ADS-B DIY Antenna

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Question: Why my Receiver Range plot by VRS has large number of spikes (sudden drop in range) in various directions?


Answer: Large number of buildings, much higher (100 feet to 300 feet) than height of my antenna (65 feet), all around my building.
My own building & 3 adjacent buildings (all 230 feet high), provide the worst obstruction, completely cutting-off 180 degrees of view

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See bird's eye view of the area below.

 

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SBSplotter - Another way of plotting receiver range
1. Using jetvision's "RTL-1090" & "SBS Plotter 1" softwares, collect receiver range data for the desired period.
(Click here to Download SBS Plotter 1 - [url]http://jetvision.de/sbs/sbsplotter1.zip[/URL])

2. Massage the data using technique described in [url]http://sonicgoose.com/superimpose-polar-plot-in-google-earth/[/URL],

3. Skip the last step of sonicgoose page, and do not create a .kml file for Google Earth by GPS Visualizer site.

4. Instead, plot the range data in Google Map using GPS Visualizer's Map site [url]http://www.gpsvisualizer.com/map_input?form=google[/URL]. Set the option "Initial Map type" to "Google Street Map", and "Colorize by" to "Track". The map will be produced in a browser window instead of Google Earth.
A screenshot of map produced by me using above method is given below.

 

[Richard Lee]

Our police would have a field day with that car. "Me! on a mobile phone, I don't think so !!"
He must have a truck battery in there somewhere to power that lot.

I did a couple 3,000 mile road trips using 2 tranceivers back in the late 90s.
One was a dual bander 2M & 70cm, and the other was a CB radio w/ AM & SSB.. (So I could hear the big rig truckers).
One radio isn't bad, but two cause too much distraction. If it gets too loud, you can't hear the alarm tone from your Radar/Laser detector.
On the second trip, I just ran the CB rig on the road, and used a handheld 2M & 70cm while I was in Texas.
In the mid 1980s, I had a 10M ham rig (11 foot antenna) that I would use in the parking lot at work, during lunch breaks.
It was only about 25w of SSB, but I could talk to taxi driving Hams in the UK.. It was fun to hear those English accents.!.

I had planned on putting a 2-meter rig in my car after retirement. But, since I don't drive my Ford Escape much these days,
it would be a waste to install a rig, and it would attract low-lifes. They break into your car, thinking it's a CB rig they can use or trade for drugs.
When it turns out to be an expensive VHF/UHF rig, they just toss it..

I do drive my wife's Prius 'C2' a lot. It's a fun car, with good MPG. But she wouldn't put up with ham rig.
She is also a licensed ham, but hasn't been on the air in many years. Never really got into the hobby.

Cheers,
Rich

 

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Flavours of CoCo
On internet, a huge variety of CoCo designs for 1090 Mhz and 2.4 Ghz are available. It is difficult to say which one is better unless all are made & tried under same conditions (i.e. Antenna Location, cable run, receiver etc to be same). Trial runs without amplifier are better as amplifier's AGC will dim the real difference. Also trial runs with antenna at window level or similar easy accessable position are practical, as it is tedious to install & uninstall trial antennas on roof.

The differences are basically in top element. Only one stem with odd number of elements can be made, and one piece each of different top elements made & installed/ tested one by one.

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I have tried to find out why CoCo:
(1) radiates
(2) requires Balun/Choke.
I don't know how far I am correct. I will call this just a "brain wave". .
Please see 2 sketches below. Comments are most welcomed.

sketch 1 of 2

sketch 2 of 2

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An abstract from "ARRL Antenna Book"

Collinear Transposed-Coax Arrays
Collinear arrays tend to be tolerant of construction tolerances, making them easy to build and adjust for VHF and UHF applications.

The most popular collinear array is the omnidirectional array of half-wave dipoles constructed of transposed sections of coaxial cable as shown in Figure 15.10B. The original array of this type is the Franklin array shown in Figure 15.10A.

The phase-reversing stubs allow multiple half-wave sections to operate in phase, creating gain at right angles to the antenna. An example of this array is the popular Cushcraft Ringo Ranger series of omnidirectional VHF and UHF antennas. While the phasing stubs make the Franklin array inconvenient for vertical stacking of more than two elements, a derivative of this array uses transposed sections of coaxial cable as in Figure 15.10B.

The phasing stub is created by the inside of each coaxial section. The outer surface of the coaxial shield forms the radiating element. The resulting antenna can be enclosed in a PVC or fiberglass tube, such as the Comet GP-series of VHF/UHF omnidirectional antennas.

