Hi there!

Most of the designs I have in mind will only need to transmit in-house, being the furthest transmission distance not longer than aprox 20 meters with up to 2 walls in between.

Therefore I was wondering if it is possible -and if any test has been drived- to use a custom-made wire, flexible antenna instead of the "bulky" plastic-cased one.
As most Z-Wave devices I've seen use this kind of short-wired antenna, I think that even not being ideal they will suffice for low-distance applications.

I've been digging around the internet, and from all I've read so far it seems soldering an aprox. 82.2mm (1/4 for 868 Mhz) cable to a SMA connector should do the trick.
The unsolved left questions I still have are:
- Which cable to use. I've read that 50 ohm coaxials such as RG-178 or RG-179 would be ok? Is that right?
- Do I need to use a 50 ohm terminator on the end of the cable?

Thanks in advance

Hello jrartamendi,

You do have a few options to reduce he antenna size:

1. Simple remove the plastic cover from the antenna you got with your Whisper Node. It'll revel a flexible antenna. It's not the smallest option but it might help if you wish to place everything inside an enclosure/box.




2. Attach the antenna wire directly into the SMA terminal. Make sure the wire doesn't touch the external part of the connector.



3. Solder the wire antenna directly to the RFM69 module pin*.



*if you decide to use the wire antenna directly into the PCB, you might wish to remove the SMA connector by cutting the connector legs first and unsoldering it. This process is easier than using a hot-air re-work station.




A 0.8mm would work fine, also the core of a coaxial cable should work as well. Regarding the length, 82.2mm is the correct for 868MHz.

Wow, thanks for the how-to!
I thought those antennas had some helicoidal section along the Paty, and that forced the plastic enclosure having that thickness.

I'll first try unmounting an existing one to test the Rangers, but I don't like it not having any isulation. Having it touching the SMA connector or the PCB itself could eventually damage it, wouldn't it?
In "production," could I use a non-shielded, with plastic enclosure, 0.75mm solid copper cable as used in electrical lines. Not 0.8mm but wouldn't it be close to 50 Ohm?

Lastly, and not least important. Wouldn't not terminating it with a resistor cause reflection and worsen the signal to noise ratio?
Is it enough to just solder a 50 ohm resistor at the end of the cable?

Thanks!

Hi again jrartamendi,

You just need to take care not to short the antenna signal with ground (or any other part of the PCB actually).

The 50 Ohms cable is fine, and to be honest you can try any piece of straight metal and it will work. You have some very short distance communication so any lost or other issue with the antenna won't have that much impact.

Regarding the resistor, I'm not an antenna designer but I never used (or heard) it. The tests we've done is always using a professional assembled antenna (as the one supplied) or the 1/4 length (which is just a wire as shown). Would the resistor be between the antenna and ground and why 50 Ohms?

(seems the mobile autocorrection played a trick on me, I meant range not Rangers )

I'll cover the antenna wire with isulating tape so there's no chance of mistakenly damaging the device.

I'm neither an antenna expert (obviously xD) but regarding the resistor/terminator I surely know it's needed on transmitting coaxial cables or the signal will bounce back affecting the quality.
I've read a lot of different pages these days and due to the previous fact and that the SNA connector needs a terminator when the antenna is not connected, I've probably mistakenly assumed the same should be applied to the antenna itself when it's connected.

Or maybe it's only needed on λ or λ/2 antennas, but I doubt it because, as you smartly pointed out, where would be the resistor be grounded!

Regarding the 50 ohm on the terminator, when transmitting on coaxial, the terminator resistance must match that of the wire to max out the reflection blocking, thus the 50 ohm I mentioned.

Happy to hear it's relatively easy to mod a custom antenna for my purposes, and thanks for helping me to learn a bit more and clearing up my misconceptions.
Waiting my FTDI-USB to arrive so I can start playing with my Whisper Nodes!

Regards