ADS-B DIY Antenna

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ab cd

Senior Member
#41
Gain = 7.15 dBi, SWR = 1.1

I have done some changes in impedance matching arrangement and then simulated the new design with 4nec2 software.

STRATEGY:
(1) Determine Antenna Impedance. This is done by Simulation of antenna without matching stub. (I have assumed 1600 ohms in the previous design, which was wrong approach, and proved to be too high).
(2) Using the antenna impedance Zant & connecting cable impedance Zcab, determine required stub impedance Zstb of matching stub using formula Zstb = √(Zant x Zcab).
(3) Using stub impedance obtained in (3) above, and the stub wire dia d, calculate center-to-center spacing between stub conductors S, using formulae Zstb = 276 log (2S/d).
(4) Simulate antenna WITH matching stub using dimensions obtained as in (3) above.

CALCULATIONS:
To get fairly accurate value of antenna impedance, I ran the simulation after removing the impedance matching stub and connecting feeder cable directly to antenna at its center.
The results show that the Antenna Impedance = 411 - j435 ohms

Using the Antenna Impedance obtained above (411 - j435 ohms)
Zant= √(square of 411 + square of 435 ) = 598 ohms. The previous design's assumed value of 1600 ohms was therefore wrong.

Based on new value of Antenna impedance obtained by simulation, I did following calculation for the dimensions of matching stub which worked out to be 3mm stub conductor spacing for a wire dia of 1mm. See calculations below.


Required impedance of Matching Stub Zstb = √(Zant x Zcab) = √(598 x 75) = 212 ohms

Stub Dimension calculation:
S= spacing between stub conductors (center-to-center)
d= dia of stub wire = 1mm (#18AWG)

Zstb = 276 log (2S/d)

which can be rearranged as S = (d/2) x 10 to power (Zstb/276)
Using d = 1mm and Zstb = 212 ohms,
S = (1/2) x 10 to power(212/276) = 3mm

I then simulated with a 3mm stub (wire #6, #7, & #8 in th first sreeshot - input geometry data. The coaxial cable connection is at wire #14)

The input/output data is shown in the 4 screenshots below (Gain = 7.15 dBi, SWR = 1.1 - see image 3, left window):


image 1 of 4 - input data
3mm stub-input-geometry.png



image 2 of 4 - input data
3mm stub-input-source load.png



image 3 of 4 - output data
3mm stub-output-gain-swr-pattern.png



image 4 of 4 - output data
3mm stub-output-sweep-swr.png



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#42
I've been following this thread with interest as I've tried to make the Franklin but can't get it to work well. I've got a slim jim which typically can see 20 aircraft but as soon as I swap to the Franklin that number drops to 3 or 4.

There was some discussion above about insulation around the wire making a difference to the lengths that have to be used. I've stripped mine back to bare metal. I'm using approx. 1mm diameter. Can you tell me what dimensions I should be using?

Keep up the good work,
Dave
 

ab cd

Senior Member
#43
I've been following this thread with interest as I've tried to make the Franklin but can't get it to work well. I've got a slim jim which typically can see 20 aircraft but as soon as I swap to the Franklin that number drops to 3 or 4.

There was some discussion above about insulation around the wire making a difference to the lengths that have to be used. I've stripped mine back to bare metal. I'm using approx. 1mm diameter. Can you tell me what dimensions I should be using?

Keep up the good work,
Dave
Please see dimensioned diagram & Camera Photos below.
Note that spacing of wires in the folds is 4 mm between centers of two parallel wires, and in the matching stub, it is 5 mm between centers of two parallel wires.

Dimension diagram
franklin-1mm wire-4mm fold-5mm stub.PNG


Picture 1
DSC03054R.JPG


Camera Picture 2
DSC03055R.JPG




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ab cd

Senior Member
#44
I've been following this thread with interest as I've tried to make the Franklin but can't get it to work well. I've got a slim jim which typically can see 20 aircraft but as soon as I swap to the Franklin that number drops to 3 or 4.

There was some discussion above about insulation around the wire making a difference to the lengths that have to be used. I've stripped mine back to bare metal. I'm using approx. 1mm diameter. Can you tell me what dimensions I should be using?

Keep up the good work,
Dave
The electric field of a single wire extends far beyond the insulation into the air and affect of insulation on velocity is negligible. However when two insulated wires are run in parallel in close proximity (fold, matchig stub,twinlead TV wire), the electric field is mainly confined between the wires and if the wires are close, major portion of field is in the insulation and partly in air. Hence velocity is affected substantially. In case of coaxial cable the field is almost entirely confined between core and braid, which is completely filled by insulation, hence affect on velocity is pronounced.

