Tested small antenna that came with Heltec Wireless Tracker with a LiteVNA.

Short black plastic rod. SMA male.

It has a peak at around 985MHz. At 868MHz it is pretty much useless.

TI design note: https://www.ti.com/lit/an/swra228c/swra228c.pdf

LoRa Fiberglass Antenna, 858-878MHz, 8dBi, 1300mm £59.00 https://uk.robotshop.com/products/lora-fiberglass-antenna-858-878mhz-8dbi-1300mm

?wire dipole: https://www.banggood.com/Original-FrSky-868MHz-Dipole-T-Antenna-IPEXI-for-R9-Slim-or-Slim+-Receiver-LBT-Version-RC-Drone-p-1380277.html?imageAb=2&rmmds=search&cur_warehouse=CN&akmClientCountry=RO