While amateur radio operators today largely employ rotating array antennas such as the Yagi dipole array and the cubical quad antenna, some still use wire antennas, some of which can afford higher gain and lower receive noise characteristics. The diamond is constructed with the narrowest ends left open for the feed point on one end and a non-inductive resistive termination on the other. In my experience both non-resonant and resonant rhombics work quite well. There exist various ways to calculate the ideal tilt angle to choose relative to the radiation angle and gain desired for a given rhombic application. Generally the tilt angle is between 65 and 75 degrees.
Height is very important for rhombics, as it is for all antennas.
Basically, the radiation angle of the main beam of any antenna is related to the height above the ground and the ground characteristics. Rhombic antennas are generally fed with open feedlines of around 400 to 600 ohm impedance. In the real world things work fine without worry related to feedlines so long as you use a good antenna tuner. Use of baluns is probably unwise as this is likely to limit the useful frequency range of this intrinsically broadbanded antenna. This exponential feeder provides a smooth transition of impedance and acts as a sort of impedance transformer.