ADS-B DIY Antenna

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airbus340

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Thank you very very much for your great explanations and your calculations, ab cd!
With all these calculations I think that we can see a pattern of whats best and that is 66 mm whip, but as you say we need to find out by trial and error to be 100% sure.

From my shorter tests it looks like can height 69 mm and whip 86 mm is best for me but I will run a week tests with different length of whip as soon the thunder season is over so I can run 24/7 for a week without to turn it off. I will come back with the results as soon I have done the tests.
 

ab cd

Senior Member
@airbus340:
Due to fluctuation of air traffic during different days of week, and during different hours of day, testing antennas one after the other cannot give reliable results. It is best to compare two antennas simultaneously, also called "side by side testing". Make two identical systems, one the benchmark or reference system, other the system under test. Both system should be identical except antennas. The benchmark/reference antenna and the antenna under test both should be palced at same location, at same height, with horizontal seperation not more than 1 meter. Each antenna should have its independent coax and independent receiver (DVB-T usb dongle). The type & length of coax for two should be same. The length of coax should be short, not more than 15 ft / 5 m, requiring in most cases an indoor installation. The gain setting of two receivers should also be same.There should be no amplifier in any of two systems.

I use Pepsi Cantenna with 66mm insulated whip as my benchmark/reference antenna. Well, it is not necessary to use Cantenna as benchmark. You can use any other antenna, which has given you good results, as your benchmark antenna.
 
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ab cd

Senior Member
@airbus340:

Below two SIMULTANEOUS screenshots of two side-by-side systems. See the time is same (within one minute) on clocks on two systems. Sketch on left of the gmap show the system pertaining to that map.

The antenna under test was a poor performing Coco, and the benchmark/reference antenna was Pepsi Cantenna.

The two antennas were installed at same location indoors, at same height, and at a horizontal distance of about 1 meter from each other. The length & type of coax between antenna & receiver dvb-t was same in two systems. The gain of two dvb-t receivers were set to same value.

Antenna Under Test
antenna under test - Coco.jpg



Benchmark/Reference Antenna
benchmark antenna - Cantenna.jpg
 
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airbus340

New Member
I understand that it would be best to compare at the same time but unfornately I only have one computer.

Is that 200+ nm with your cantenna and no amplifier? If so it´s amazing, my planecount without amplifier is really bad with every antenna I have ever tried.

One thing though with your test setup that is wrong. You can not run the antennas on same gain. Some of my antennas need gain 28 and some 38 to work their best, so with same gain its a unfair competition. To find the sweet spot, start ADSB# and start from max gain and lower the gain number by number. When you have found the gain with the highest message rate you have found the antennas sweet spot and that is the gain you should use in Dump1090.
 

ab cd

Senior Member
I understand that it would be best to compare at the same time but unfornately I only have one computer.
Which software you use to display aeroplanes on a map?

If you purchase 2 Raspberry Pi and 2 dvb-t usb dongles, the dump1090 on 2 Raspberry Pi will simultaneously plot gmap in 2 browser tabs on your computer through local area network (LAN)/Router.

My RPi 1 has a local address of 192.168.2.17, and RPi 2 has local address of 192.168.2.27. Hence my windows desktop computer's browser simultaneusly display two dvb-t in two tab as follows:

(1) http://192.168.2.17/dump1090/gmap.html

(2) http://192.168.2.27/dump1090/gmap.html


Is that 200+ nm with your cantenna and no amplifier? If so it´s amazing, my planecount without amplifier is really bad with every antenna I have ever tried.
Yes, it is WITOUT Amplifier. My installation is strictly indoor as I live in an apartment. My antennas are also indoor, located near a large window. The height of apartment, hence height of antenna is 60 ft above ground.

One thing though with your test setup that is wrong. You can not run the antennas on same gain. Some of my antennas need gain 28 and some 38 to work their best, so with same gain its a unfair competition. To find the sweet spot, start ADSB# and start from max gain and lower the gain number by number. When you have found the gain with the highest message rate you have found the antennas sweet spot and that is the gain you should use in Dump1090.
You need to adjust receiver/tuner gain because your amplifier makes the signal so strong that it overloads front end of your dvb-t receiver and plane count drops unless you remove overload by reducing receiver gain. For normal use, this is ok, but when comparing two antenna, using amplifier+adjusting gain masks the actual difference between antennas. That is why when COMPARING antennas, I NEVER use amplifier, and ALWAYS set two dvb-t receivers to same gain.
 
