Feed Point Impedance vs. Feedpoint Height Above Ground for End-Fed Antennas
End-fed antennas are widely used in amateur radio due to their simplicity and effectiveness. However, the feed point impedance of these antennas can vary significantly based on feedpoint height above ground, affecting performance and matching efficiency. This article explores the impedance characteristics of three common end-fed configurations: the End-Fed Half-Wave (EFHW) with a 49:1 transformer, End-Fed Off-Center (EFOC) with a 4:1 transformer and End-Fed Random Wire (EFRW) with a 9:1 transformer.
End-Fed Half-Wave (EFHW) with a 49:1 Transformer
The EFHW antenna is designed to be a half-wavelength long at its lowest operating frequency. The feed point impedance of an EFHW typically ranges from 2000 to 5000 ohms, necessitating the use of a 49:1 autotransformer to bring the impedance down to approximately 50 ohms for standard coaxial feed lines.
Effect of Feedpoint Height Above Ground:
- At low heights (< 10 meters): The impedance tends to be lower than expected due to ground coupling and losses.
- At intermediate heights (10-15 meters): The impedance stabilizes around the expected range (~2000-4000 ohms), with radiation efficiency improving.
- At higher elevations (> 15 meters): The impedance is generally more stable, but minor variations occur due to ground reflection effects and the influence of the environment.
- Sloping vs. Vertical vs. Horizontal: A sloping EFHW can have slightly lower impedance than a horizontal one, while a vertical orientation tends to show more variability due to interactions with nearby objects.
End-Fed Off-Center (EFOC) with a 4:1 Transformer
An end-fed antenna designed for near-resonant operation (e.g., a wire length close to λ/2, 3λ/2, or 5λ/2) typically exhibits a feed point impedance in the range of 150 to 600 ohms. A 4:1 current balun (unun) is used to transform this down closer to 50 ohms.
Effect of Feedpoint Height Above Ground:
- At low heights (< 5 meters): The impedance can be lower than expected due to ground losses and capacitance effects.
- At optimal heights (5-10 meters): Impedance values stabilize within the expected range (150-500 ohms), allowing the 4:1 transformation to work efficiently.
- At higher heights (> 15 meters): The impedance remains stable, with minor variations based on frequency and surrounding objects.
End-Fed Random Wire (EFRW) with a 9:1 Transformer
A random wire antenna with a 9:1 unun is often used as a multi-band antenna requiring an antenna tuner. The impedance at the feed point is highly variable and depends on wire length, frequency, and feedpoint height above ground.
Effect of Feedpoint Height Above Ground:
- At very low heights (< 2 meters): The impedance fluctuates significantly, often dropping to 100-1000 ohms due to strong ground effects.
- At optimal heights (2-5 meters): Impedance varies between 500 and 5000 ohms, depending on frequency and wire length.
- At higher elevations (> 10 meters): The impedance variations become smoother, though still unpredictable across a wide frequency range.
- Counterpoise Considerations: A counterpoise helps stabilize impedance by reducing ground losses and capacitive coupling effects.
Summary Table: Feed Point Impedance vs. Feedpoint Height Above Ground
| Feedpoint Height (m) | EFHW (49:1) | EFRW (9:1) | EFOC (4:1) |
|---|---|---|---|
| 1-2 | - | 100-1000 | - |
| 2-5 | - | 500-5000 | 100-300 |
| 5-10 | - | - | 150-500 |
| 10-15 | 2000-4000 | - | 150-500 |
| 15+ | 3000-5000 | 1000-6000 | 200-600 |
Conclusion
The feedpoint height above ground significantly affects impedance. The EFHW (49:1) performs best when placed 10 meters or higher, while the random wire antenna (EFRW 9:1) is most effective at 2-5 meters. The near-resonant EFOC (4:1) works optimally at 5-10 meters.
Understanding these variations allows for better impedance matching and optimization of antenna performance. Whether using an EFHW, EFRW, or EFOC, selecting an appropriate feedpoint height above ground can improve efficiency and reduce losses in the feed system.
Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.