Most of
the writing on the Internet isn't about EFHW "End Fed Half
Wave" antennas, but about EFW "End Fed Wires".
If you really want to find an article about End Fed Half Wave antennas, this is the best place to find information:
A half
wave wire fed at the center is low impedance and has high
current at the center, at the ends where the current goes to
zero (very low current) , it becomes "voltage fed" or high
impedance.If you really want to find an article about End Fed Half Wave antennas, this is the best place to find information:
The End Fed Half Wave Antenna from Steve Yates - AA5TB
The radiation of any antenna is maximum at the current peak which occurs at 1/4 wave points - which is the center of a 1/2 wave dipole.
See the gif showing the standing waves on a half wave dipole, the RED is voltage - maximum occurs at the ends where current goes to ZERO because there is NO more conductor to travel, so current flow goes to ZERO. When current is zero, voltage by necessity is maximum.
Blue is the current, where there is maximum current that is where maximum radiation occurs. The center of the center fed dipole is at a high current point and a low voltage point. A high current point / low voltage point is low impedance. A high voltage, low current point (the opposite) is a high impedance. So the ends of the 1/2 wave wire are high impedance which is an End Fed Half Wave. The practical measurement of a half-wave end fed wire at a half wave is around 3,000 ohms at the center it will be about 70 ohms.
Half wave antennas will have larger bandwidth because the impedance doesn't change as rapidly as the low impedance at the center does.
Although resonance is important, for practical radio working, it's much more important for power to be accepted by the antenna, which means that the characteristic impedance of the feedline (or feeder) matches the impedance of the antenna feed point.
A half-wave antenna end fed (impedance = 3,000 ohms) by 50 ohm coax will have a VSWR of approximately 60:1.
A half-wave antenna center fed (impedance = 70 ohms) by 50 ohm coax will have a VSWR of approximately 1.4:1.
A half-wave antenna end fed (impedance = 3,000 ohms) by 50 ohm coax using a 49:1 transformer to elevate its impedance 2,450 ohms will have a VSWR of approximately 1.2:1.
Antenna books will have more information on transmission lines - high tension lines are high voltage and are best for long runs - high voltage = high impedance - which is why open wire feeders are lowest loss.
Shane Nelson, KD8UJM under the ebay seller name "NelsonAntennas" sells excellent quality transformers like this one https://www.ebay.com/itm/