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airbus340

New Member
Thanks for the answer, very impressive with 250 nm without amplifier! Yes, I always use amplifier so I guess thats why I need to adjust the gain then.

I use VRS and a Windows machine, I have never ever used MAC.
 

ab cd

Senior Member
Thanks for the answer, very impressive with 250 nm without amplifier!
250 nm without amplifier is because of following reasons:

(1) Un obstructed view of horizon in directions of 250 nm, but in many directions range is much less or negligible due to surrounding high rise buildings.

(2) Short length of coax, hence negligible loss/attenuation due to coax cable.

(3) Cantenna's impedance is close to 75 ohms, which matches with impedance of coax (75 ohms) and impedance of dvb-t dongle (75 ohms). Hence SWR is 1.5 or less, a very decent figure.

I use VRS and a Windows machine, I have never ever used MAC.
I also use Windows Desktop/Laptop. Earlier I used adsbScope, then VRS to plot range map, but it requires leaving computer running for at least 24 hrs to get a meaningful plot. Couple of months ago I purchased 2 Rasoberry Pi and 2nd dvb-t dongle, and that made things very easy, as I can see two plots simultaneously. Also on RPi I have installed rrd collectd tool which collects the data and draws message rate and maximum range graphs. Now I dont have to run Windows Desktop for 24 hrs, as all this is done on RPi which I have left running 24 hours a day, 7 days a week, 52 weeks a year :). I use Windows Desktop for a short period only when I have to see the planes plotted on a map by RPi (http://192.168.27/dump1090/gmap.html)

The RPi uses Linux operating system. I have installed Debian Wheezy fron Raspberry Pi's official site.
 
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Oz Wald

New Member
Hi, first post, so why not start with what is probably a dumb question; they says there's no such thing, but hey!
I've only recently started looking into ADS-B and have now built and tested a few antennas.
3 have been of the monopole +gp design put on the web by atouk, the other a design by my next door neighbour, that is a basket on the Yagi principle.
I'll explain now that although I studied engineering in the early '70s, mechanical was my forte, electrical, I had a basic understanding, but electronics went way over my head and other than as a keen user, still pretty much does.
So, basically (I hate that), give me a drawing or a concept and I'll make it out of pretty much nothing. Ask me to understand it to any depth and you'll get a 1000 yard stare and a bit of dribble.
Here's my dumb question:
Nearly all the designs for 1090Mhz, 1/4 wave, 75ohm Cantennas and Platennas, I've seen on here, plus the length of the elements, my next door antenna guru (I'll call him Kang for a groan) came up with are around the 69mm mark. The lengths that atouk quotes are 2.575in (65.4mm).
I have built 2x 1/4 wave and 1x 1/2 wave monopoles using the 65.4mm length and they work; indeed the latest 1/4 has a general range of 240nm, reasonably circular without spikes; no amplification is used.
Why the differences? Kang couldn't answer me, not knowing enough about ADS-B, bandwidth, sidebands etc (I'm quoting here), so I thought I'd ask.
Please don't hurt my brain.

coverage0309.jpg range 0309.jpg
 

trigger

Member
Hi Oz,
I'm sure that ab cd will chip in at some point but my understanding is that any length of wire will pick up 1090Mhz. What we are trying to do is optimise the different types of antenna (cantenna, ground plane, CoCo). Your 65.4mm works fine but will 65.3mm be better? ;) Theory will suggest particular lengths for an antenna but location, feed line properties, day of the week, will all have a bearing on how well the antenna works. A LOT of trial and error is involved. Start from a known position, monitor, chop a bit off, monitor, chop a bit more off, ooops gone too far!, start again ........

Your coverage looks brilliant to me.
 

Oz Wald

New Member
Thanks for that Trigger. I get the trial and error bit, but the 3.6mm difference in the two figures quoted seemed so vastly different to me that I was wondering if there was a reason for it.
I was going to (and probably still will) make a 69mm version of atouk's design and see if that works better or worse. I'm using parts of 15mm brass plumbing end caps, f-type male joiners and 2mm wire from meter tails for the monopoles. Having a lathe, a mill and time on my hands helps.
monopole.jpg

I am surprised about the range so far, as I am situated close to sea level, in an estuary with low hills both sides. The antennas are roughly 30ft up, just above chimneys and tv aerials. The basket only gets a spiky 180nm max and about just over half the number of aircraft/min.
 

ab cd

Senior Member
Hi Oz!
WELCOME ABOARD the (crazy) ads-b bandwagon.

