I bought these antenna from Amazon. I’m satisfied with my performance, getting messages from up to 200 miles, but I resd somewhere these sntennas should be trimmed to 13.7cm.
Will I improve my range with such mod?
You could also remove the tip, because it is screwed only.
Then replace it with a piece of strong wire in the exact length.
Be aware that you have to add the socket as well in your calculation.
There is a correct guide somewhere documented by @abcd567, but i cannot find it at the moment.
Most generic dongles come with a single element antenna (one wire from the base). This works ok from VHF through UHF adequately (most of the time!).
When we listen for ADS-B, we no longer have use for a broadband antenna, and trimming the single wire to be 1/4 wave of 1090MHz “should” improve it’s performance (but often doesn’t).
The antenna you’ve got could be just some random lengths styled like a 5/8 over 1/4 - if so, it’s already a single frequency antenna, you’d need to trim both the top and bottom to get the best out of it. Of course, the further from 1090MHz it was manufactured, the more inappropriate the phasing coil will be.
Try measuring just the top section and if you feel like it, trim this to 1/4 wave.
This antenna seem to be a bit different to the typical DVB-T antennas
Jetvision is also selling this version bundled with a cheap dongle.
For a mini project i bought three of them and on one i replaced it with a wire in correct length.
Finally the difference was not recognizable.
However as it can be tested non-destructive, give it a try
The top length should be lambda/2 and the bottom part lambda/4. The coil length is also lambda/4.
I think that the exact length is not that critical. We are talking about maximum 10% sensitivity loss and in my experience, the actual obstacles will limit the distance much earlier than the signal level (noise).
Earth curvature if anything limits the distance faster than the signal level. So… install height is more important than anything.
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You know…There is not a single flat-earther that will agree with that statement. 
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So… that’s all of those interested about precise antenna size, impedance matching, SWR graphs, and gain?
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Do NOT cut the whip. Unscrew it, and use a thin wire as whip
If the wire is thin, it will be easy to wrap it over the threaded brass stud of the magnetic base. With thick wire it is a bit difficult.
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Another reason for not cutting the whip is so you can use it as a control and switch back and forth to a whip of your own making.
This is where theory and practice start to go their separate ways.
If you had (say) a 1/4λ over a 1/4λ, you’d want a 180º phase shift so they work together rather than canceling each other. This commonly employs a coil, or hair-pin.
The trouble is, a coil also introduces significant induction which alters the phase angle and must be compensated for by altering the coil length.
If the top section is not 1/4λ, but rather 1/2λ or 5/8λ, then the coil also needs to offer an impedance match as well as a phase shift (a 1/4λ is a low impedance antenna, while a 1/2λ is very high)
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Hi there,
I have the same antenna as you. I would NOT recommend to cut it (for the reasons mentioned above), but instead build your own one!
I follow many guides online and tried build couple of antennas myself. As far as my skills and equipment go, this setup works best for me Collinear Dipole Antenna Design Calculator
You can screw out the antenna you have, build the collinear-dipole one and just screw back into the bolt. JUST BE CAREFULL NOT TO PUSH THE BOLT INSIDE !!! as it happens to me. The way I did that was I created a little ring down the bottom and then squeeze it around (pictures to follow)
As you can see, there is a small diff. (about 2-3mm) between those two. These are quarter wave length antennas. You have to test it yourself and consider that the bolt connector down the bottom of your antenna’s stand might be 2-5mm long. Therefore you can adjust the custom antenna by cutting the tip.
This is how I did it…
and here is the collinear-dipole antenna (using calculator from the website)
I was trying to build a spider antenna this week… updates later 
Just to add - that after you change your antenna, sometimes you must reboot your raspberry pi or wait about 30 minutes, as the flight aware software is automatically adjusting gain (dBi) to the new antenna.
You can run a script on your raspberry pi to see that in action … but this is for another topic.
You just explained why the coil works. The idea is to change the phase angle with 180 degree. A coil can do that. A hairpin is a simplified coil too. All it matters is to have that 180 degree (that’s actually not a real angle but an actual time delay) provided as length of lambda/2 - in some shape.
Also… precise tuning of those simple antennas is not that critical. They have a fairly wide band, with slow slopes anyway.
IMO obstacles and Earth radius will intervene sooner to cut the signals than the signal/noise ratio, so all this quest for antennas is kind of misleading. Raise the antenna above obstacles and you can use the antenna from above pic.
My son used to live in an apartment block at 15th floor, open sky on one side. The antenna from above picture, in the window, was giving him 250 miles reception on one direction, with a blue FA dongle.
Now he moved somewhere lower, in a valley, and all he can see is 40 miles away.
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I think you missed gecko’s point. Because the coil is coil-shaped, it has a non-trivial inductance. The inductance affects the shape of the standing wave. You need to compensate for this.
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It also has distributed capacitance between coils. Can’t tell exactly what will do in the end, but empirically I saw that it works.
Is it better than a short lambda/4 stub? Don’t know, I didn’t tested it in both configurations.
At GHz frequencies, it is very difficult for an amateur to make a good wire collinear antenna with a coil. It can be done only if designed, tested, manufactured and tuned in a factory environment. At GHz frequencies an error of a millimeter or two can be disastrous.
Hairpins or U-stubs are used as alternative by amateurs as they are easy to control.
Recently I have introduced a V-stub as alternate to U-stub. The V can be formed more easily & precisely than the U-stub. I have built & tested the pilot, both with 4 horizontal radials, and with magnetic base on a metal sheet, and both give good results.
In the first diagram see the current distribution in green color along the wires which confirms that the V has provided required 180 deg phase shift. This diagram also shows in red color the 3D the plot of Far-Field.
Current distribution along wires in green color
3D Far-field in red color
Note: Paper placed behind V-Stub while taking snapshot to make the thin wire visible in photo.
CLICK ON IMAGE TO SEE LARGER SIZE
The V has components on vertical (22.5mm) that would affect the lengths of those arms…
The fact that is not that visible, shows that for ADS-B the antenna gain and impedance matching is not important.
OK, maybe like 10% important.
The free clear sight of the horizon is the rest of 90%.
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Emperical Rule of thumb-1
For minimum SWR with varying lengths of wires:
Lower wire+ Width of V+Upper wire = constant
Example for best SWR:
82 mm + 45 mm + 150 mm = 277 mm
If width of V is reduced by 10mm (i.e. from 45mm to 35mm), then to get best SWR, the upper wire lemgth should be increased by 10mm (i.e. from 150mm to 160mm)
Emperical Rule of thumb-2
The bigger the gap between two vertical wires, the higher the gain. Hence increasing V stub’s width from 35mm to 45mm will increase gap between two vertical wires, and improve gain.
Hidden truth.
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That’s not working really well. Ex: If you increase that gap to the max, you will have a straight wire.
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Emperical Rule of thumb-3
Don’t increase the gap until it’s a straight wire.
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