Pretty sure it sees “over” nothing in terms of immediate surroundings. (See image). But I could see that with the forested areas surrounding at distance. With its compact size it handles the surrounding dense tree attenuation better, and maybe reflections too. There’s no question I get a lot of signal distortion from the tree trunks. Airspy also handles this significantly better than other receivers I’ve tried. It also gets low altitude craft better. In addition to trees, I also have hills and mountain range limitations, particularly in the one direction where the trees aren’t.
The funny thing is most of my range and message rate limitations don’t come from the trees next to the antenna, rather the rows of trees 100 feet away with dense long pine needles. There are a few studies out there showing long pine needles impact UHF signals quite a bit. And even more so when wet!
The only reason I got the Active Diapason antenna was because the DPD had water in cable issues for the third time… I’ve tried every tape method you can imagine with no luck.
I did run adsbcompare a couple weeks after to look at the data. There was other change going on with near daily airspy_adsb betas etc. But most of all, most metrics hard to compare due to significant daily variance in traffic and weather.
I think the message rate per aircraft graph captures the benefit of the Active Diapason antenna best. But that could also be influenced by airspy_adsb improvements, though that benefit is mostly tested in the 2 or 3% increased message rate range. (Disclaimers, I tested lots of things, and the active diapason antenna was on a 10’ shorter backup mast during the first week or so, and there are a couple re-tests for 6 or 8 hours at a time with the DPD antenna mixed into the active diapason section of the graphs below…)
Well if you have issues with water ingress on the DPD one could say it’s not a fair comparison if it’s not an inherent issue with the antenna.
Also the coax used will be a factor.
It seems like the diapason is on top of the mast while the DPD is on an arm of the mast?
That can also cause a performance difference.
Well there was the first rain / dark weather in about 3 months for a few days before the leak visibility impacted the signal. Traffic was down due to weather, but I think the DPD antenna worked fine until the fateful Friday night where the arrow points. Immediately switched to the FA antenna and different cable I had.
I ALWAYS give the 1090 antenna the best mount position. So DPD was at the top. The FA 978 UAT antenna gets the bad spot below whatever is mounted at the top of the mast. Doesn’t matter much for metrics for UAT. I designed and 3D printed this mast standoff myself.
Eventually I replaced my backup cable and raised the top of the Diapason antenna close to where there top of the DPD antenna was - that’s a couple weeks later into the Diapason part of the graphs. Before that the base of the Diapason was about the same height has the base of the DPD was, or 10 feet lower during the first week to 10 days. All metrics of the past few weeks have tons of rain with only a few sunny days. Not unlike the weather just before I got the DPD leak.
Keen eye. I was there a decade ago, but now I’m back home.
Today watching aircraft circle as the lowest pressure ever recorded in this region spins just off the WA coast. Very far away, basically a cat 3 hurricane, yet I still lost a couple trees and some branches in my yard from wind!
Odd thing, the Active Diapason antenna gives me better tracking of aircraft near the airport at a lower altitude usually. But with this exceptionally low pressure today, aircraft near the airport aren’t tracking below 1500 feet all day. Usually 700 to 800 feet above the 400+ foot elevation runway…
Also at least 10 aborted landings on this map of the last 2 hours and lots of aircraft circling. Crazy day! The storm stretches from Mexico to Alaska. Listening to my airband feed, it’s a “heavy chop” nightmare up there.
NOTE: I am now using a varient with only 2 radials (please see photo below). The radials are in the same plane as V, so I can tape it to window glass (I live in an apartment and am restricted to indoor antennas only).
CLICK ON PHOTO TO SEE LARGER SIZE CLICK AGAIN TO SEE FULL SIZE
Swept another Amazon special, the![AEcreative_RTL_SDR ADS-B|690x404]
which is a quarter wave on a mag mount. It has a long 6.5’ feed line of RG-174-ish cable. The return loss bridge showed a lot of interaction with the long feed line. The valleys change amplitude but not frequency/location when the feed line was moved around or bunched up.
The antenna was on a roughly 8" x 12" metal plate. But at 1090 MHz the return loss and VSWR were OK given the limits of a quarter wave on such a long feed line.
Keep in mind that as the feed line gets longer and has more loss, there can be an effect where the return loss looks better, I’ve used a 100’ spool of RG-174 as a “dummy load” before, it capitalized on the loss of the line.
So this was better than the 5/8-ish mag mount I looked at, might be OK to see if things work, but for a final solution you can do better. It cost $7.50 in the US, so there is that going for it if you are trying to keep costs down to see if you can get things working.
Yes, that’s the antenna from Amazon.
