i'm using FM tranmitter V2.0 (70-108MHz) and tea5767 fm receiver.The rough ballpark for a "long wire:" antenna is 1 wavelength optimal. Which is usually too long for many installations so the minimum is usually 1/4 wavelength. The trick though as Albert mentioned is matching the impedance between the RCVR/XMTR and the antenna with a device called an antenna tuner. Some "store-bought" antennas have this incorporated into the antenna. Then there are external tuners and some radios have them built-in. With a proper antenna tuner, I can impedance match an aluminum lawn chair as an antenna but it's effective receive/transmit propagation would be horrible. What frequency are you wanting an antenna for?
The transmitter uses 3.3 V it a short range transmitter, Any single wire can act as Antenna.Pretty broadband. 70Mhz ~14 feet to 108Mhz ~9 feet for full wave. What is your XMTR power? Is this a stereo low watt unit? They usually come with a telescoping whip antenna. For high watt, they get up to around 1K$ for a directional yagi. What are you using this for, what watt out and what range? eBay has a good used Harris military surplus antenna for ~$40. What happens is for space-saving and portability they start using coiled wire in fiberglass to get the long-wavelength in a compact antenna to save space.
Not as an effective antenna. A proper antenna will probably cost more than the XMTR. There are some short, wire-wound antennas such as this https://www.aliexpress.com/item/400...chweb0_0,searchweb201602_7,searchweb201603_60Any single wire can act as Antenna