868 MHz antenna question

[kobuki]

I have this kind of "air coil" antenna running on several 433 MHz motes (see attachment). Is it possible to make a similar one for 868 MHz? It's not a simple case of halving the lenght and such things, it needs proper sizing and number of turns for a matching impedance, if nothing else. The one I use for 433 MHz in place of a simple wire monnopole practically produces the same RSSI values for me on a moteino using the same base, so I thought it might be useful to make one for the other important frequency if possible. Or as alternative, a simple spring antenna is easy to make - does someone have advices regarding any of these?

[captcha]

If you're after a really small antenna search this forum (or elsewhere) for a loop antenna.

Anything else seems to work with these radio modules but every design would be far from perfect because of the relative smallness of the system and it's unavoidable proximity to objects that will alter the antenna system's efficiency, impedance, radiation pattern etc..

The good thing is that these radios are pretty forgiving and you can do all sorts of things to the antenna and it all still seems to work; add a center loading coil, change its shape to the contours of your favourite country or just keep it a straight monopole.

In short, without a VNA (Vector Network Analyzer) your pretty much blind in testing the effectiveness of your design. RSSI is unfortunately not the whole story.

[kobuki]

I'm after something that doesn't need a matching network and is simple to make without antenna design and is small. I don't have a VNA (neither want to buy one for this). I've found something for 868 MHz, what do you think about these (image attached)? Though I will make one and try, regardless.

[captcha]

What I think is that you're probably wasting your time to get it perfectly right. Especially without any test equipment to confirm the actual workings of your design once you built it.

The image you posted looks beautiful but doesn't show a complete antenna system. The ground plane is missing, the top 'whip' of the antenna seems to be missing and you're not mentioning anything about feedline either.

The antenna from your first post shows insulated solid core copper wire. But do you know the dielectric constant of the material? The coil in your second antenna seems to only mention the diameter of the copper wire, but doesn't explicitly say that it is bare copper wire. For the inductance of the loading coil it probably doesn't matter too much but anything straight will be subject to velocity factor.

The easiest DIY antenna that does not require a matching network for these radios is a quarter-wave vertical with 4 ground radials bent downwards 45 degrees. That roughly gives you a feedpoint impedance of 50 ohms. You can then feed it with any length 50R characteristic impedance coax (RG-174 etc.).

I believe the Bowtie antenna is also 50ohms feedpoint impedance, but a lot more difficult to make (on PCB) and not small either.

If you want really small then just use a 50R carbon resistor. Not matching network required. Range is minimal but hey, it's small.. ;-)

[kobuki]

Oh, I think you misunderstood me a little. Though I didn't exactly specify what I want, either. I don't want something "perfectly right" but something that's more robust than a wire randomly folded in a small project box, to sum up:
 - as small as possible to fit in small project boxes wit the mote, batteries, etc.
 - not (or minimally) directional
 - tuned
 - simple to make
 - has a "decent" or predictable range

"Decent" because it's vague, but be largely comparable to the defaul monopole. The antenna with the ground plane radials you describe is huge for my outlined purposes, but would be probably a good choice as a base station antenna. It's not for a mass-market product, if I ever make one I'll hire a pro for designing the RF part.

The image is part of this page. It's bare copper as I gather. I think the difference is not great enough to mind the coating, though I might be mistaken.

[captcha]

Cool, thanks for the clarification. It definitely helps to hear what you're trying to accomplish.

The antennas in your link look like helicals (similar to what you'll find in a 'rubber ducky' antenna). If they are tuned for that frequency you might be all right but you'll still need a ground plane or counterpoise in order to make it work properly. In most cases the alternative ground plane as provided by the GND tracks on the Moteino circuit board will be your next best friend. Because of this the antenna system will be unbalanced and create RF all over your GND tracks but it seems that not many people are reporting issues with that.

If size is your biggest concern I would seriously look at PCB-based antenna designs. The Moteino with trace antenna seems like what you need.
https://lowpowerlab.com/shop/MoteinoANT

Or is this still too big?

[kobuki]

Thanks for the hints. The Moteino+trace antenna would be great, but I've a bunch of the normal Moteinos and want to use the antenna for other boards, too. The counterpoise in case of the simple wire monopole and the Moteino seems to be the GND plane/tracks of the board, and it works fine and has pretty good reception (unfolded straight wire). So as you noted, it will probably be fine. I'll report back with results later.