Build a Satellite Crossed Yagi Antenna
Amateur radio satellites in low Earth orbit pass overhead in 10–15 minute windows, transmitting with circular polarisation from an antenna tumbling in zero gravity. A ground station using a linearly polarised Yagi loses up to 3 dB every time the satellite's polarisation crosses 45° from the antenna's orientation — a loss that cycles multiple times per pass, causing annoying signal fading. The solution is a crossed Yagi: two identical Yagis mounted at 90° on the same boom, fed with a 90° phasing harness to produce circular polarisation. The result is a steady, fade-free signal throughout the entire satellite pass. This guide covers circular polarisation theory, construction of a 2m receive Yagi and 70cm transmit Yagi for Mode J (uplink 145 MHz, downlink 435 MHz) satellite operation, the phasing harness, azimuth-elevation mounting, and operating with amateur LEO satellites.
Why Satellite Signals Fade with Linear Antennas
Amateur satellites in LEO (Low Earth Orbit) tumble freely in space — their antennas point in continuously changing directions as the satellite rotates. The satellite's antenna polarisation relative to the ground station changes several times per pass:
How Crossed Yagis Produce Circular Polarisation
Two identical Yagis mounted at 90° to each other on the same boom, fed with equal power but with a 90° phase difference between them, produce circular polarisation. The two linear polarisations (horizontal and vertical) combine at 90° phase offset to trace a circle:
The 90° Phasing Harness
The phasing harness creates the 90° phase difference between the two Yagis. The most common approach uses a quarter-wave length of 75 Ω coax (or a specific length of 50 Ω coax chosen for the correct electrical length) as a delay line for one Yagi, combined with a power splitter for the other:
Mode J vs Mode V/U — Which Satellites to Work
Amateur satellites use different frequency combinations for uplink and downlink. Understanding the modes helps configure the antenna system correctly:
- Mode J (most common for FM LEO satellites): uplink on 2m (145.9 MHz), downlink on 70cm (435.300 MHz). The ground station transmits on 2m and receives on 70cm. Satellites: SO-50, AO-91, AO-92, various CubeSats. The 2m uplink is lower frequency and longer wavelength — more forgiving of pointing errors. The 70cm downlink is the critical receive path.
- Mode V/U (linear transponder satellites): uplink on 2m (145 MHz), downlink on 70cm (435 MHz) with a linear (SSB) transponder. Satellites: FO-29, AO-7, AO-73. Requires SSB capability. The crossed Yagi benefits these satellites the most because the signal from a linear transponder satellite is especially susceptible to polarisation fading.
- Mode U/V: uplink on 70cm, downlink on 2m. Less common for LEO. The 70cm Yagi transmits and 2m Yagi receives — opposite of Mode J. Requires knowing the satellite's mode before configuring the antenna.
- ISS and ARISS: the International Space Station uses 145.800 MHz downlink, 144.490 MHz uplink for voice. A 2m Yagi alone often suffices for high passes. For low passes, circular polarisation helps significantly.
| Band | Use | Design freq | Elements | Boom length | Gain (each Yagi) | Element diameter |
|---|---|---|---|---|---|---|
| 2m | Receive (Mode J) / Transmit (Mode U/V) | 145.9 MHz | 5 | 3.5 ft (1.07 m) | ~7 dBd | 3/16-inch aluminium rod |
| 2m | Same — more gain | 145.9 MHz | 7 | 6 ft (1.83 m) | ~9 dBd | 3/16-inch aluminium rod |
| 70cm | Transmit (Mode J) / Receive (Mode U/V) | 435.300 MHz | 7 | 2 ft (0.61 m) | ~9 dBd | 3/16-inch aluminium rod |
| 70cm | Same — more gain | 435.300 MHz | 11 | 3.5 ft (1.07 m) | ~12 dBd | 3/16-inch aluminium rod |
Satellite Crossed Yagi Calculator
Materials for a 2m + 70cm crossed Yagi system for LEO satellite operation — Mode J (uplink 2m, downlink 70cm)
Building the 2m + 70cm Crossed Yagi System
Build the two Yagis independently first, verify SWR on each, then assemble the crossed configuration and build the phasing harness. The Yagi construction procedure is identical to the 2m Yagi and 70cm Yagi guides — this guide focuses on the satellite-specific elements: satellite frequencies, crossed configuration, phasing harness, and azimuth-elevation mounting.
