I got bitten by a new bug, looking at a comms system called Meshtastic. Think of it like an open source pager using a mesh network of LoRa devices. Ordered my ESP32's expecting a rough spiral antenna with heatshrink on it. What arrived was a stubby antenna of much nicer quality with an SMA connector, meaning I can try different antenna. Soooo, of course I am going to test this little stubby antenna, I rebuilt my electronics lab over Covid thinking about keeping my hand in and also being able to service kit we already have. Yeah, also playing about ranks pretty highly too.
The stubby was listed as being 868Mhz suitable, well my VNA test would disagree. Anyone familiar with CB should know getting the SWR low is the aim of the game, around 1:1 being ideal, 1:3 not great but workable. The VSWR here is bad, 1:15.21 .... WTF....
Yep, looks like they grabbed these off the shelf regardless of frequency. So I scratch my head for a bit and think, been playing winding my own copper inductors to play with. My thoughts drift to magnetic loops where a small inductive loop resonates a larger loop. Could I transpose that ? After several attempts with different coils I settled on double helical coil, first thought was that the inner would cancel the outer, but since I hadn't tried it, give it a go.
After juggling the coil position, on the plastic toothpick I found the sweet spot VSWR now 1:2.52 That's a hell of a lot better than where I started. The stubby is now exciting my little coil that is now acting as the antenna, it also provides a better impedance match to the stubby making it appear more like 50-ish ohms.
Proof is in the pudding, a great feature of these NanoVNA's is that you can also use them as a signal generator, you just need a spectrum analyser to view the result....
I was lucky getting some work in during Covid that gave me the opportunity to buy a spectrum analyser, yeah I have a TinySA and I use that to check what I hook up as it's much cheaper to blow up a TinySA than the Rigol. Set the telescopic antenna to the wavelength of 868Mhz, about 345mm and look at the RF signal. I saw an increase of 5dbm over not using the coupling coil, the coil has paper insulation to stop the coils shorting against the inner and outer windings.
Conclusion, it was great to play with my toys get an actual result, but by far I would be better making my own antenna's from scrap wire, they are hardly huge at these frequencies. Enough distraction, back to getting Meshtastic understood and working.
