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/4000103452164.html?spm=a2g0o.productlist.0.0.245527af7rkAlU&algo_pvid=95c0fac4-98da-42d8-a70e-3c44dab0e3ff&algo_expid=95c0fac4-98da-42d8-a70e-3c44dab0e3ff-50&btsid=13f84e68-dc20-4d99-914b-6f694f0f9fa3&ws_ab_test=searchweb0_0,searchweb201602_7,searchweb201603_60Any single wire can act as Antenna
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