Hi,
Lambda/2 is not a minimal requirement for a transmitter. You can use lambda/4 with ground radiants. The more important is the impedance adaptation between the output stage of the transmitter and the antenna.
If your transmitter has an output impedance of 50ohms, the antenna must have an impedance of 50ohms. If it is not the case you can connect an T or PI impedance adapter (also known as matcher) which will adapt transmitter to antenna. With this kind of material, you can connect any antenna to your transmitter.
This is done to avoid reflective energy not absorbed by the antenna to return to the transmitter and destroy the output stage, specialy when the output power is high.
In the receiver case, the length of the antenna is less critical, because the ernergies are too small. But if you try a lambda/4 without radiant and your compare with the same antenna by adding some radiants, you will note a best signal strength with the radiants. That's because the transfert of the received energy from the antenna to the receiver is better due to the impedance matching.
Regards
Papyaki