For this reason I removed insulation of folds and matching stib, but did not bother to remove insulation of vertical limbs. Removing or leaving limb insulation does not make any difference. Please see photos of franklin I have posted few hours ago.

The stub wire dia & spacing of stub conductors greatly affects impedance matching & swr. I have used 5mm center to center from practical cosiderations (terminal block for tap-off had 5mm spacing between terminal screws). Using 4 mm or 3 mm will improve swr.
 
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#45
Hi ab cd, Many thanks for your replies and photos. I'll have try using coax exactly like yours and another using the dimensions for 1mm wire.
Regards
Dave
 

ab cd

Senior Member
#46
I have now done re-modelling on software 4nec2 (available free of cost at http://www.qsl.net/4nec2 ) , using VARIABLE parameters to see how these affect the gain & SWR.
The variable chosen were
1. tap position from shorted end of stub (symbol 'tap' in input data, see screenshots below).
2. center-to-center spacing of of fold wires (symbol 'foldH' in input data, see screenshots below).
3. center-to-center spacing of of stub wires (symbol stubH' in input data, see screenshots below).

I then ran a trial run of built antenna, and a simulation as well, using tap = 18 mm, foldH = 4mm, stubH= 5 mm (i.e. exactly same as built by me, and shown in diagram/pictures of my yesterday's post #43). See screenshot 1, 2, 3, 4 & 5 below.

I then ran the software in sweep mode, for values of variable 'tap' changed from 10 mm to 25 mm, and the results plotted as graph of SWR & Gain vs tap position are in screenshot 6 & 7 below.

Screenshot 1 of 7 - Maximum Range trial Run
franklin-fold4mm-stub5mm-tap 18mm-04Feb2014.png



Screenshot 2 of 7 - Input - wire geometry data (wire #14 is connection point/tap where Coaxial Cable is connected)
franklin-fold4mm-stub5mm-tap 18mm-input data geometry.png



Screenshot 3 of 7 - Input - source/load data (point where Coaxial Cable is connected to the antenna)
franklin-fold4mm-stub5mm-tap 18mm-input data source.png



Screenshot 4 of 7 - Input - Variables (called symbols in 4nec2)
franklin-fold4mm-stub5mm-tap 18mm-input data variables.png


Screenshot 5 of 7 - Output - Geometry, Pattern, Gain, SWR, Impedance

franklin-fold4mm-stub5mm-tap 18mm-geometry pattern gain swr.png



Screenshot 6 of 7 - Output - Plot of SWR vs Tap position (10 mm to 25 mm, shown in meters as 0.01 to 0.025)
franklin-fold4mm-stub5mm-SWR-sweep tap 10mm to 25mm.png



Screenshot 7 of 7 - Output - Plot of Gain vs Tap position (10 mm to 25 mm, shown in meters as 0.01 to 0.025)
franklin-fold4mm-stub5mm-Gain-sweep tap 10mm to 25mm.png





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#48
Excellent work :)

I shall make this antenna this afternoon. I'm trying to get better low altitude coverage rather than distance.
Regards
Dave
 

ab cd

Senior Member
#49
I've been following this thread with interest as I've tried to make the Franklin but can't get it to work well. I've got a slim jim which typically can see 20 aircraft but as soon as I swap to the Franklin that number drops to 3 or 4.

There was some discussion above about insulation around the wire making a difference to the lengths that have to be used. I've stripped mine back to bare metal. I'm using approx. 1mm diameter. Can you tell me what dimensions I should be using?

Keep up the good work,
Dave
Hi Dave,

I would love to get the dimensions & construction details of your SLIM JIM.
If I get the details, I will :
1. Run a computer simulation using software 4nec2.
2. Built one and put it on trial run, to see it's performance, and make a comparison with other 3 antenna types I have built and tried (1/2 wavelength Dipole, Franklin Collinear, & Coaxial Collinear).

Best Regards,
ab cd
 
#50
Hi ab cd,

My Slim Jim is made to this design --> Slim Jim

I've also made this one --> Vertical Antenna

I made the Franklin this afternoon and I have it running with SBS Plotter at the moment. I'll report on how it compares with the above 2 antennas.

Regards
Dave
 
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#51
It looks like the Franklin is better than either of the other 2 antennas I've tried. There was limited traffic this afternoon so the comparison is not valid. The Franklin has an extra 20 to 30nm in all directions. However the low altitude <10,000 ft seems to be the same as the other 2. That may be due to my location rather than antenna design.

