The whip at top is ¼ wavelength i.e. 69mm (see dimension drawing below)
The whip is core wire of coax cable.
It can be bare copper wire OR copper wire + core insulation.
Shield (braid+foil) MUST be removed from the whip.
Lots of people talk about building CoCo (Collinear antennas made from co-axial cable) - yes they are cheap to make, but it is also very easy to make a bad coco that doesn’t work well.
Essentially ADS-B is line of sight, the maximum range is to the horizon, and one of the simple antennas above will get you most of the way there.
You can check your antenna performance by going to web site heywhatsthat.com/ do a new panorama Mark your exact antenna location and enter it’s height above the ground. The backend will calculate your horizon taking into account earth curvature, mountains, hils etc. On the map that is produced select ‘In the Air’ then zoom out 'till the map is probably 1000km wide (closer you won’t see the range rings since they’ll be outside the map segment you’re viewing). now in the bottom of the map say you want an outer ring at 40,000 feet … that will be about as far as you will see. If the horizon is over the ocean you seem to get a better range ???
How what is that will not take into account the shading of the signals that is caused by surrounding buildings, bags of water (also called trees), placing the antenna under something that cuts down the signal (like roof tiles or a long downlead), etc.
Apply a rapid setting (5 minutes) sealant like 2-part epoxy, or silicone, or hot melt glue, or similar at the point where radials enter the outer jacket of coax.
After installation. wrap the F-connector in tape to prevent moisture ingress. You may also use outdoor type F connector which are water resistant
For the panel mount connector Spider antennas, use an N connector if you have a choice. I’m a single data point, but I’ve made several with N and UHF (SO-239) and the N connector versions provide more position tracks. YMMV, but it stands to reason since UHF connectors are only intended for <300MHz.
For those of you nervous to cut a soda can or don’t have the tools I have found that a Bush’s 8.3oz baked beans can is just over 70mm long.
The can has a pull off lid and so therefore saves you cutting the can.
All you have to do is drill a hole in the center of the closed end to mount the antenna.
Made one of these tonight and will let it run a few days.
In my Cantenna, I use an insulated whip (core wire of RG6 coax with core’s insulation). The length of whip was 69 mm. Few days ago I cut 3 mm off the top of whip, making it 66 mm long. I noticed not very big, but noticeable improvement in performance.
With bare copper wire whip, a lesser chop-off may be required, say 1 or 2 mm instead of 3 mm (making whip 68 or 67 mm long). I have yet to try the chopped-off bare wire whip.
For cutting the aluminum drink can (Pepsi/Coke/Beer), I have used:
Scissors as described by Peter.
Stanley knife.
The can wall being very thin, it was easy to cut it using scissors or knife.
Cutting a food cans made of iron sheet is a bit tough & requires tin-snips.
The sharp cut edge can easily injure fingers during cutting & subsequent handling. A layer of tape should be applied around the cut rim of food/drink can.
An example of cutting the tin: “the cutting edge technology”
I have tried all these designs, and more. Even did a stripline antenna out of printed circuit board, which simulated a coaxial collinear design. This has been the best one so far, best range. Almost double any other one I have tried. Note, I use BNC connectors instead of N: http://www.topsecretbases.com/images/coaxial.colinear.jpg