Build an APRS Antenna
APRS — Automatic Packet Reporting System — operates on 144.390 MHz in North America (144.800 MHz in most of Europe) and uses FM voice radio infrastructure to carry GPS position packets, messages, weather data, and telemetry across a nationwide network of digipeaters and iGates. The antenna for an APRS station has specific requirements that differ from a voice radio station: it must be omnidirectional (you never know what direction the next packet will come from), vertically polarised (standard for VHF FM), optimised for low elevation angles (packets arrive from vehicles and portable stations at or near ground level), and mounted as high as practical to maximise the station's coverage footprint. This guide covers three APRS antenna options — a quarter-wave ground plane for the simplest build, a Slim Jim for better gain, and a collinear for an iGate or digipeater installation — with specific dimensions for 144.390 MHz, installation guidance, and coax routing for a permanent APRS station.
What APRS Needs From an Antenna
An APRS station antenna has different priorities from a voice radio station. Understanding these differences helps choose the right antenna and installation for your specific role — mobile tracker, fixed iGate, or digipeater:
Choosing Between Ground Plane, Slim Jim, and Collinear
Three antenna types dominate APRS installations, each with a different cost/complexity/performance trade-off:
iGate vs Digipeater — Antenna Implications
APRS stations serve different functions in the network, and the function affects antenna choice:
- Fixed tracker (home station): transmits position beacons at intervals and receives acknowledgements. An iGate function is optional but useful. A Slim Jim or 5/8-wave ground plane at 25–40 ft is fully adequate — you need to reach nearby digipeaters, not the entire region.
- iGate: receives APRS packets and uploads them to the APRS-IS internet server. An iGate only receives (or also transmits with a full-duplex iGate). Coverage footprint directly determines how many mobiles the iGate serves — a collinear on a high mast serves the most stations. An RTL-SDR SDR dongle iGate needs good antenna sensitivity; a collinear or Slim Jim is the minimum standard.
- Digipeater: receives packets and re-transmits them at higher power to extend coverage. A digipeater needs both transmit and receive coverage. A high collinear at maximum height is the correct antenna — more height means more coverage from both directions. Digipeaters on hilltops with collinear antennas form the backbone of the APRS network.
- Mobile tracker: on a vehicle, a 5/8-wave magmount or a roof-mounted quarter-wave with radials is standard. Mobile stations need wide radiation angle — height is provided by terrain avoidance as the vehicle moves.
Coax and Feedline for APRS
At 144 MHz, feedline loss is measurable but not as critical as at 432 MHz or 1296 MHz. However, for an iGate or digipeater running continuously, coax quality matters for both performance and long-term reliability:
| Antenna type | Element length | Total height | Gain | Construction material | Notes |
|---|---|---|---|---|---|
| Quarter-wave ground plane | 19.3 in (49.0 cm) vertical + 4× 19.3 in radials | ~24 in | 0 dBd | 1/4-inch copper pipe or #14 AWG wire | Radials angled 45° downward for ~50 Ω match |
| 5/8-wave ground plane | 32.2 in (81.8 cm) vertical + 4× 19.3 in radials | ~38 in | ~3 dBd | 1/4-inch aluminium or copper rod | Requires matching coil at base; best standalone APRS antenna |
| Slim Jim (144.390 MHz) | Half-wave: 37.6 in + Quarter-wave stub: 18.8 in | ~57 in (4.75 ft) | ~3 dBd | 300 Ω twin-lead | VF = 0.82; tap position at ~1.5 in from shorted end |
| 2-element collinear | 2× 5/8-wave sections with phasing coil | ~7 ft | ~5–6 dBd | Coax construction or aluminium rod | Best for iGate and digipeater; see collinear guide |
| Copper Cactus (3× collinear) | 3× phased elements | ~10 ft | ~7–8 dBd | 3/4-inch copper pipe | High gain; see Copper Cactus guide for full details |
For 144.800 MHz (European APRS): all dimensions scale by factor 144.390/144.800 = 0.997 — less than 0.5% shorter. The same antenna dimensions work on both frequencies without modification.
Materials for a 144.390 MHz APRS Slim Jim — recommended as the best single-band APRS homebrew antenna
Building the 144.390 MHz APRS Slim Jim
The APRS Slim Jim uses the same construction procedure as described in the 2m Slim Jim guide, scaled precisely for 144.390 MHz. The procedure is summarised here with APRS-specific notes. For the full Slim Jim theory and detailed step-by-step, see the 2m Slim Jim build guide.
