VK4FUQ’s optimised sloper antenna for 3.5MHz. Radiator and counterpoise radial are both one-quarter-wave in length (length in metres = 71.37/(frequency in MHz) or in feet 234/MHz). The radial should be at least 5ft (preferably about 8ft) above ground and immediately underneath the radiator. Good quality insulators are need at the top of the preferably non-conductive mast and at the end of the radial.
Critical Dimensions for 3.5 MHz
Sloping Wire Length: 20.5 meters (67 feet) – start here and trim Mast Height: 12-15 meters minimum (as shown in your diagram) Radial Length: 10-20 meters each (quarter to half wavelength) Number of Radials: Minimum 4, preferably 8-16 for better efficiency
Feed Point Details
Location: At the base of the mast where sloping wire connects Impedance: Typically 35-50 ohms (needs matching) Matching: Use 2:1 or 3:1 balun, or tune with L-network
Performance Characteristics
- Radiation Pattern: Directional in direction of slope
- Low Angle Radiation: Excellent for DX
- Bandwidth: ~50-70 kHz at 2:1 SWR
- Gain: 2-3 dB over simple vertical
Tuning Procedure
- Install with calculated length
- Check SWR across 3.5-3.8 MHz band
- Trim wire if resonant frequency too low
- Add loading coil if wire gets too short
The configuration in your diagram is spot-on for 80m operation. The key to success will be a good ground system with those radials!