

ADS-B_screencapture

The above screen capture was from my Raspberry Pi running PiAware and connected to a homebrew 1090 MHz antenna up on the roof of the house. Nothing special in the setup, but look at the range! A/C at altitude are hearable out over the Grand Canyon and into California. Occasionally I get a/c into or out of Mexico, and I can see traffic in the southwest corner of New Mexico.

I’m planning to put one of these receivers on a local hilltop, about 3000′ above my house and the Valley floor, and yesterday installed the antenna on a temporary mount on the tower at the site. I connected it to my most recent ADS-B receiver setup, seen below, and was awestruck at the coverage. Was seeing a/c over Los Angeles and Albuquerque!

IMG_4253

It’s a bit tricky to put a cheap SDR dongle anywhere near radio transmitters, and the hilltop that I was on is loaded with them. In fact, the building in which I have some current monitoring equipment is only 50′ away from a huge comms tower with dozens of two-way radio antennas, and a lot of potential interference. The ADS-B receiver antenna is right in the center of the picture, on the end of a piece of unistrut attached to the tower legs. In the background, nearly a dozen towers bristling with antennas.

IMG_4229

The secret to success is a very good filter in front of the SDR receiver. That black square near the middle of the picture is exactly that. It’s a custom-made cavity filter, only 50 MHz wide, centered at 1090 MHz. Extremely sharp rolloff and ultimate rejection about 100 dB. Really helps the RTL-SDR receiver.

However, I wasn’t able to leave the receiver up on the hill yesterday, I was troubleshooting other issues and didn’t have time to set up the network connection to my receiver. Next time I will hopefully get it installed and on the air!

Getting rid of unused COM ports in Windows

Ok, this isn’t new, but Windows has the memory of an elephant, and remembers serial dongles that no longer exist. I was up to COM21 and only had two external USB-serial dongles, so it was time for housecleaning.

Found this page that absolutely was spot-on and that I’d never have divined on my own. I think it was the combination of running Device Manager as Administrator, and that’s apparently not straightforward to do (even though I have admin privileges).

After deleting 6 GPS serial instances,  8 FTDI instances, and another 5 Prolific instances, I rebooted the box, waited for it to find the two USB-serial adapters, and now am happily back to COM3 and COM4.

Small victories are good.

PSA livery here at Sky Harbor


Growing up in LA, used to see these smiling birds all the time until gobbled up by America West. Now of course it’s American, but nice to see the colors flying again. Only saw it when an American pilot and first officer noted it with “check out those wild colors”… They must not be from around here!!!