I'll do the test again tomorrow with all the Monday morning commuting and report back.
Regards
Dave
 

ab cd

Senior Member
#52
Hi ab cd,

My Slim Jim is made to this design --> Slim Jim

I've also made this one --> Vertical Antenna

I made the Franklin this afternoon and I have it running with SBS Plotter at the moment. I'll report on how it compares with the above 2 antennas.

Regards
Dave
Hi Dave!

THAKS for SLIM JIM Diagram.
I will stimulate it by the 4nec2 software, and also build it and put on a trial run.
Also thanks for the link to other antenna.

Regards
ab cd
 

ab cd

Senior Member
#53
It looks like the Franklin is better than either of the other 2 antennas I've tried. There was limited traffic this afternoon so the comparison is not valid. The Franklin has an extra 20 to 30nm in all directions. However the low altitude <10,000 ft seems to be the same as the other 2. That may be due to my location rather than antenna design.

I'll do the test again tomorrow with all the Monday morning commuting and report back.
Regards
Dave
1. The 5 mm spacing of stub wires is between centers of two wires. This means the air gap between wires of 1 mm dia will be 4mm. Please see image 1 (stub wire spaacing diagram) below.

2. If you reduce spacing of stub wires from 5 mm to 3mm both SWR and Gain will improve slightly. SWR will decrease from 1.9 to 1.5 and Gain will increase from 7.26 to 7.45 dBi, both changes are an advantage. (I have used 5 mm spacing since the cable tap-off block I used had a screw spacing of 5mm). Please see images 2 & 3 (screenshots) below.

Image 1 of 3 - Stub Wire Spacing Diagram
stub wire spacing.PNG



Image 2 of 3 - Screenshot SWR
franklin-fold 4mm-stub variable-sweep swr.PNG



Image 3 of 3 - Screenshot Gain
franklin-fold 4mm-stub variable-sweep gain.PNG




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ab cd

Senior Member
#54
It looks like the Franklin is better than either of the other 2 antennas I've tried. There was limited traffic this afternoon so the comparison is not valid. The Franklin has an extra 20 to 30nm in all directions. However the low altitude <10,000 ft seems to be the same as the other 2. That may be due to my location rather than antenna design.

I'll do the test again tomorrow with all the Monday morning commuting and report back.
Regards
Dave
Hi Dave!
Make another Franklin antenna with 2mm dia wire using same dimensions as given in my yesterday's post for 1 mm wire dia (post #47).
Give the 2mm antenna also a try and compare results with 1 mm antenna.

The reason I am asking you to do so is that I have run another simulation, this time increasing the wire dia from 1 mm to 2 mm in 4 steps, and keeping all dimensions same as the one I have posted yesterday (post #47). The simulation shows that higher wire size gives higher Gain & lower SWR, both making antenna better. see the screenshot below:

franklin wire size.PNG


Regards,
ab cd
 

ab cd

Senior Member
#55
Excellent work :)

I shall make this antenna this afternoon. I'm trying to get better low altitude coverage rather than distance.
Regards
Dave
I you want better low altitude coverage, try the Tiny Tot, Cutie Pie , 1/2 wavelength Dipole shown in the picture below (ignore amplifier shown in picture and try without amplifier):
DSC02542C.GIF




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#56
Hi ab cd,
OK, I'll make a dipole and try a Franklin with 2mm wire. It will be a few days before I can do this.

Is the idea of the Franklin to get the air gap as small as possible?

Many thanks for all your help.
Dave
 

ab cd

Senior Member
#57
Hi ab cd,
OK, I'll make a dipole and try a Franklin with 2mm wire. It will be a few days before I can do this.

Is the idea of the Franklin to get the air gap as small as possible?

Many thanks for all your help.
Dave
Simulations in which the stubH (center-to-center distance between stub wires) is varied, show reducing the distance between wires improves the Gain & SWR. However the spacing cannot be kept too little due to practical consideration. See screenshots 1 & 2.

Simulations in which the D (diameter of stub wires) is varied, show increasing diameter of stub wires improves the Gain & SWR. See screenshot 3. (Thicker wire has also an added advantage of better mechanical strength)

Screenshot 1
stub Height sweep - 1 mm wire.PNG


Screenshot 2
stub Height sweep - 2mm wire.PNG


Screenshot 3
franklin wire size.PNG



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#58
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
 
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ab cd

Senior Member
#59
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
Possibly a combination of both thr higher wire size & better bendiing.

I ram simulation of your SlimJim, and it is shown inferior to Franklin (Gain is lower & SWR is much higher than Franklin).
 
#60
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?
 
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