What is the minimum height for the center and ends of an inverted V antenna for 7135 kHz?
The height of your inverted V antenna is crucial because it significantly affects the antenna’s radiation pattern (where the signal goes) and its feedpoint impedance (which affects SWR).
Your frequency of 7135 kHz is in the 40-meter band. The wavelength (λ) is approximately:
λ = 300/f(MHz) = 300/7.135 ~ 42 meters
Therefore, one-half wavelength (λ/2) is about 21 meters. This is the ideal height for the center, but rarely practical for the 40-meter band.
Here is a breakdown of the suggested heights for the center and ends, along with the trade-offs:
1. Inverted V Center Height (Apex)
The center of the antenna, where the coax feeds into the wire, should be as high as you can reasonably get it.
| Height Factor (Fraction of λ) | Height in Meters (for 7135 kHz) | Primary Effect on Performance |
| Ideal (λ/2) | ~ 21 meters (69 ft) | Excellent DX (long-distance) performance with a low take-off angle. Very difficult to achieve in most settings. |
| Good (λ/4) | ~ 10.5 meters (34 ft) | Good all-around performance. Lower angles for DX are improved compared to lower heights. |
| Minimum (Practical) | 6 to 8 meters (20 to 26 ft) | Acceptable. Tends to favor NVIS (Near Vertical Incidence Skywave) for local/regional contacts (up to about 500 km). This is a common height for 40-meter inverted V installations. |
2. Inverted V End Height
The height of the ends and the resulting angle are a major advantage of the inverted V, allowing it to fit into smaller spaces.
- Minimum End Height: The ends must be kept out of reach of people and pets, especially since they are energized with RF power. A minimum of 2 to 3 meters (6 to 10 feet) above ground is a common safety and practical minimum.
- Optimum Apex Angle: The angle between the two legs at the center (the apex angle) is a key factor.
- An angle of 90° to 120° is generally recommended.
- A wider angle (closer to a straight dipole) provides a slightly more directional pattern and higher impedance (closer to 75Ω).
- A sharper angle (closer to 90°) provides a more omnidirectional pattern and a lower feedpoint impedance, often closer to 50Ω, which is ideal for matching standard 50Ω coaxial cable without a complex matching network.
In summary for your 7135 kHz antenna:
- Center Apex: Aim for 10.5 meters or higher for best DX, but 6 to 8 meters will still be very effective for regional communication.
- Ends: Anchor them 2 to meters off the ground, ensuring the total distance between the ends is manageable for your space while maintaining an apex angle of 90° to 120°.