Antenna Design and Selection

The antenna is one of the most critical components of any radio system. This guide covers antenna selection, design considerations, and optimization for the Aegis-Beacon.

Stock Antenna

The BOM includes a 433 MHz rubber duck antenna with the following specifications:

Parameter	Value
Type	Helical rubber duck
Frequency	433 MHz center
Gain	~2 dBi
Impedance	50 ohm
Connector	SMA male
Length	~17 cm

This antenna provides adequate performance for most scenarios and is compact enough for portable use.

Alternative Antenna Options

Quarter-Wave Vertical

The simplest effective antenna for the Aegis-Beacon:

• Length: ~17.3 cm at 433 MHz (225 / frequency in MHz = quarter wavelength in meters)
• Gain: ~2 dBi
• Pattern: Omnidirectional in horizontal plane
• Best for: General-purpose use, beacon deployment

J-Pole Antenna

A J-pole provides slightly better gain without requiring a ground plane:

• Gain: ~3 dBi
• Advantage: No ground plane needed
• Construction: Two pieces of 300-ohm twin-lead or copper tubing
• Best for: Fixed installations, base stations

Yagi-Uda Antenna

For定向 search operations, a Yagi provides significant gain:

• Gain: 7-10 dBi (3-4 element)
• Beamwidth: 40-60 degrees
• Best for: Directional search, finding beacons at maximum range
• Construction: Aluminum rod elements on a boom

Dipole Antenna

A half-wave dipole is the reference antenna:

• Length: ~34.6 cm total (2 x 17.3 cm elements)
• Gain: 2.15 dBi
• Pattern: Figure-8 in the E-plane, omnidirectional in the H-plane
• Best for: Testing, benchmarking, permanent installations

Antenna Tuning

SWR Measurement

Standing Wave Ratio (SWR) measures how well the antenna is matched to the transmitter:

SWR	Quality	Action
1.0 - 1.5:1	Excellent	No adjustment needed
1.5 - 2.0:1	Good	Acceptable for operation
2.0 - 3.0:1	Fair	Trim or extend antenna elements
> 3.0:1	Poor	Significant mismatch, redesign needed

Field Tuning Procedure

Without an SWR meter, you can approximate tuning:

1. Set the device to Beacon mode at your target frequency.
2. Measure the DC current draw during transmission.
3. A properly tuned antenna will show the lowest current draw.
4. If current is higher than expected, the antenna is likely mistuned.
5. Make small adjustments to element length and retest.

Connector Types

Connector	Typical Use	Notes
SMA	Stock antenna, most modules	Thread-on, compact
RP-SMA	Some WiFi antennas	Reverse pin — not compatible with SMA
BNC	Quick-connect applications	Bayonet mount, faster to connect
N-type	High-power base stations	Larger, weather-resistant
SMA female	Panel mount	For enclosure bulkhead connectors

Coaxial Cable

For remote antenna mounting, use low-loss coaxial cable:

Cable Type	Loss at 433 MHz	Recommendation
RG-174	0.4 dB/m	Acceptable up to 1m
RG-58	0.17 dB/m	Good for 1-3m runs
LMR-200	0.13 dB/m	Best for 1-5m runs
LMR-400	0.04 dB/m	For permanent installations >5m

Weatherproofing

For outdoor installations:

• Use self-amalgamating tape around all connectors.
• Apply silicone sealant at the cable entry point.
• Use a drip loop to prevent water running down the cable into the connector.
• Consider a weatherproof enclosure (IP65 rated minimum).

Ground Plane

The ground plane affects antenna performance significantly:

• Rubber duck antennas have an internal ground plane (the operator’s body or the PCB).
• External antennas mounted on a pole need an artificial ground plane.
• A metal sheet or radial wires (4 x quarter-wavelength) below the antenna provides an effective ground plane.
• For the Aegis-Beacon, the PCB ground plane is adequate when using the stock antenna directly on the device.