Passive antennas increase range
Inexpensive
Easy to add
SmartThings V3 hub, other hubs should be similar

I was unimpressed with the range and reliability of my SmartThings v3 hub and z-wave plus sensors so I experimented with attaching passive antennas on the sensors. Adding an 8" (5/8 wavelength US frequency) wire close to the internal antenna of the sensor increased the power by ~4x. Other countries use different z-wave frequencies so the antenna length will vary. See the sensor Intstructable below for information on how to calculate the antenna length and how to put antennas on the sensors.

https://www.instructables.com/id/Z-Wave-Antenna/

Next I experimented with adding passive antennas to the hub. At first an 8" wire was taped to the front of the hub near the internal antenna (picture 3), then a hole was drilled to get the wire closer (picture 2). I recommend taping to the front first and if you need more range try the hole method.

Without passive antennas on the sensors and hub I could not get consistent alerts from more than 20 feet away. With antennas on both I was having reliable alerts from about 100 feet outside my home. The hub was in the line of site through a window, walls and objects will affect the range.

Odd Behavior!

I encountered odd behavior during my testing, when the sensors are near the range limit it seems like they will stop trying to transmit until you get much closer. A sensor was tested successfully 10 times at ~100 feet, then went to 120 feet at which point the sensor stopped working. Went back to 100 feet and it still wouldn't work. I had to get really close to the hub before it would work again. Maybe they go into a timeout state for a while.

Step 1: Drill a BIG HOLE!!!

I could not get the hub apart so I ended up drilling a large hole under the rubber foot and placing an 8" (5/8 wavelength) wire next to the internal antenna. It is about 1/2" closer than where it was taped to the front of the hub. The internal antenna looks like a spring about the same diameter as a pencil. Use a stiff, insulated wire and do not strip the end. Use tweezers to hold about 1/4" of the end of the wire against one of the coils and hot melt glue it as in picture 3. I don't currently have any way of taking measurements with this antenna but my testing showed an increase in range. I'm trying to borrow the Hackrf One SDR so I can take measurements and post them. NOTE: Do not solder the wire to the internal antenna, modifying the antenna is illegal per the FCC. Passive antennas are allowed.

Disassembly Drama!

I unsuccessfully tried to take the hub apart. There are no screws under the label or the squarish rubber bottom grip. It looks like the bottom slides on, there's even an arrow showing which direction to slide. Mine would not slide, I even tried using a hammer and large flat bladed screw driver to force it to slide in both directions, it would not budge. I ended up using a large drill bit to make a hole that will be covered up by the rubber grip. Note the board is mounted upside down, you need to drill in from the bottom to have access to the antenna. The antenna is directly in front of the power plug, see picture 2.