I’ve actually had pretty good success with the fixed antenna that came with my Nooelec RTL-SDR once I trimmed and then magnetically attached it to a piece of metal bar stock. I’ve since built a cantenna which is working well, but today while I was cleaning up my workbench I disassembled the little antenna that came with the RTL-SDR and in the process I think I may have stumbled across what may be the easiest beginner antenna possible, which should give great results with very minimal effort. The antenna that came with my RTL-SDR is below.
Here is the exploded view of the parts:
Now, the only tricky part about just cutting this element to size, is that you have no idea where the coax is separated inside the unit without disassembly. I’ve measured mine, and it looks like 15mm would be the appropriate amount to deduct from the element length to obtain that magical 68-69mm element length. If you have a similar antenna, it doesn’t look like the manufacturer could vary more than a few mm from this unit to unit.
Step 1: Unscrew the element from the base, remove the rubber tip and then measure 53mm (68-15, because 15mm of the effective element is inside the antenna base), measure the 53mm again (knurled part of the antenna included (you always measure twice )) and then cut it with a heavy side cutter or hacksaw. Replace the rubber tip.
Now the problem for step 2 is what to do with that tiny included ground plane… Mine was attached with just a small dab of glue, and it appears to be mostly press fit into the plastic base. I think if you were more gingerly in your approach than my “side cutter brute force technique”, and just worked a guitar pick or small precision screwdriver around the base you could separate it easily. Once your inside, this is what you’ll see:
Not very impressive, they just wedged the loose ends of the coax ground in between the lip of the base and their tiny ground plane.
Step 2: Remove the ground plane and make sure no loose strands of copper are contacting the center coax.
Now, Step 3 is a little more open ended, but essentially we would take this trimmed unit and attach it to a proper ground plane. After some thought, I think the best method would be:
Step 3: Discard the ground plane cover sticker and then drill a small hole in the center of the ground plane, a matching hole in a small bean/soda/etc can (preferably trimmed to 68mm in height), and then screw or bolt the two togather. You could also just skip drilling all together and solder or weld the ground plane and can together (conductive JB weld might be a great solution for this). When your done you should have something that looks and functions like the popular “cantenna” design, only with the coax coming out of the original antenna base and not the bottom of the can.
Also, since the ground plane has a small magnet hastily stuck on it you could just try magnetically attaching it to various ground planes to experiment with what works best. Just make sure you get good electrical contact between the two pieces of metal (great use for your multimeter! ). With a little ingenuity you could also fabricate a 68mm/68mm 90 degree dual purpose mounting bracket/ground plane from of a piece of flat steel for a very clean looking antenna.
Happy antenna modding! - KB9OVP