The practical limit for gain in this type of array is about 10 dBi. A choke balun or other method of decoupling such as a set of λ/4 radials is required at the feed point of the array to prevent current from being induced on the outer surface of the coaxial feed line.

Collinear Omnidirectional Array for 70 cm
Figure 15.11 shows the basic construction of a transposed-coax array for the 70 cm band with dimensions in millimeters for accuracy. The λ/4 whip at the end of the array is optional. The gain of this array is approximately 9 dBi (slightly less without the whip). The original design of this antenna is credited to the Radio Amateur Society of Norwich (www.rason.org). More information is available via the “Projects” page of the RASON website.

The physical length of each λ/2 section of coax must account for the velocity factor of the cable which should be measured accurately before cutting any cable. Once the physical length of λ/2 has been determined, add 8 mm to allow for creating the 4 mm connecting surfaces on each end.
For a VF = 0.66, the λ/2 sections should be 223 mm long plus 8 mm for a total of 231 mm. RG-58, RG-8, RG-8X or RG-213 can be used for this antenna. Do not remove the outer jacket from the cable other than at the connecting ends as this will allow the individual braid strands to loosen, reducing the shield’s effectiveness as a continuous conductor.

Use a 169 mm segment of #16 AWG copper wire for the top whip section.

A λ/4 coaxial sleeve balun is attached at the feed point of the antenna. (See the Transmission Line Coupling and Impedance Matching chapter.) The balun is made from copper tubing that is soldered to the shield of the feed line using strips of brass or copper shim. If 5⁄8-inch tubing is used, the length should be 160 mm. The feed line should be centered in the balun tubing by using small pieces of plastic inserted between the coax jacket and the tubing’s inner surface. Approximately λ/4 beyond the end of the balun’s closed end add an additional choke balun of three type 43 ferrite beads (choose the ID to fit the feed line coax).

The entire antenna should be enclosed in a length of PVC or fiberglass tubing to protect it from the weather. If necessary for mechanical stability, support the antenna sections with a length of wooden dowel or plastic rod, secured with electrical tape.

 

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Another abstract from "ARRL Antenna Book"

DETUNING SLEEVES
The detuning sleeve shown in Figure 24.61B is essentially an air-insulated λ/4 line, but of the coaxial type, with the sleeve constituting the outer conductor and the outside of the coax line being the inner conductor.

Because the impedance at the open end is very high, the unbalanced voltage on the coax line cannot cause much current to flow on the outside of the sleeve. Thus the sleeve acts just like a choke to isolate the remainder of the line from the antenna. (The same viewpoint can be used in explaining the action of the λ/4 arrangement shown at Figure 24.61A, but is less easy to understand in the case of baluns less than λ/4 long.)

A sleeve of this type may be resonated by cutting a small longitudinal slot near the bottom, just large enough to take a single-turn loop which is, in turn, link-coupled to a dip meter. If the sleeve is a little long to start with, a bit at a time can be cut off the top until the stub is resonant.

The diameter of the coaxial detuning sleeve in Figure 24.61B should be fairly large compared with the diameter of the cable it surrounds. A diameter of two inches or so is satisfactory with half-inch cable. The sleeve should be symmetrically placed with respect to the center of the antenna so that it will be equally coupled to both sides. Otherwise a current will be induced from the antenna to the outside of the sleeve. This is particularly important at VHF and UHF.

In both the balancing methods shown in Figure 24.61 the λ/4 section should be cut to be resonant at exactly the same frequency as the antenna itself. These sections tend to have a beneficial effect on the impedance-frequency characteristic of the system, because their reactance varies in the opposite direction to that of the antenna. For instance, if the operating frequency is slightly below resonance the antenna has capacitive reactance, but the shorted λ/4 sections or stubs have inductive reactance. Thus the reactances tend to cancel, which prevents the impedance from changing rapidly and helps maintain a low SWR on the line over a band of frequencies.

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Source: ECE Dept, McMaster University, Ontario, Canada. Antenna Lectures by Prof. Natalia K. Nikolova
Balanced-to-unbalanced feed
Sometimes, when high-frequency devices are connected, their impedances might be well matched, and still we may observe significant reflections. This is sometimes referred to as “field mismatch.”
A typical example in antennas is the interconnection between a coaxial line of Zc = 75Ω and a half-wavelength dipole of Zin= 73Ω. The reflections are much more severe than one would predict using equation Γ2 = {(SWR-1)/(SWR+1)}2. This is because the field and the current distributions in the coaxial line and at the input of the wire dipole are very different [see figure below]. The unequal currents on the dipole’s arms unbalance the antenna and the coaxial feed and induce currents on the outside of the coax shield which are the reason for parasitic radiation. To balance the currents, various devices are used, called baluns (balanced-to-unbalanced transformer).

