Meshtastic

for the handheld portables like a tdeck stick to flexible whips. it's lesson you want to learn before breaking your first tdeck.

generally though these pocket devices are not stand alone solutions. they become useful in an established mesh network. If your area doesn't have an existing community for this stuff, you dont travel to places with established meshes or you're just getting started. plan on picking up at least one other small device. a heltec or rak with decent antenna would let you place it at a high point and experiment with what kind of range the tdeck gets.

I spent some time with a cheap spectrum analyzer going through different antenna. Most of the ones designed for carry radios were not very good, but there is a limit to how much you can do with something that small. The two that I've had the best luck with are a 17cm whip and a 4.5cm "glue stick" stubby.

The specific models are available from many different sources, but here is a link for each, to get you started.

The 17cm Whip is actually a surprisingly good antenna. Unfortunately, I still find it too awkward for everyday use. On the other hand, my wife has no problem with it on the radio in her purse. https://www.etsy.com/listing/1713089150/gizont-915mhz-17cm-whip-antenna-with?crt=1&sts=1

This 4.5cm "Glue Stick" Stubby antenna is the same one that Heltec ships with their kits. It's small enough to use anywhere. The performance is okay, which is actually pretty good for one that small. I got much worse results with a similar 5cm Stubby. https://muzi.works/products/stubby-antenna?_pos=2&_ss=r

Be aware that there are often big differences between antennas that appear to be identical. I found cases where the only obvious different was the branding printed on the outside, but where the SWR values weren't even close (1:1.2 vs 1:9 in one case).

Good luck!

I'm also not that experienced with Meshtastic, or radio communication in general, but I think you should consider another antenna for a few reasons.

1. The size is a bit small. At least it looks like it from the pictures. The description says 36 cm, but that's including the connection cable which you won't use on your T-Deck, right? It's probably incorrect to say that a longer antenna is always better, but I've solved issues with one of my Meshtastic devices by increasing the size of the antenna a few times.

2. It's made of fiberglass. There are flexible antennas which I think would fit better with the portable nature of your T-Deck. I couldn't find one on the website you linked, unfortunately, but just searching for "868MHz Antenna" on Amazon/Aliexpress should give you some results like this:
   

3. The price is quite steep. 45€ vs ~10€ for other options.

4. And least important point, it's white, but your T-Deck is dark gray I assume?