Antenna System
The antenna is the most influential part of a SimTX station. Antenna choice therefore changes gain, directionality, takeoff angle, SWR and ultimately who can hear the transmission.
Choose and tune an antenna
For a first setup, use the 31 m dipole example. For other frequencies:
- Set the operating frequency first.
- Select an antenna for the intended coverage. A dipole is a useful general comparison; a vertical favors low angles; a Yagi concentrates coverage in one direction.
- Set its main dimension for the new frequency, then check the local SWR estimate. For a dipole,
143 / frequency in MHzmetres is a starting length, not an exact solver result. - Adjust length in small steps if needed. Recheck after changing height or antenna type.
- For a directional antenna, set Azimuth toward the intended receiving area.
Length zero is a template shortcut, not an automatic tuner. In the current geometry solver it selects a nominal half wavelength for dipoles and inverted-Vs, a quarter wavelength for verticals, about 0.476 wavelengths for a Yagi driven element, one wavelength for a loop, and two wavelengths for longwires and Beverages. The EFHW template uses 32.5 m when zero is entered. Actual SWR still depends on the solved geometry and ground.
An antenna with a good SWR can still be weak at a distant receiver if it radiates in the wrong direction or the path is closed. See Propagation Basics.
Configuration fields
| Field | Meaning |
|---|---|
| Type | One of the built-in antenna types below, or a custom geometry via NEC File Import. |
| Wire Length (m) | The main dimension. Its label adapts to the type (circumference for the loop, element length for the vertical, driven element for the Yagis). 0 selects the nominal template length; see the type-specific rules above. It does not guarantee a resonant impedance or low SWR. |
| Height (m) | Height of the feedpoint or apex above ground. Height strongly shapes the vertical radiation pattern of horizontal antennas: low antennas radiate upward (short range), high antennas radiate at low angles (long range). |
| Azimuth (°) | Orientation, 0 is north, 90 is east. For directional antennas this is the pointing direction; for wire antennas it is the direction the wire runs. Disabled for the vertical. |
| Feedline Loss (dB) | Fixed loss of the cable between transmitter and antenna. Subtracted from the radiated signal in both cases. |
Built-in antenna types
| Type | Typical gain (dBi) | Directional | Typical takeoff angle | Default length (m) | Default feedline loss (dB) | Page |
|---|---|---|---|---|---|---|
| Dipole | 5.5 | No | 30° | 10.1 | 1.0 | Dipole Antenna |
| Inverted-V | 1.8 | No | 40° | 10.8 | 1.0 | Inverted-V Antenna |
| Vertical | 1.76 | No | 18° | 5.1 | 0.5 | Vertical Antenna |
| Yagi 3-element | 7.5 | Yes | 14° | 10.1 | 0.8 | Yagi Antenna |
| Yagi 5-element | 10.2 | Yes | 10° | 10.1 | 0.8 | Yagi Antenna |
| Full-wave Loop | 1.5 | No | 55° | 1.0 | 0.3 | Full-Wave Loop Antenna |
| Longwire | 4.0 | Yes | 20° | 21.0 | 1.2 | Longwire Antenna |
| Beverage | 8.0 | Yes | 8° | 80.0 | 2.0 | Beverage Antenna |
| EFHW + 49:1 unun | 2.1 | No | 28° | 32.5 | 0.6 | End-Fed Half-Wave Antenna |
The gain and takeoff figures are nominal type-reference values, not predictions for every setup. The current app solves the actual wire geometry; performance varies with frequency, length, height, azimuth, and ground. The Directional column distinguishes beam-like types, not whether every other antenna is equal in all directions.
How the antenna affects the link
What reaches a receiver depends on the antenna gain in the exact direction and elevation of the path, minus feedline loss, mismatch loss and any protection foldback, at both ends of the link. Consequences:
- Takeoff angle matters as much as gain. Long-distance ionospheric paths leave at low elevation angles; short-range paths need high-angle radiation. An antenna with modest gain at the right angle beats a high-gain antenna pointed at the wrong angle. See Propagation Basics.
- Directional antennas must be aimed. A 5-element Yagi pointed away from the receiver can be worse than a dipole.
- Polarization matters. Ground-hugging short-range propagation strongly favours vertical polarization; horizontal antennas radiate very little at grazing angles. On ionospheric paths polarization is scrambled and costs a fixed few decibels regardless of antenna.
- Height shapes the pattern. Raising a horizontal antenna lowers its main lobe. As a rule of thumb, half a wavelength of height gives a useful low-angle lobe.
Local estimate
The Radio Deck shows an SWR and power estimate for the current configuration before any session starts, recomputed whenever frequency, power or antenna fields change. Use it to sanity-check a design before transmitting.