Although I am quite happy with the performance of my inverted-vee dipole antenna, I never stop looking for portable antennas that can be deployed speedily without compromising their core function. Consequently I keep trying various vertical end-fed antenna designs, as I’ve also done with my EFHW for 15 m.
Recently a post in the SOTA forum pointed me towards 3D designs for a End-Fed Random Wire antenna which can be deployed speedily, due to the antenna wire being wrapped around a neat spool. Links to the design files can be found at the end of this post. Here are some pictures from the build process:
After printing I’ve inserted the BNC connector and attached antenna wire to it. The antenna wire is 24 AWG (0.25 mm²) highly flexible silicon insulated litz wire.
Unfortunately I got the position of the BNC connecter wrong on my first attempt. It had to be unsoldered, flipped and resoldered. The picture below shows the connector’s correct mounting position.
Next I tentatively arranged the complete design by putting all components together and check for any problems.
No problems found I proceeded by inserting three mounting screws (M2.5, 20 mm, countersunk head screw) connecting all parts of the spool. Beware that the 20 mm is slightly too long for the 3D print, either use shorter screws or a 2.1 mm drill bit for extending the drill holes. Winding up the antenna is a breeze.
A s-carabine at the end of the antenna wire allows easy and quick attachment to the top of an extensible GFK mast.
A few lengths on the length of a „random wire“ antenna. Turns out, the length of a EFRW can not be totally random. It’s advisable that the antenna is NOT resonant on any of the frequencies you intend to use it. Mike, AB3AP, has some more details on his website, including a small Python script for EFRW antenna design. You can get it here:
cd /tmp
wget https://udel.edu/~mm/ham/randomWire/endfed.py
chmod +x endfed.pyBy providing the desired ham radio bands you want to use the antenna with you get a graphical chart which antenna lengths are save to use. If you want use it for 40 and 20 m (my staple bands for SOTA activations) everything up to roughly 19 m is find.
./endfed.py -m 40 20
Here’s the result if you want to use your antenna for 40, 20 and 15 m.
./endfed.py -m 40 20 15
And here’s the result if you want to use your antenna for 40, 20, 15 and 10 m.
./endfed.py -m 40 20 15 10
I’ve tested the antenna during a recent SOTA activation of Ruine Ruttenstein (OE/OO-438). The antenna was deployed within a very short time-span (less than 5 minutes) and I got enough QSOs for a valid activation. That being said, the signal reports were 4-5 S-levels below the ones I’ve come to expect from my inverted-vee dipole. However, drawing conclusions from a single antenna outing would be premature, I’ll keep deploying the antenna and gathering more experience with it.
Resources
In order to print the spool yourself you can download the STL files for the antenna spool from Printables. This should be your primary source, however, in case of the site going down or the content no longer being available I am storing a copy on this site:
digitenna-stl-files.zip (153 downloads )
If you do not want to go the DIY-route you can purchase a fully assembled „Digitenna“ here.
In case the original website of Mike, AB3AP about the design of an endfed random wire (EFRW) is going offline I am storing a copy of his endfed.py EFRW design tool here.