(1) Antenna dimensions: As Dave (trigger) has rightly summerized, any length of wire acts as antenna & picks radio signals, but for each frequency/wavelength, there is an optimum length which gives best result.

(2) The Beauty of ¼ wavelength: The ¼ wavelength whip with a groundplane has the beauty of having an impedance close to 75 ohms, and hence is naturally matched to 75 & 50 ohm systems of coax cable & receiver. The whip is connected to core wire of coax cable, and the groundplane is connected to shield of coax. The groundplane is can be in numrous forms, some are listed below:

(a) Three or four Horizontal radials, ¼ wavelength. The impedance of antenna in this configuration is 30 ohms, a close match to 50 ohm system used by hams.

(b) A metallic disc, ¼ wavelength RADIUS. The impedance of antenna in this configuration is 30 ohms, a close match to 50 ohm system used by hams.

(c) Spider - Three, four, six or eight radials, ¼ wavelength, bent down 45 degrees below horizontal. The impedance of antenna in this configuration is 70 ohms, a close match to 75 ohm system of satellite/tv coax & dvb-t usb receiver. It is also not far from 50 ohm system and gives a reasonable match to it.

(d) Cantenna - Cylendrical/sleeved groundplane made of an inverted food or drink can, vertical cylinderical wall cut to ¼ wavelength. The impedance of antenna in this configuration is 65 ohms, a close match to 75 ohm system of satellite/tv coax & dvb-t usb receiver. It is also not far from 50 ohm system and gives a reasonable match to it.

(3) The correct length of ¼ wave whip : The 1090 Mhz wave in air is 275 mm. Hence ¼ wavelength is 68.75 mm which is rounded to 69 mm. When RF curren travels in a wire, the length of wave is slightly less (1% to 5%), and depends on dia of wire use. The figure of 65.4 mm originates from ARRL book used by hams, and mainly pertains to lower freqencies of 440 mhz & below (uhf, vhf, hf).

I have tried to trim the BARE 69 mm whip, 2 mm a time, and at 67 mm it showed slight improvement, but at 65 mm it showed slight degradation. I tried this with INSULATED 69mm whip & got best results at 66mm.

As Dave has rightfully mentioned, make a whip 70 mm, start trimming a mm a time till you say oops, it is bad, then go back to length which gave you the best results.

(4) The ½ wavelength vs ¼ wavelength: The ½ wavelength has higher Gain (dB) than ¼ wavelength, but it also has much higher impedance (several hundred ohms). This makes the ½ wavelength whip a poor performing antenna on 75 & 50 ohm systems, unless some form of impedance matching device is used (or an amplifier is inserted between antenna & coax cable to compensate for reduced signal transfer from antenna to system due to impedance mismatch).
 
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toml

New Member
My apologies if I'm asking this on the wrong thread but maybe someone can point me to the right one. I know there was a discussion about Raspberry Pi a while ago. I'm attempting to connect my RPi 1 to one of several dongles I have including a Nooelec R820T and MicroADSB. I went through the procedure at

http://www.satsignal.eu/raspberry-pi/dump1090.html#fails-after-upgrade

under the heading of Preparing the Raspberry Pi. All went as described until I tried to run "rtl_test -t" at which point I get the error "No Supported devices found" .

When I type "lsusb" it returns "Microchip Technology, Inc. CDC RS-232 Emulation Demo for bus device 006, so I think the Pi sees the dongle.
 

ab cd

Senior Member

For benefit of other members, answer to @toml 's question:
The "rtl_test" command will execute only when you stop dump1090, which is using the dongle. When dump1090 is running, the "rtl_test" command gives the response "No Supported devices found"
 

Oz Wald

New Member
Oz,

Take a look at this write up and you can workout your theoretical max range http://discussions.flightaware.com/post180667.html?hilit=heywhatsthat#p180667
Wow! I just did as you suggested, entered 1000 & 40000ft as limits and just about got my coverage. That'll stop me spending a lot of time trying to improve it.
I do seem to get a lot of "Unreported" altitude figures in the data.
Thanks again Trigger.

p.s we used to have a security guard where I worked, called Dave. My mate and I used to call him Rodney and it was ages before he cottoned on why.

View attachment 2329
coverage0309.jpg
maxrange.jpg
 
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Oz Wald

New Member
Hi Oz!
WELCOME ABOARD the (crazy) ads-b bandwagon.

