I just replaced the 24" adsb exchange antenna with DPD’s 4ft+ “ADS-B Vertical Outdoor Base Antenna” in my attic and actually saw a very slight decrease in range, messages, and planes spotted. The antenna is connected by a 4ft cable to a flight aware pro stick which is attached to a raspberry pi zero 2 W. I’m trying to figure out 1.) why the DPD antenna is actually performing slightly worse and 2.) what I can do to improve the number of messages and planes I’m picking up. I know various situations can see improvements with a sysmocom filter, LNA, and/or airspy mini, but I’m not sure if any of these would help my specific situation? Here’s the graphs1090 output from 6 days before I made the switch (Monday at 11pm~) and the first day afterward, as well as the output Optimize Gain.
(I realize putting the antenna outside will get me tons more planes and messages than any other thing I could possibly do, but that’s not an option for me right now so I’m doing the best I can with my current situation and trying to use this as a learning experience)
ADSB CPU utilization is consistently around 27-28%, while the Pi’s CPU is around 11.5-12%.
I replaced an FA 5.5 db antenna (~26" length) with a Vinnat 5/8 0db antenna (~9" length) and reception increased.
Assuming you installed both so that the top of the antenna was as high as can be placed in the attic I think a shorter antenna will perform better because it’s effectively higher.
Any apirspy will help.
I have two receivers in my attic, one is a RTL-SDR v3 with their 1090 LNA, the other an Airpy Mini with the RTL-SDT 1090 LNA, both with a Vinnat 5/8 antenna.
Aircraft, blue is the Airpy Mini, orange is the RTL-SDR v3.
Both antennas receive most of the signal in the part closest to the connector.
The DPD antenna being much longer will sit lower thus have more ‘stuff’ in the attic to deal with.
That’s my best guess.
Then there is the matter of where your electronics are and if they are possibly now closer to the antenna. If that’s the case the antenna might be picking up the noise from your electronics.
If you’re gonna get an airspy you should get a filtered LNA with it as the airspy isn’t that sensitive on its own.
I’ve been trying to read up on the idea of hanging the antenna upside down, and so far it seems like the big downsides for doing so indoors are missing a few more signals that are directly overhead and dealing with interference from the coax cable to the SDR. flipping the tall antenna upside down would be pretty simple given how I’ve mounted it, so then the only question is how far to let the RG8x cable run along the eaves before it drops from the ceiling down to the floor where the Pi is? Any thoughts on how far a parallel coax cable should be from the antenna so it doesn’t cause interference?
The way the brackets are currently secured it should be pretty simple to just remove the u bolts, flip the antenna, place the ‘top’ of the antenna (pointing down) in the existing u-bolts, and then tighten them gently back into place. The only question is, how to do this without getting a ton of coax interference
I tied a string at the top of the antenna with a clove hitch. I fastened the other end of the string to an eye screwed into the interior roof midway between trusses.
Hmmm… can’t imagine two 600mm (or whatever they are!) are going to be effective, or happy mere inches apart? Or are they?
Only one way to find out I guess. I’d do it now if it weren’t for the fact I’d have issues getting a second cable through a wall and then getting a cable to the second Pi. Definitely something to think about of the coming weeks though.
The Cell Phone antennas are special design done by specialists using novel techniques to cramp so many antennas in such a small space.
Our ADSB antennas are not of such special design. Further we dont have space limitation like a Cell Phone has. In this situation, it is better to have ample clearances between antennas. There is no hard and fast rule, but the rule of thumb is to keep a horizontal distance of one wavelength between the antennas. At ADSB frequency (1090 MHz), one wavelength is approximately 30cm or 1 ft…
With the antenna upside down, the coax will exit the antenna into the pinnacle of the attic. The pi/sdr are located on the floor of the attic (a few feet away as of now, though i could increase or decrease that distance a bit). For the coax to travel from the pinnacle of the attic ceiling to the attic floor, it’s going to have to essentially be parallel to (or diagonal away from) the antenna. What I’m wondering is how far I should away should the coax be from the antenna (i.e. how long do I string it up through the upper rafters) before it drops down to meet the pi on the floor.
Antennas should be at least one wavelength away from other metal parts. Coax cables, masts, other antennas, and even attic metal reinforcement rafter parts. All will detune and block the signals we want. Hope that helps. Have fun and be safe.
Gene