Thank you for the link to the post! Lots of good points, and I really like the coverage charts and 3D field images.
Close to my RF test gear is a small printed chart I made decades ago that is SWR to Equivalent Attenuation. I kind of wish that Rigol added that info to their display, It’s so easy to get “Lost in the Sauce” when you look at things with logarithmic/dB scales.
I try to keep in mind that the focus needs to be as balanced as the antenna itself. It’s too easy to focus on the radiator and forget about the equally important grounds plane that is needed to give firm footing that gives the radiator something to “push against as it launches your signal out into the cruel world.” (or the opposite on receive)
As was discussed in the link you provided, all “Mag Mounts” struggle with the ground plane issues, and the feed line become the hidden passive component in the antenna system. This antenna was good enough for easy to set up test receive antenna. I didn’t bother to play with or worry about feed line, triming the radiator, putting a ferrite choke at the right point to quiet down the 2M long feed line resonance, etc.
For $45, the SRDS-RF_1090 from Amazon does just fine for me, and there are other antennas in the $30 to $60 price range that are good. I do think it’s nice to experiment with antennas just to learn about them. I liked your meter and it’s ability to show the reactive component of the S11 measurement, test equipment to do that is a bit pricey and that device is great for the 1% percent world that most people live in and what you see when you implement just about anything.
I’ve got the parts in to make a LNA as I start to focus on getting my antenna outside. Not sure what the outcome will be, for me there is fun in experimenting and I need to put up a very small (30’) tower for some year when I can get the StarLink disk going. An LNA installed a half wave down from the feed point into the vertical antenna also lets me stop worrying about feed line loss. I do know a group that has a good network analyzer, and hopefully can put the LNA on there to see how good it is. Time will tell, will advise in a few months…
I’m also spending time on software. I have a program that connects to the Pi and grabs the Base Station stream .csv formatted information. It builds up the info on all the messages, then write out and keeps a database of current aircraft being tracked. That is pushed out to another server that lets Emergnecy Responders see the info. Another database on that server with a web interface lets you set up the info for the medical helicopters so the entire system can work in deployed scenarios without internet access and you can enter what info you want to see when you click on the aircraft icon. Feeding data to Flight Aware in exchange for the raw ADS-B info is a most equitable situation. The hardware and software on the PI represents a lot of hard work.
So, all good stuff. I just wanted to post the sweep of an inexpensive quarter wave. It’s a good contrast to a more expensive vertical or your V-Stub that “makes it’s own” ground plane, so to speak. It was also a very striking example of when your feed line becomes part of your antenna system, the perils of considering RG-174-ish cable as “feed line” not withstanding…
It may come down to the location of the filtered pre-amp. At the receiver vs at the antenna.
In my case, DPD was compared with RTL-SDR filtered LNA or Uputronics filtered LNA positioned in the box with the receiver. In my harsh conditions, the Active Diapason, with amp at the antenna, performs better than the DPD, with filtered preamp at the receiver, over 35 feet of LMR400, especially for positions stats.
For many ADS-B situations, filtered preamp at the antenna is difficult sustain reliably. There aren’t many great compact, structurally strong and fully weather proof preamp-at-the-antenna devices available at 1090 MHz.
I have an Airband DPD antenna which performs fantastically with this weather-proof filtered LNA at the antenna. The good news is that this maker does plan to offer a 978-1090 MHz version of this compact device at some point in the future. (Was being tested as of a few months ago when I asked. I lobbied for N type connectors on both sides, but the same hardware enclosure across all frequencies has SMA female out ).
Below I am copying a year 2014 post (from planefinder forum) by designer of Diapasone Antenna, Late Francois (F5ANN):
1 You can’t compare an active antenna with a standard passive antenna. You will received nothing with this passive antenna and 30 meters of standard coax cable…
2 An active antenna is designed to be used, and only to be used, when you have a long run coaxial cable and no low loss cable (hard-line or such). The active antenna can support without signal degradation up to 40 meters of standard RG-58 coax cable, or up to 100 meters of Satellite TV cable RG-6 - we don’t mind the 50/75 ohms mismatch-).
3 Choice of antenna is dependant of the antenna location (height above ground), lenght of coax cable between antenna and receiver, horizon clearance from the antenna …
4 If your antenna is indoor, with a short coax cable, it is a non-sense to put money in an active antenna.
I think the best value way to do this is get the rtl-sdr preamp and put it in a waterproof box. I used a small electrical junction box that cost about £4 from Amazon. It’s been up for 2 years now with no problems.
That’s about the best option I considered. Mounting 30 - 35 ft up on a mast is a challenge but doable with some effort. Might get hot in some places though?
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