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

In the SimTX desktop app, antennas belong to a station. Add and edit them in the station editor: in the Stations tab, select the station and press Edit, then go to Antennas. See Stations.

For a first setup, use the 31 m dipole example. For other frequencies:

1. Set the operating frequency first.
2. 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.
3. Set its main dimension for the new frequency, then check the SWR with Check SWR at on the antenna card. For a dipole, 143 / frequency in MHz metres is a starting length, not an exact solver result.
4. Adjust length in small steps if needed. Recheck after changing height or antenna type.
5. For a directional antenna on a Fixed mount, set Points toward to the intended receiving area. On a rotator mount, turn it while you transmit; see Rotators and tracking.

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).
Feedline Loss (dB)	Fixed loss of the cable between transmitter and antenna. Subtracted from the radiated signal in both cases.
Mount	Fixed, Azimuth rotator or Az-El rotator. See Rotators and tracking.
Points toward (°)	On a Fixed mount: the orientation, 0 is north, 90 is east. For directional antennas this is the pointing direction. Shown only for directional types.
Parks at azimuth, Elevation (°)	On a rotator mount: where the antenna points when a session starts.

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.

The types Monopole, Turnstile, Crossed Yagi and Helix are meant for satellites and for ground stations with a rotator. See SimTX Guide/Satellites and SimTX Guide/Rotators and tracking.

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

Before any session starts, the antenna card in the station editor shows the SWR at a frequency you choose (Check SWR at), and the Station section of the Transmit tab shows the SWR around your transmit frequency. Use them to check a design before transmitting.

See also

• Antenna Calculations
• NEC File Import
• SWR and Transmitter Protection