Cut Twin-Lead to APRS Dimensions
For 144.390 MHz with 300 Ω twin-lead at VF 0.82, the Slim Jim dimensions are:
Solder Short Circuit and Find Tap Position
Solder the short circuit at the bottom of the twin-lead (both conductors joined). Start with the coax tap at 1.5 inches from the short circuit. Connect the NanoVNA and sweep 143–146 MHz. Adjust the tap position in 5mm steps until the SWR minimum falls at 144.390 MHz specifically — not at 146 MHz as a standard 2m Slim Jim would be tuned. The SWR at 144.390 MHz should reach 1.1:1 to 1.5:1 at the optimum tap position.
Install in PVC and Mount for Maximum Height
House the tuned Slim Jim in the 3/4-inch or 1-inch PVC pipe. As noted in the 70cm Slim Jim guide, PVC shifts resonance slightly — test inside the unsealed housing before cementing the end caps. For APRS, the installation height is the most important performance factor after the antenna build itself. Mount the antenna at the highest practical point:
Connect to APRS Equipment and Verify
Connect the antenna to the APRS station equipment — TNC, radio, or SDR — via the coax feedline. Verify packet reception by monitoring aprs.fi for packets appearing from your station's location. For an iGate, check the APRS-IS connection and verify packets are uploading correctly. For a digipeater, verify the station appears in the digipeater network list:
Building a Complete APRS iGate Station
The antenna is just one component of an APRS iGate. A complete station for contributing to the APRS network consists of several elements working together:
- Antenna: the Slim Jim or collinear described in this guide. Mount at maximum height.
- Coax: RG-8X for runs under 50 ft; LMR-400 for longer runs. Keep as short as practical.
- Radio or SDR: a standard 2m FM transceiver (any brand) or an RTL-SDR dongle. The RTL-SDR approach requires a Raspberry Pi running Direwolf software for packet decoding and iGate functions. Commercial TNC-equipped radios (Kenwood TM-D710, TM-D72) have built-in APRS functionality requiring no separate TNC or SDR.
- TNC (Terminal Node Controller): decodes AX.25 APRS packets from the FM demodulated audio. Software TNCs (Direwolf on a Raspberry Pi) are now more common than hardware TNCs and are free. Direwolf + Raspberry Pi + RTL-SDR is the most popular modern APRS iGate configuration.
- Internet connection: the iGate software (Direwolf, APRX, or others) connects to APRS-IS servers and uploads received packets. A standard home broadband connection is sufficient.
- Power reliability: a continuously operating iGate benefits from a UPS (uninterruptible power supply) to keep it running during power outages — especially useful in emergency communication contexts where APRS nets are most needed.
Optimising for Receive vs Transmit
An iGate's primary function is receive — hearing as many APRS stations as possible and uploading packets to APRS-IS. A digipeater must both receive and re-transmit effectively. These different roles affect antenna system priorities:
- Receive-only iGate: antenna gain benefits receive equally to transmit — a Slim Jim hears 3 dBd better than a ground plane. Additionally, a low-noise amplifier (LNA) at the antenna dramatically improves weak packet reception for an SDR iGate. The NooElec or similar RTL-SDR preamps at the antenna input add 15–20 dB gain with low noise figure, enabling the iGate to decode packets from stations that would otherwise be below the noise floor.
- Full-duplex iGate (transmit and receive): the same antenna is used for both. The LNA must be bypassed or protected during transmit — this requires a TR switching relay or a circulator. Most small iGate stations using low power (1–5W) omit the LNA and accept slightly reduced weak-signal receive performance in exchange for simpler system design.
- Digipeater: runs at 5–25W typically. The antenna must handle continuous carrier at this power level — all antennas described in this guide do so without concern. The gain of the antenna benefits both receive sensitivity and transmit effective radiated power equally, making the collinear the preferred digipeater antenna.
