Passive antennas increase power and range
No disassembly or soldering necessary
Inexpensive
Easy to install

I’ve been experimenting with my Z-Wave Plus system to increase the range of my battery powered door/window sensors. I use them to monitor my rat/mouse traps and needed more distance. See my other instructable https://www.instructables.com/id/Wireless-Mouse-Tr...

If you don't want to read all the tech info below, try taping a straight 8" copper or piano wire to a sensor as close to the internal antenna (see the black pen marks in picture 1 & 2) as possible and see if it increases the range. Also tape an 8" wire to your Z-Wave hub near the internal antenna, for the Smartthings V3 hub tape it to the front on the right side as you are looking at the face of the hub, directly in line with the power connector. All passive antennas should be vertical. To test, trigger the sensor 10 times before and after adding the antennas then check history to see how many events are recorded.

Placing the wire close to the internal antenna is critical. Both my sensors were taken apart to find where the internal antennas are located, they are opposite the battery. The antenna in picture 1 looks like a small spring under the black mark. The antenna in picture 2 is almost a complete rectangle, marked by the black lines. Put the passive antenna directly over any of the black marks. By moving the passive antenna in the first picture over the internal antenna the sensor went from recording 0/10 to 10/10 successful open close events recorded in history from the same location.

My SmartThings Antenna Instructable, most info is repeated in the last step of this instructable.

https://www.instructables.com/id/SmartThings-Anten...

Testing Procedure

Passive antennas (thin copper wires) were attached to the sensors and tested for signal strength with a Hackrf One SDR (Software Defined Radio). Lengths of ¼, ⅝ and 1 wavelength were compared with no antenna. The ⅝ wavelength antenna performed the best.

Below are the results in dB. dB is a log scale so a 3dB increase in power is 2x the power. For example if you go from -50dB to -47dB the power has increased by 3dB or doubled.

Power Levels were measured at 916MHz US Z-Wave PLus frequency. For each measurement separated by a comma in the data below, the door sensor was triggered ~10 times and the peak of the 10 triggers was recorded.

916MHz Peak Power Test Results

5/8 wavelength passive antenna (Best Performance!)

-55.6db, -55.4db, -55.6db, -55.6db, -56.3db

1 wavelength (Longer is NOT better!)

-59.9dB, -59.4db, -59db

1/4 wavelength (Most internet info recommends 1/4 wavelength passive antennas, 5/8 is MUCH better!)

-64.7dB, -66.4db, -62.8db

No passive antenna (stock)

-71db, -68.5db, -69.1db, -67.4db

Note

Z-Wave and Z-Wave Plus use the same frequency. Z-Wave Plus is the newer version with longer range and better battery life so get Z-Wave Plus if you can.

These are the two sensors I'm using.

Monoprice Z-Wave Plus Door/Window Sensor, NO LOGO (This one had the best measured power gain with the 5/8 antenna) Update: The sensor was taken apart to locate the internal antenna, place the passive antenna on top of any of the black marks in photo 2.

https://www.monoprice.com/product?c_id=122&cp_id=1...

Monoprice Z-Wave Plus Door and Window Sensor, No Logo (Had a much lower power gain) Update: The sensor was taken apart to find where the internal antenna was located and showed a huge improvement in the amount of recorded test events (0/10 recorded events old location, 10/10 new location) by moving the antenna over the black dot in the 1st picture.)

https://www.monoprice.com/product?p_id=24259

Step 1: Making a Passive Antenna

Find your Z-wave frequency in the wikipedia list below. A lot of countries have 2 frequencies, if you are not sure what frequency your Z-wave device is on pick the average of the 2. Most documentation says that the US frequency is 908.42MHz, I measured both my sensors at the second frequency 916MHz.

Z-Wave frequency list https://en.wikipedia.org/wiki/Z-Wave

Take your frequency and put it in this calculator link below to get 1 wavelength and then multiply it by ⅝ to get 5/8 wavelength, this will be your antenna/wire length.

https://www.everythingrf.com/rf-calculators/freque...

Here is how to calculate length for the 2 US frequencies

Z-wave plus 908.42MHz : wavelength = 0.33001526m = 12.99inches : ⅝ wavelength = 8.11inches

Z-wave plus 916MHz : wavelength 0.32728434 = 12.88inches : ⅝ wavelength = 8.05inches

Cut a small gauge copper wire or piano wire to the 5/8 wavelength you calculate. You can get piano wire at the hardware store, get the thinnest they have.

Tape the wire to the side of the sensor and test!

Tip: For longer range, try pointing the front of your z-wave hub at the furthest sensor. I have Samsung Smartthings v3 hub, their website says the strongest reception is from the front.

Step 2: How the Measurements Were Made

The first picture is with the passive 5/8 wavelength antenna taped to the front, the second picture is with no antenna.

I used a Hackrf One because I could borrow it from a friend. It’s overkill for measuring Z-wave, there are cheaper SDR’s that should work though I’m not familiar with them. You need one that will measure frequencies in the z-wave range, ~850Mhz to ~950Mhz. If anyone figures out how to measure with a less expensive SDR please comment.

Measurement Equipment

Hackrf One ~$300 https://greatscottgadgets.com/hackrf/

https://www.seattletechnicalbooks.com/hackrf

I think this is the antenna, will update after I ask my friend. https://greatscottgadgets.com/hackrf/

Measurement Software

I used this free spectrum analyzer, ‘Pavsa hackrf spectrum analyzer’. It was easy to setup, was reliable and worked well. https://github.com/pavsa/hackrf-spectrum-analyzer

Measurement Settings

Set the spectrum analyzer to start just below and end just above your z-wave frequency, for me it was 915Mhz to 917Mhz. I played around with the number of samples and FFT Bin (Hz) until I got consistent measurements, 5000 Bin (Hz) and 65536 samples worked well. On the chart options tab choose ‘Waterfall enabled’, ‘Show peaks’ and ‘Persistent display’, and set ‘Persistence time’ to 60 seconds.

Step 3: Smartthings V3 Hub Passive Antenna

Try taping a 5/8 wavelength wire (8" for US Z-wave) to the front right side of your Smartthings V3 hub, the testing below shows that it extends the range.

I found a picture of the Smartthings V3 hub circuit board, see the link below. The Z-wave antenna is in front of the hub, directly forward of the power connector. The picture below is the bottom of the circuit board so the passive antenna needs to go on the right side. For other brands of hubs, try to find where the internal antenna is mounted and place the wire close as possible.

https://community.smartthings.com/t/should-i-wait-...

I couldn't find a way to force the Smartthings hub to send an outgoing signal for measuring with the SDR so I triggered my sensors 15 times each with no passive hub antenna and 15 times again with a passive hub antenna at ~40' from the hub. The history was checked to see how many events were recorded. My results with the passive hub antenna were near perfect.

Results without Passive Hub Antenna

Rectangular Sensor 11/15 (recorded events/total times sensor was triggered)

Rounded Sensor 13/15

Results with Passive Hub Antenna

Rectangular Sensor 15/15

Rounded Sensor 14/15

Step 4: Smartthings V3 Passive Antenna Experiment

With the sensors I found that placing the passive antenna closer to the internal antenna increased the signal. Placing the passive antenna closer to the internal antenna of the hub should help increase the signal/range too. 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 (see picture 3). I don't currently have any way of taking measurements with this antenna. I'm trying to borrow the Hackrf One SDR so I can take measurements and post them.

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 3.