(1) Antenna dimensions: As Dave (trigger) has rightly summerized, any length of wire acts as antenna & picks radio signals, but for each frequency/wavelength, there is an optimum length which gives best result.

(2) The Beauty of ¼ wavelength: The ¼ wavelength whip with a groundplane has the beauty of having an impedance close to 75 ohms, and hence is naturally matched to 75 & 50 ohm systems of coax cable & receiver. The whip is connected to core wire of coax cable, and the groundplane is connected to shield of coax. The groundplane is can be in numrous forms, some are listed below:

(a) Three or four Horizontal radials, ¼ wavelength. The impedance of antenna in this configuration is 30 ohms, a close match to 50 ohm system used by hams.

(b) A metallic disc, ¼ wavelength RADIUS. The impedance of antenna in this configuration is 30 ohms, a close match to 50 ohm system used by hams.

(c) Spider - Three, four, six or eight radials, ¼ wavelength, bent down 45 degrees below horizontal. The impedance of antenna in this configuration is 70 ohms, a close match to 75 ohm system of satellite/tv coax & dvb-t usb receiver. It is also not far from 50 ohm system and gives a reasonable match to it.

(d) Cantenna - Cylendrical/sleeved groundplane made of an inverted food or drink can, vertical cylinderical wall cut to ¼ wavelength. The impedance of antenna in this configuration is 65 ohms, a close match to 75 ohm system of satellite/tv coax & dvb-t usb receiver. It is also not far from 50 ohm system and gives a reasonable match to it.

(3) The correct length of ¼ wave whip : The 1090 Mhz wave in air is 275 mm. Hence ¼ wavelength is 68.75 mm which is rounded to 69 mm. When RF curren travels in a wire, the length of wave is slightly less (1% to 5%), and depends on dia of wire use. The figure of 65.4 mm originates from ARRL book used by hams, and mainly pertains to lower freqencies of 440 mhz & below (uhf, vhf, hf).

I have tried to trim the BARE 69 mm whip, 2 mm a time, and at 67 mm it showed slight improvement, but at 65 mm it showed slight degradation. I tried this with INSULATED 69mm whip & got best results at 66mm.

As Dave has rightfully mentioned, make a whip 70 mm, start trimming a mm a time till you say oops, it is bad, then go back to length which gave you the best results.

(4) The ½ wavelength vs ¼ wavelength: The ½ wavelength has higher Gain (dB) than ¼ wavelength, but it also has much higher impedance (several hundred ohms). This makes the ½ wavelength whip a poor performing antenna on 75 & 50 ohm systems, unless some form of impedance matching device is used (or an amplifier is inserted between antenna & coax cable to compensate for reduced signal transfer from antenna to system due to impedance mismatch).
For once I pretty much understand the clever bit behind the metalwork, I'm going to take a bit of time digesting this. It certainly explains why the 1/2 wave spider I made first didn't work as well as the 1/4 that followed it. Damned impedence!

A couple of days ago, to save changing plugs at the antenna end between f and pl259, trying new versions of my original, I bored out the cable entry of the pl259, threaded it to 3/8 x32, screwed in an f coupler and soldered a length of core through the centre. I showed it to Kang who said "Pl259 is 50 ohm, f is 75 - might not work."
Such is life!

Thanks for the info.
Oz (Chris)
 

ab cd

Senior Member
"Pl259 is 50 ohm, f is 75 - might not work."
Hi Chris!
No, it works perfectly well.
For connectors, impedance is not a problem.


P.S. Your nickname Oz kept on pinching my memory, till I finally recalled why it sounded so familiar. My school story book "The Wizard of Oz" !!
 
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ab cd

Senior Member
Wow! I just did as you suggested, entered 1000 & 40000ft as limits and just about got my coverage. That'll stop me spending a lot of time trying to improve it.
I do seem to get a lot of "Unreported" altitude figures in the data.
Thanks again Trigger.

p.s we used to have a security guard where I worked, called Dave. My mate and I used to call him Rodney and it was ages before he cottoned on why.

View attachment 2329
View attachment 2330
View attachment 2331
Good to know that at least one person benefited from my post.
I have posted heywhtsthat.com procedure in Flightaware forum couple of days ago, but not a single response/feedback yet, except your's ! Thanks to Dave (trigger) for guiding you to the post.


EDIT: Just now I re-visited the post and found one response.
 
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