Build the 2m Yagi for 145.9 MHz
Build a 5-element Yagi for 145.9 MHz (the standard Mode J satellite uplink frequency, also used as the downlink reference for Mode U/V). Use the dimensions from the 2m Yagi build guide but adjust the design frequency to 145.9 MHz rather than 144 or 146 MHz. The element dimensions scale directly with frequency:
Build the 70cm Yagi for 435.300 MHz
Build two identical 7-element Yagis for 435.300 MHz (the standard Mode J satellite downlink frequency). Use the 70cm Yagi build guide dimensions adjusted for 435.300 MHz. The 70cm satellite Yagi requires the same construction precision as described in the 70cm Yagi guide — element lengths to ±2mm, positions to ±3mm:
Assemble the Crossed Configuration
Mount the two 2m Yagis at 90° to each other on the main boom — one with elements vertical, one with elements horizontal. Both booms point in the same direction (the satellite direction). The two booms are mounted perpendicular to each other using a cross-boom plate at the balance point:
Build and Connect the 90° Phasing Harness
The phasing harness creates the 90° phase difference between the two Yagis of each crossed pair. Use the λ/4 delay line method for simplicity, or a commercial 90° hybrid for precision. Build a separate phasing harness for the 2m pair and the 70cm pair:
Install on Azimuth-Elevation Mount
The crossed Yagi system requires an azimuth-elevation (az/el) rotator mount that can point the antenna at any position in the sky — from horizon to zenith. This is fundamentally different from a standard HF/VHF beam rotator that only rotates in azimuth. The az/el rotator combination allows the antenna to track a satellite as it crosses the sky from one horizon to the other:
Configure Tracking Software and Make First Satellite Contact
With the antenna system complete, configure the satellite tracking software. Gpredict is free, open-source, and excellent for amateur satellite operation. Load current Keplerian elements (TLE data) from NASA/NORAD, configure the rotator interface, and set up the radio control for Doppler correction:
FM Satellites — SO-50, AO-91, AO-92
FM satellites use a simple repeater — you transmit on the uplink frequency and the satellite re-transmits your signal on the downlink. Operating procedure is similar to a terrestrial FM repeater, but with specific challenges:
- Doppler shift: the downlink frequency changes by ±10 kHz during a 70cm pass. Most operators either use Gpredict auto-correction or manually tune the receiver slightly as the pass progresses. A radio with a Doppler correction interface (CAT control) makes this automatic and transparent.
- Access tone: SO-50 requires a 67 Hz CTCSS tone to open the repeater, plus a 2-second burst of 74.4 Hz to arm it before the pass. AO-91 and AO-92 open with 67.0 Hz CTCSS only. Check the current satellite guide for the access tone of each satellite.
- Contest etiquette: FM satellites are shared by all operators simultaneously. Keep contacts short — callsign, grid square, signal report. Avoid long transmissions that prevent others from accessing the repeater.
- Handheld operation: many operators make their first satellite contacts with a 5W handheld and a simple 2m/70cm yagi — the crossed Yagi system in this guide is the next level of capability after the handheld + portable Yagi stage.
Linear Transponder Satellites — SSB Operation
Linear transponder satellites (FO-29, AO-7, AO-73) use an inverting linear transponder — a 100 kHz segment of 2m uplink is translated to a 70cm downlink segment. SSB operation on these satellites is more technically demanding but more rewarding:
- Full duplex operation: you must hear your own signal on the downlink while transmitting on the uplink. This requires a dual-band radio capable of simultaneous transmit on 2m and receive on 70cm (IC-9700, FT-847, TS-2000).
- Inverted operation: most linear transponders invert the sideband — if you transmit USB on 2m, you receive LSB on 70cm. Check the satellite's documentation for the specific inversion mode.
- Doppler on both bands: both the uplink and downlink have independent Doppler shifts. You must tune the uplink to compensate for the uplink Doppler while also monitoring the downlink for the downlink Doppler. Tracking software handles this automatically with CAT control.
