ADS-B DIY Antenna

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ab cd

Senior Member
I'm still testing my last antenna build but I have been doing some reading. I found an article --> here which suggests taking 2% off the ends of the last elements to give a downward radiation pattern. I wonder if this will work for a receiving antenna too?
The down tilt is useful for high located repeaters to reach down located stations, as mentioned in the document:
"One additional characteristic of this type of antenna is that the radiation pattern can be tilted downwards from horizontal by as much as 3 degrees by reducing the calculated half-wave element lengths 2 percent shorter. Repeaters that are located at very high elevations can take advantage of this tilted pattern to improve repeater access (down-link and up-link) by mobiles and base stations, located far down below the repeater site."

In case of ASD-B reception with collinear, it is of no use, as the maximum gain of collinear is already in horizontal direction where the farthest planes are picked.
However, for 1/2 wavelength Dipole (my Tiny Tot) it may give some advantage. See diagram below. The dark blue elliptical curve is for our 4 element Franklin, and light blue circular curve is for 1/2 wavelength Dipole.

Franklin-Co-Linear Antenna Directivity.png



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ab cd

Senior Member
Hi ab cd,
I didn't have any 2mm wire so I built another one with 1.5mm. It again improved my distance by about 20nm. I wonder if this may be due to me getting more accurate at bending wire?

I'll be running some back to back tests with adsbScope in the next few days and report back.

Dave
Hi Dave,
It will be great if you post Camera pictures of your antennas & setup.
Regards,
ab cd

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trigger

Member
Here are some photos.

This is the 1.5mm Franklin
DSCF0942.JPG


The dongle
DSCF0940.JPG


Connection to coax
DSCF0948.JPG


An early attempt
DSCF0949.JPG
 
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trigger

Member
Sorry, I haven't got the hang of uploading images.

I'm hoping to eventually get better coverage to the East so I can watch the holding patterns at Heathrow. The planes drop off the screen at about 8000 feet.
 
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trigger

Member
Hi ab cd,

I've added an extra section to both ends of the Franklin so it is now 6 x 138mm with

5 x ¼ stubs. If you get a chance, can you tell me what distance the feed for the coax should be along the middle stub?

My temporary fixing at 18mm suggests I have a few extra Km and better coverage at below 8000 feet.

I'm also going to try to fold the stubs into a circle so I can get the antenna into a tube. What effect will this have on the gain & swr?

Thanks
Dave
 

ab cd

Senior Member
Hi ab cd,

I've added an extra section to both ends of the Franklin so it is now 6 x 138mm with

5 x ¼ stubs. If you get a chance, can you tell me what distance the feed for the coax should be along the middle stub?

My temporary fixing at 18mm suggests I have a few extra Km and better coverage at below 8000 feet.

I'm also going to try to fold the stubs into a circle so I can get the antenna into a tube. What effect will this have on the gain & swr?

Thanks
Dave
I have run a simulation of 6-element franklin you have built (1.5mm dia wire, 6 vertical 138mm elements, 4 folds 67mm x 4mm, 1 stub 69mm x 5mm).
The results are:
Gain = 8.5 dBi
Minimum SWR = 1.41 at cable tap position 23mm

(at 18 mm, SWR is 1.81)

Move your tap from 18mm to 23mm. This will reduce SWR from 1.81 to 1.41, and you will notice an improvement in range.

Do not wrap the folds yet. First try above setting.

See screenshots below for simulation

Screenshot 1 of 3
franklin - 6 element - 1.5mm dia wire - gain swr pattern.png



Screenshot 2 of 3
franklin - 6 element-1.5mm dia wire-geometry-variables.png



Screenshot 3 of 3
franklin - 6 element-1.5mm dia wire-geometry-wires.png






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trigger

Member
Hi ab cd,

Many thanks for the calculations. I'll move the tap to 23mm and report back.

I have the dongle attached to a Raspberry pi at the moment and I'm in the process of installing all the software for that. It may be a couple of days before I can set it up again on my laptop.

Regards
Dave
 

ab cd

Senior Member
Hi ab cd,

Many thanks for the calculations. I'll move the tap to 23mm and report back.

I have the dongle attached to a Raspberry pi at the moment and I'm in the process of installing all the software for that. It may be a couple of days before I can set it up again on my laptop.

Regards
Dave
Hi Dave,
I will wait for your results.

I just want to tell you that the spacing between wires of Matching Stub (to which you connect your coaxial cable) affects the SWR appreciably.
Smaller the spacing, smaller SWR and hence better Range.

I have run two plots of SWR & Gain. One with 5mm stub spacing, other with 10mm stub spacing (see attached screenshots below)
Results show that
For 5mm Stub Wire Spacing: SWR = 1.41, Gain = 8.51 dB, Tap = 23mm (for lowest SWR)
For 10mm Stub Wire Spacing: SWR = 2.14, Gain = 8.32 dB, Tap = 25mm (for lowest SWR)

Conclusion: Avoid using large spacing of stub wires. Try to keep 5mm to 3mm. (This is for Matching Stub only. For other Folds, the wire spacing is not very critical and 4 mm is ok)

Image 1 of 2 - Stub Wire Spacing 5mm
franklin-6 element-1.5mm dia wire-5mm stub-swr gain sweep.png



Image 2 of 2 - Stub Wire Spacing 10mm
franklin-6 element-1.5mm dia wire-10mm stub-swr gain sweep.png






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trigger

Member
I've just got the RPi working so put the antenna back onto the laptop for consistency. The 6 element with the tap at 23mm doesn't seem to give as good results as the tap at 18mm. I'll re-check the dimensions tomorrow and run another ADSBScope plot.
 

ab cd

Senior Member
I've just got the RPi working so put the antenna back onto the laptop for consistency. The 6 element with the tap at 23mm doesn't seem to give as good results as the tap at 18mm. I'll re-check the dimensions tomorrow and run another ADSBScope plot.
The tap position calculated by simulation is exact if the antenna is cut & bent precisely as in simulation. Since home made antennas cannot be very precise, the best tap position may differ and to be adjusted by trial. Try 25, 23, 20, & 18 mm to find best position.
 