Sleeve (bazooka) balun 1:1
The sleeve and the outer conductor of the coaxial feed form another coaxial line, which has a characteristic impedance of Z′c. This line is shorted quarter-wavelength away from the antenna input terminals. Thus, its input impedance is very large and results in: (i) suppression of the currents on the outer shield (I3), and (ii) no interference with the antenna input impedance, which is in parallel with respect to the coaxial feed. This is a narrowband balun, which does not transform the impedance (1:1 balun).

 

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I have just now finished 8 hour trial runs of my old CoCo (3.5 element, 11cm element length) with & without Sleeve. The screenshots of VRS are given below. To me it looks that adding sleeve has improved near-range slightly, while removing Sleeve has improved far-range slightly, but the differences are marginal.

Note: The range rings are 50 kms apart. The outermost blue range ring is 450 kms from receiver.

Screenshot 1 of 2 : No Sleeve Balun

Screenshot 2 of 2 : With Sleeve Balun

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..... And here are the construction details of the Sleeve....
The length of Sleeve (i.e. cut length of Pepsi tin) is 6.9 cm (λ/4 in air VF=1)

picture 1 of 5

picture 2 of 5

picture 3 of 5

picture 4 of 5

picture 5 of 5 - Fully Assembled

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[Richard Lee]

Looks like a good job on the Sleeve. Not sure if the diameter should have been that large. Most of the sleeves I've seen are just a little fatter than the coax..


IIRC, this was made originally for 915 MHz. I hacked it a little, trying to make it work better on 1090 MHz.
The coax is some kind of 141 copper HardLine, the sleeve (~10mm OD copper pipe) is soldered at the top.

I believe it is called a coaxial half wave dipole.
http://1313poppy.home.comcast.net/~1313poppy/easytuneantenna/index.2.jpg
The one on the right in the linked pic..

I've seen people make these for UHF, by simply stripping off the outside plastic jacket off RG-8 and pull the copper braid down over the coax.
Leaving the center conductor sticking up as the upper 1/4 in the 'dipole'..

When I used this on 915 MHz, I tested it at work and was surprised at it's -20dB return loss at 915.. Super good match..
And very good performance on a handheld scanner on the 900 MHz comm freqs..

 

[ab cd]

........I believe it is called a coaxial half wave dipole.
http://1313poppy.home.comcast.net/~1313poppy/easytuneantenna/index.2.jpg
The one on the right in the linked pic..
......When I used this on 915 MHz, I tested it at work and was surprised at it's -20dB return loss at 915.. Super good match..

Thanks Richard. Yes it is a halfwave dipole. As 1/2 wave dipole has an impedance of 75 ohms, it gives a very good impedance match to both 75 ohm (1:1) & 50 ohm (1.5:1)

 

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Another Experiment: A 1/2 λ sleeve dipole
After Failed experiment of Sleeved CoCo, I decided to utilize the Pepsi sleeve for another experiment: A 1/2 λ sleeve dipole. The Pepsi can is the down 1/4 λ limb of the dipole, and upper limb is a whip made out of core of coaxial cable 69 mm long (with insulation) + 20 mm bare conductor for insertion into the connector.

The Pepsi can serves dual purpose (1) Lower limb of the dipole & (2) Decoupling Sleeve

Please see below:
(1) Three hrs trial run VRS coverage screenshot.
(2) 4 photos, showing construction details.

Image 1 of 6 : VRS Coverage
The range rings are 50 kms apart, the outermost blue ring is 450 kms from the receiver.

Image 2 of 6 : Construction Details 1

Image 3 of 6 : Construction Details 2

Image 4 of 6 : Construction Details 3

Image 5 of 6 : Fully Assembled

Image 6 of 6 : Dimensions

 

[Richard Lee]

Not bad for just three hours. I'm impressed by the almost complete coverage over Lake Ontario.
If you let it run longer, you might see much better lake coverage than on the two plots taken with the Sleeved CoCo. (above).
Distant targets don't seem too bad either. Maybe a longer scan would reveal a bit more range.

Besides it's limited gain, I think the problem with this kind of antenna is it's floppiness.
Which could be fixed by running the feed cable up to the sleeve using a 1/2" PVC pipe mast..

It's not the best antenna for extremly long range, but the pattern seems prefect for folks who live in high air traffic areas
and want good high and low altitude coverage of local airports.
Assuming you have a sensitive system and the antenna is mounted up in clear air, with minimum nearby blockage.