- Coverage vs selectivity: APRS iGates benefit from wide receive bandwidth (the Slim Jim and ground plane have flat SWR across the entire 2m band) but do not benefit from narrow-band selectivity. Avoid adding bandpass filters between antenna and receiver unless there is a specific strong interferer — filters add insertion loss that reduces sensitivity.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| iGate hearing very few stations — under 5 per hour | Antenna too low; high coax loss; SDR gain too low; or software configuration problem | Check aprs.fi for nearby iGates and compare their heard counts; check coax connector weatherproofing; adjust SDR gain | Raise antenna if possible; replace RG-58 with LMR-400; optimise SDR gain to 35–45 dB; verify Direwolf is decoding and uploading |
| Packets heard but not uploading to APRS-IS | Internet connection issue or iGate software configuration | Check Direwolf or APRX log for connection errors to APRS-IS server | Verify internet connection; check APRS-IS server address and port in software config; verify callsign and passcode are correct in iGate software |
| High SWR — radio reports fault or foldback | Coax connector corroded or water in coax; antenna connection failed | Inspect all outdoor coax connectors for green corrosion; measure SWR from radio end | Replace corroded connectors; re-weatherproof all outdoor connections; verify antenna is still mechanically intact |
| Station beacons not appearing on aprs.fi | Not reaching any digipeater; or APRS path set incorrectly in TNC/radio | Check APRS PATH setting — should be WIDE1-1,WIDE2-1 for standard home operation | Verify APRS PATH in TNC software; increase transmit power if signal is not reaching nearby digipeater; check aprs.fi for nearest digipeater distance and bearing |
| RTL-SDR iGate decoding rate very low — many packets missed | SDR overloading from strong local signal; or gain set too high | Listen to 144.390 MHz audio from SDR — distorted audio indicates overload; reduce gain 5 dB at a time | Reduce SDR gain; add attenuator if strong FM broadcast or other signal near 144.390 MHz is causing overload; add 2m bandpass filter before SDR |
| iGate works in summer, poor performance in winter | Water in coax or connectors due to thermal cycling; ice on antenna | Inspect connectors after first freeze/thaw; measure SWR | Re-weatherproof all outdoor connections before winter; use PL-259 caps on unused connectors; fill PVC housing with foam to prevent condensation |
What is the difference between an iGate and a digipeater?
A digipeater (digital repeater) receives APRS packets and re-transmits them on the same frequency at higher power, extending the range of the packet across the RF network. It does not require an internet connection. An iGate (internet gateway) receives APRS packets and uploads them to the APRS-IS internet server, making them visible on aprs.fi and connecting the RF network to the global APRS database. Most modern installations combine both functions — a station that acts as both an iGate and a digipeater provides the most complete service to the APRS network in its coverage area.
Can I use the same antenna for APRS and voice?
Yes — any 2m antenna tuned to the 2m band works for both APRS (144.390 MHz) and voice (146–148 MHz) simultaneously. A Slim Jim tuned to 144.390 MHz has SWR under 2:1 across the full 2m allocation, so it works equally well for both purposes. Many operators connect both their APRS TNC/radio and their voice radio to the same antenna via a coax splitter — this works at low power levels but reduces sensitivity on both receivers slightly. A better approach is a separate antenna for APRS (dedicated iGate antenna) and a separate antenna for voice, each optimised for its frequency segment.
How much gain do I need for a good APRS iGate?
Height matters more than gain, but gain is worth having once the antenna is at a good height. A ground plane (0 dBd) at 50 ft reaches the same area as a Slim Jim (3 dBd) at 25 ft. If you can only mount the antenna at 25 ft, use a Slim Jim or collinear to compensate with antenna gain. If you can mount at 50 ft or higher, a ground plane is adequate and a Slim Jim or collinear makes your coverage noticeably better. For a dedicated digipeater on a high tower or hilltop, a 2-element collinear or 2m Copper Cactus is the standard choice — 5–7 dBd gain from a prominent location produces coverage of 20–40 miles in flat terrain.
What is the best low-cost APRS iGate setup?
The most popular low-cost APRS iGate configuration is: RTL-SDR dongle (~$25) + Raspberry Pi (~$35–50) + Direwolf software (free) + homebrew Slim Jim antenna (~$10) + RG-8X coax. Total cost: approximately $80–100 for a complete permanently operating iGate. The RTL-SDR provides the radio receiver; Direwolf decodes packets and connects to APRS-IS. This combination runs continuously on about 5W of power from the Raspberry Pi, making it very inexpensive to operate. For transmit capability (full iGate or digipeater), add a 2m FM transceiver connected to the Raspberry Pi via its audio interface and a PTT control line — many operators use a Baofeng or similar inexpensive radio for this purpose.
Should I use a high-gain antenna or just go higher?
Both — but height first if you must choose. The effect of antenna height on coverage area is larger than the effect of antenna gain, especially at low heights where every additional foot of elevation angle reduction matters significantly. At heights above 50 feet, antenna gain starts to become more important relative to additional height increments. The practical advice: get the antenna as high as your situation allows (rooftop, mast, existing tower), then choose the best-performing antenna you can build or buy for that mounting point. Never sacrifice height to use a larger antenna structure that requires ground-level mounting.
Does antenna polarisation matter for APRS?
Yes — always use a vertically polarised antenna for APRS. All APRS mobile stations use vertical polarisation (standard for FM), all digipeaters and iGates use vertical polarisation, and the entire infrastructure is designed around this standard. A horizontally polarised APRS antenna would suffer 20 dB of polarisation mismatch loss against every station it communicates with — effectively making it appear 100× less powerful and 100× less sensitive. There is no scenario in fixed APRS operation where horizontal polarisation is appropriate. The Slim Jim, ground plane, collinear, and all other antennas described in this guide are vertically polarised.