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ab cd

Senior Member
I've just got the RPi working so put the antenna back onto the laptop for consistency. The 6 element with the tap at 23mm doesn't seem to give as good results as the tap at 18mm. I'll re-check the dimensions tomorrow and run another ADSBScope plot.
Also try to change the approach of your coax so that tap wires (red & white) are not closely parallel to last part of stub (see image below).
It will most likely improve results. :
DSCF0948-Dave-Rev.JPG
 
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trigger

Member
OK I'll give that a try. Nice work in Photoshop by the way ;)

I've taken the insulation off the connector block and removed the little metal connector. I sawed it in half and have now used the 2 pieces to connect the coax to the stub. This has given me a bit more flexibility with getting the stub separation. I have to use a small bit of insulation tape to keep them apart!
 

ab cd

Senior Member
OK I'll give that a try. Nice work in Photoshop by the way ;)

I've taken the insulation off the connector block and removed the little metal connector. I sawed it in half and have now used the 2 pieces to connect the coax to the stub. This has given me a bit more flexibility with getting the stub separation. I have to use a small bit of insulation tape to keep them apart!
yes, I have noted that you are usin connector block used for electrical wiring in homes, and the separation (center of screw to center-of-screw) is about 10 mm. This made your stub 5mm all the way upto connector, then widened to 10mm at connector, and then reduced again to 5 mm in the tail portion. Good that you have removed insulation & seperated the two metal blocks, so that you can get a closer spacing of stub wires.
Waiting to see your results. It would be nice if you can post photo of antenna with torn-down connectors.
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trigger

Member
Hi ab cd, I hopefully have attached a couple of photos. One shows the torn down connector with the 90 degree attachment, and the other shows the antenna against a ruler.

I've tried various positions for the tap on the 6 element and it seems to have less range than the 4 element. I'll have a go at building another one.
DSCF0951.JPG
DSCF0954.JPG
 

ab cd

Senior Member
Hi ab cd, I hopefudlly have attached a couple of photos. One shows the torn down connector with the 90 degree attachment, and the other shows the antenna against a ruler.

I've tried various positions for the tap on the 6 element and it seems to have less range than the 4 element. I'll have a go at building another one.View attachment 115 View attachment 116
hi
Hi ab cd, I hopefully have attached a couple of photos. One shows the torn down connector with the 90 degree attachment, and the other shows the antenna against a ruler.

I've tried various positions for the tap on the 6 element and it seems to have less range than the 4 element. I'll have a go at building another one.View attachment 115 View attachment 116
It seems you have made the gap between FOLD wires almost zero. This will have two negative affects:
1. The Fold wires may touch each other at some point, causing reduction in gain.

2. Such a narrow spacing of FOLD wires, and 6 elements will result in a very high antenna impedance, which cannot be matched with 5mm or even 3mm stub.

Conclusion: Increase the spacing of FOLD wires. Make the GAP at least 2.5mm (center-to-center will become 2.5+1.5=4mm).
STUB wire spacing seems ok. Check, if they are touching each other at some point, separate them or slightly increase spacing
 
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trigger

Member
Hi ab cd, I think the photo didn't show the gaps properly. They are all similarly spaced as the stub wire.

I will re-check the spacing and make sure the gap is 2.5mm along the whole length of each fold.

Many thanks for your observations.
Regards,
Dave
 

trigger

Member
Latest findings ....

I've adjusted the tap between 18mm and 28mm in 2 mm steps. The best position seems to be about 26mm but there is very little difference between any of the positions. I made a 2.5mm thick plastic template and used that to form the stub wires so they are all similarly spaced.

In your diagram above you have the stub lengths of 67mm whereas the tap length is 69mm. How important is the 67mm? My 6 element has stub lengths of 68mm but the tap is 69mm.

The 6 segment Franklin has a small range benefit (~10 nm) over the 4 segment but not as much as I expected. I'll try folding the stub wires into a circle today.

Many thanks
Dave
 

ab cd

Senior Member
Latest findings ....

I've adjusted the tap between 18mm and 28mm in 2 mm steps. The best position seems to be about 26mm but there is very little difference between any of the positions. I made a 2.5mm thick plastic template and used that to form the stub wires so they are all similarly spaced.

In your diagram above you have the stub lengths of 67mm whereas the tap length is 69mm. How important is the 67mm? My 6 element has stub lengths of 68mm but the tap is 69mm.

The 6 segment Franklin has a small range benefit (~10 nm) over the 4 segment but not as much as I expected. I'll try folding the stub wires into a circle today.

Many thanks
Dave
FOLD's length is not very critical and 68mm instead of 67mm will not make any noticeable difference.
STUB's Length & Spacing are very critical and variations may result in noticeable difference in performance.
The theoretical gains are as follows:
4-element Franklin, 1.5 mm wire dia = 7.57 dBi
6-element Franklin, 1.5mm wire dia = 8.51 dBi.
The difference is 0.93 dBi, not much.
 
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