Because it's not too large, (and inexpensive) this might be a good antenna to use with lap-top portable rigs.
For those days when you don't want to take along a 6 foot antenna..

 

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Test Setup for 1/2 λ Sleeve Dipole

Picture 1 of 2

Picture 2 of 2

 

[ab cd]

Not bad for just three hours. I'm impressed by the almost complete coverage over Lake Ontario.
If you let it run longer, you might see much better lake coverage than on the two plots taken with the Sleeved CoCo. (above).
Distant targets don't seem too bad either. Maybe a longer scan would reveal a bit more range.

Besides it's limited gain, I think the problem with this kind of antenna is it's floppiness.
Which could be fixed by running the feed cable up to the sleeve using a 1/2" PVC pipe mast..

It's not the best antenna for extremly long range, but the pattern seems prefect for folks who live in high air traffic areas
and want good high and low altitude coverage of local airports.
Assuming you have a sensitive system and the antenna is mounted up in clear air, with minimum nearby blockage.

Because it's not too large, (and inexpensive) this might be a good antenna to use with lap-top portable rigs.
For those days when you don't want to take along a 6 foot antenna..

Thanks for your comments & opinion. These discussions are very enlightening.

Just now I have trimmed the Pepsi can by about 5 mm, and put it on trial run. It will take couple of hours to know the result.

The reason for trimming is that the Pepsi can's bottom is not flat. It is cup shaped depressed inside. The lowest point is almost same level as the edge of blue paint. I have originally taken 69mm from that point. Later I realized that the outer part of bottom is at rim level, which is about 10 mm above the center. Hence the disk equalent position should be not lowest, not highest, but some intermediate position. I took average of the two which makes it 5 mm above the bottom of cup/blu edge. From that reference point, the length of Pepsi can is 69+5 mm. So I trimmed the can by 5 mm to make the length of sleeve 69 mm from reference point. If there is substantial difference of coverage between trimmed & untrimmed sleeves, I will post the results.

 

[Richard Lee]

I wonder if the flat part on top of the can is part of the overall electrical wave-length?
Is the distance between the coax to the edge (OD) of the can going to effect the gain pattern?

I've seen some ground plain antennas that used a flat disk. Some were to simulate being installed on the roof of a car..
I can't recall what the OD of the disk was, relative to the wave-length. Might have been 1/2 wave..?.

http://www.chem.hawaii.edu/uham/hnet.html


Anyways, using a coke can might be okay at VHF, but at 1090 Mhz..?.
Maybe the OD of the can should be much smaller.. Seems almost logical to my old brain..

Edit:
Hey I just found out what's inside of a WiFi rubber-ducky!

http://martybugs.net/wireless/rubberducky.cgi

 

[ab cd]

I wonder if the flat part on top of the can is part of the overall electrical wave-length?
Is the distance between the coax to the edge (OD) of the can going to effect the gain pattern?

I've seen some ground plain antennas that used a flat disk. Some were to simulate being installed on the roof of a car..
I can't recall what the OD of the disk was, relative to the wave-length. Might have been 1/2 wave..?.

http://www.chem.hawaii.edu/uham/hnet.html


Anyways, using a coke can might be okay at VHF, but at 1090 Mhz..?.
Maybe the OD of the can should be much smaller.. Seems almost logical to my old brain..

No, the circular bottom of can is not part of wavelength. In a dipole, the lower & upper limbs start at the same point i.e. where central conductor leaves the shield.

 

[ab cd]

I wonder if the flat part on top of the can is part of the overall electrical wave-length?
Is the distance between the coax to the edge (OD) of the can going to effect the gain pattern?

I've seen some ground plain antennas that used a flat disk. Some were to simulate being installed on the roof of a car..
I can't recall what the OD of the disk was, relative to the wave-length. Might have been 1/2 wave..?.

http://www.chem.hawaii.edu/uham/hnet.html


Anyways, using a coke can might be okay at VHF, but at 1090 Mhz..?.
Maybe the OD of the can should be much smaller.. Seems almost logical to my old brain..

Edit:
Hey I just found out what's inside of a WiFi rubber-ducky!

http://martybugs.net/wireless/rubberducky.cgi

Great discovery! So λ/2 Sleeve Dipole is really a good antenna, else it won't be so popular & widely used.

 

[Richard Lee]

So, if I took an old 16" cooking pot, drilled a hole in the center for a double female and cut the pot down to 69 mm.?.

It sure seems like that flat cooking pot bottom would look like the roof of a car, at 1 Ghz..

Humm, maybe I should put my money where my mouth is, and start building some antenna test equipment!