feedpoint

The feedpoint is the physical location where a transmission line connects to an antenna and delivers radio frequency (RF) energy into the radiating element. This is where the coaxial cable or waveguide from your transmitter terminates, making it the critical interface between the powered feedline and the unpowered antenna structure. Impedance matching at this junction determines how efficiently energy radiates rather than reflects back down the line.

The feedpoint location determines the antenna's radiation pattern and feed impedance. For a simple dipole antenna, the feedpoint sits at the electrical center, typically between the two arms of the radiator. For loop antennas, end-fed configurations, or phased arrays, the feedpoint may be offset or distributed across multiple locations. Moving the feedpoint along the radiating element shifts the impedance seen by the transmission line, changing how well it matches the standard 50 ohm or 75 ohm characteristic impedance of the feedline.

Impedance and matching

A poor impedance match at the feedpoint wastes power: RF energy that cannot be radiated reflects backward into the transmitter, creating heat in the cable and reducing efficiency. This reflection coefficient is expressed as return loss (measured in dB) or voltage standing wave ratio (VSWR). Typical design targets range from VSWR 1.5:1 to 2:1 across the intended operating bandwidth. Matching networks, stub tuners, or ballun transformers can be inserted at the feedpoint to bring mismatched antennas into acceptable range, though they consume some power and add complexity.

The mechanical structure around the feedpoint matters: mounting hardware, weather sealing, and nearby conductive surfaces all affect impedance and radiation performance. In array antennas and phased systems, multiple feedpoints must be fed in phase and with controlled amplitude ratios; phase errors at individual feedpoints corrupt the overall beam pattern and nulls. The feedpoint is also where corrosion, water ingress, and connector degradation occur most readily on outdoor installations, making it a common maintenance hotspot.

In microwave and satellite systems, the feedpoint may be a waveguide horn, a probe within a waveguide cavity, or a slot opening in a metallic structure. At lower frequencies (AM broadcast, shortwave), the feedpoint is typically a simple coaxial connector or a pair of heavy conductors bolted to the antenna base. The term applies equally to transmitting and receiving antennas: reception impedance matching at the feedpoint is just as critical as transmission matching for sensitivity and noise figure.