- Power discipline: linear transponder satellites are easily overloaded by strong signals. Use the minimum power needed to just hear your own signal in the transponder passband — typically 10–25W into a 9 dBd Yagi system is more than adequate.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Signal fading still occurs with circular polarisation | Phasing harness incorrect length; or one Yagi not resonant at satellite frequency | Verify each Yagi SWR independently at the satellite frequency; measure phasing harness physical length | Re-trim phasing harness to exact λ/4 length using VF-corrected calculation; re-tune Yagis to correct satellite frequency |
| Cannot access FM satellite — no response to uplink | Uplink Doppler uncorrected; wrong access tone; or power too low | Check that CTCSS tone is set correctly for the specific satellite; verify Doppler correction is active | Add correct CTCSS tone; enable Doppler correction in software; increase transmit power toward 25W; verify 2m Yagi is pointing correctly |
| Hearing satellite but signal choppy and fading — circular pol not working | Phasing harness wrong sense; or one Yagi's feedline has an unexpected length | Try reversing the phasing harness polarity — swap which Yagi has the delay line | Swap delay line to other Yagi (changes RHCP to LHCP); if signal improves, the original sense was wrong for this satellite |
| Rotator tracking not following satellite smoothly | Software/rotator interface communication problem; or TLE data is out of date | Command manual az/el position from software and verify rotator moves correctly; update TLE data | Verify GS-232 or hamlib interface settings (port, baud rate); update TLE from Celestrak for current satellite positions |
| SWR high on one Yagi but not the other in crossed pair | One Yagi built to slightly wrong dimensions; or interaction with the adjacent Yagi | Test each Yagi separately before crossing — SWR should match between pair; measure interaction when assembled | Check element lengths of high-SWR Yagi individually; increase physical separation between crossed Yagis to reduce coupling |
| Zenith gap — satellite passes directly overhead and is lost | Normal az/el system limitation — the azimuth rotator cannot track fast enough near zenith | This is a known limitation of az/el mounts; passes near 90° elevation lose tracking briefly | Accept as normal; pre-position antenna at 90° before the zenith crossing and let the satellite drift through; or use a third elevation axis (az/az mount) to avoid the gap |
Can I work satellites with a handheld and no Yagi?
Yes — many operators make their first FM satellite contacts with a 5W dual-band handheld and a simple tape measure Yagi or even just the rubber duck antenna on a high-elevation pass. The satellites are designed to be accessible with modest stations. The crossed Yagi system in this guide represents the next level of capability after the handheld stage — for operators who want to work more satellites, have better contact quality, and eventually try linear transponder satellites where antenna gain matters more.
Do I need full duplex capability?
For FM satellites: not strictly required — you can operate simplex (push to talk, listen for reply) since the satellite retransmits on a different frequency. However, full duplex is very helpful for confirming you are accessing the satellite and hearing your own signal. For linear transponder satellites: full duplex is mandatory — you must hear your own signal while transmitting to know you are in the transponder passband and adjust your uplink frequency correctly.
How important is the az/el rotator vs just hand-tracking?
Many experienced satellite operators begin with hand-tracking — manually pointing a handheld Yagi at the satellite by watching its predicted track on a phone app. This works surprisingly well for FM satellites where pointing accuracy of ±10–15° is sufficient. Computer-controlled az/el tracking is more important for the 70cm receive path (where the narrower beamwidth of the shorter-wavelength antenna demands more precise pointing) and for linear transponder satellites where you need both hands for the radio controls. For a serious fixed station, the G-5500 with computer control is the standard — but hand-tracking works for initial satellite operating.
Which FM satellite is best for a first contact?
SO-50 has been operational since 2002 and is one of the most reliable FM satellites for first contacts. AO-91 and AO-92 are also active and popular. Before a pass, verify the satellite is operational on the AMSAT status page (amsat.org) — satellites do occasionally go into safe mode or have solar panel issues that temporarily stop operation. Choose a pass with at least 45° maximum elevation for your first attempts — high-elevation passes last longer and require less precise antenna pointing than low-elevation passes near the horizon.
What power do I need for the uplink?
For FM satellites via Mode J (2m uplink), 25–50W into a 7–9 dBd Yagi is comfortable for accessing most passes. 5–10W works for high-elevation passes. The linear transponder satellites require less power — 10–25W into a crossed Yagi is usually more than enough; too much power overloads the transponder and degrades the signal for everyone. A common mistake for new satellite operators is using too much power — the first thing to try if your signal is not coming through is reducing power and confirming the Doppler correction and access tone are correct, before assuming more power is the answer.
Can I use a dish antenna instead of crossed Yagis?
A dish antenna with a circularly polarised feed provides higher gain than crossed Yagis and works well for satellite operation. The practical challenge is that dishes are heavier and present more wind load than Yagi arrays, requiring more robust az/el mounting. A 1-metre dish with a helical feed produces approximately 20 dBd gain — far more than any Yagi system — and is used by serious satellite DXers working weak CW beacons from deep-space probes and high-altitude balloons. For standard LEO FM and linear transponder satellites, a crossed Yagi system provides more than enough gain and is mechanically simpler to mount and track.