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.
Configuration fields[edit | edit source]
| 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 means resonant at the operating frequency: the length is derived automatically. |
| 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[edit | edit source]
| 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 typical values; actual performance varies with frequency, length, height and azimuth.
How the antenna affects the link[edit | edit source]
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[edit | edit source]
The Radio Deck shows the expected peak gain (dBi) and the VSWR at 50 ohms 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.