Build a 70cm Yagi Antenna
The 70cm Yagi at 432 MHz is the compact, high-gain companion to the 2m Yagi for satellite operation, UHF weak-signal SSB, and terrestrial microwave work. At this frequency a 10-element Yagi spans just 30 inches of boom yet delivers 14–16 dBd of gain — performance that at HF would require a tower-mounted multi-element monster. The short wavelength (70cm) means every dimension is cut to within 1mm, connector quality becomes critical, and feedline loss per foot is three times higher than at 2m. This guide builds a 10-element 432 MHz Yagi using a hairpin match driven element optimised for low-noise weak-signal operation, covering the full 70cm weak-signal segment from 432.0 to 432.5 MHz.
How 432 MHz Differs from 144 MHz Construction
Every precision requirement from the 2m Yagi build is amplified further at 432 MHz. The wavelength is exactly one-third that of 2m — all dimensions are one-third the size, and all tolerances are one-third as generous:
Boom Effect is More Critical at 432 MHz
At 432 MHz, elements mounted through a metallic boom experience a significantly larger boom correction than at 144 MHz. Using insulated element mounting (elements on top of boom with HDPE spacers) completely avoids this complication and produces more predictable, reproducible results:
Feed System Options at 432 MHz
The 70cm Yagi's feedpoint impedance (typically 20–35 Ω with parasitic elements) requires a matching network for 50 Ω coax connection. Three practical options:
- Hairpin match (this guide): a short shorted stub across the driven element feedpoint, similar to the beta match used in the 3-element HF Yagi. At 432 MHz the hairpin dimensions are very small (10–20 mm) and must be built from rigid wire or rod. Extremely effective and reliable once set correctly.
- Gamma match: a tap on one element half with a series capacitor. Adjustable after installation but introduces some pattern asymmetry. Less common on 70cm than on HF Yagis.
- Coax stub match: a calculated length of 75 Ω coax transforms the feedpoint impedance to 50 Ω. Elegant, no moving parts, but very sensitive to coax length precision at 432 MHz. Suitable for builders with accurate NanoVNA measurement capability.
Operating Applications for 70cm
The 70cm Yagi opens several operating modes unique to UHF amateur radio:
- Weak-signal SSB/CW: 432.000–432.300 MHz — the primary weak-signal segment on 70cm. Tropo and aircraft scatter contacts of 100–500 miles are routine during band openings. A 10-element Yagi is competitive for regional contest operation.
- Satellite downlink: most FM satellites downlink on 70cm (435–438 MHz). A 10-element Yagi is more than adequate for FM satellite downlinks — many operators use a simple 3-element 70cm Yagi and still copy the satellite clearly.
- ATV (amateur television): 70cm is the primary band for analog and digital fast-scan ATV (Amateur Television). A Yagi pointed at the transmitter provides dramatically better picture quality than an omnidirectional antenna.
- Meteor scatter: 432 MHz meteor scatter is possible but requires high gain and low-noise receive chains. A 10-element Yagi is a starting point — serious meteor scatter operators typically use arrays of 4+ Yagis.
- EME (moon bounce): 70cm EME requires significant antenna arrays (typically 4–16 Yagis) and high power. A single 10-element Yagi can receive EME signals from the most powerful stations using WSJT modes — useful for learning EME without the full hardware commitment.
| Element | Function | Length (mm) | Length (inches) | Position from reflector (mm) | Position from reflector (inches) |
|---|---|---|---|---|---|
| El. 1 | Reflector | 345 mm | 13.6 in | 0 mm | 0 in |
| El. 2 | Driven element | 320 mm | 12.6 in | 125 mm | 4.9 in |
| El. 3 | Director 1 | 311 mm | 12.2 in | 250 mm | 9.8 in |
| El. 4 | Director 2 | 306 mm | 12.1 in | 390 mm | 15.4 in |
| El. 5 | Director 3 | 302 mm | 11.9 in | 538 mm | 21.2 in |
| El. 6 | Director 4 | 299 mm | 11.8 in | 694 mm | 27.3 in |
| El. 7 | Director 5 | 296 mm | 11.7 in | 856 mm | 33.7 in |
| El. 8 | Director 6 | 294 mm | 11.6 in | 1023 mm | 40.3 in |
| El. 9 | Director 7 | 292 mm | 11.5 in | 1195 mm | 47.0 in |
| El. 10 | Director 8 | 290 mm | 11.4 in | 1370 mm | 53.9 in |
| Total boom length | ~1430 mm | ~56.3 in (4.69 ft) | |||
Uhf Yagi 70cm Calculator
Materials for a 10-element 432 MHz Yagi with hairpin match driven element
Why the Hairpin Works at 432 MHz
The hairpin match principle is identical to the beta match described in the 3-element HF Yagi guide — a shorted stub across the driven element feedpoint. At 432 MHz the physical dimensions are very small but the electrical function is the same. The driven element is made slightly shorter than resonance (creating capacitive reactance), and the hairpin stub adds inductive reactance to cancel it while also transforming the feedpoint resistance to approximately 50 Ω:
Building the Hairpin at 432 MHz
The very small dimensions of the 432 MHz hairpin require careful fabrication — at this frequency, small asymmetries in the stub produce measurable pattern distortion:
Building the 10-Element 70cm Yagi
At 432 MHz, measurement precision is the difference between an excellent antenna and a mediocre one. Use calipers for every element measurement. Work in millimeters. Do not rush the element cutting — once an element is cut too short it must be replaced. Cut every element slightly long and trim to exact length with a file.
Prepare the Boom
Cut the 3/4-inch OD aluminum boom to 1450 mm (57.1 inches) — 80 mm longer than the last element position to provide support beyond director 8. Mark all ten element positions using a steel rule and marking pen. At 432 MHz, element position accuracy matters — mark to ±1 mm:
Cut All Elements to Exact Length
Cut all 10 elements from 3 mm OD aluminum rod to the lengths in the table. At 432 MHz, a ±1 mm element length error corresponds to approximately 1.4 MHz resonance shift — meaningful at this frequency. Target ±0.5 mm:
Fabricate Element Insulators and Mount All Elements
Cut 10 HDPE insulator strips from a scrap of 8 mm thick HDPE sheet. Each strip should be 20 × 20 mm with a 3 mm hole through the center. Alternatively, use purchased nylon standoff blocks — many electronics suppliers stock appropriate sizes.
Mount each element through its insulator strip on top of the boom at the marked position. Use an M3 nylon screw through the insulator hole (the element passes through this hole) and secure with a nylon nut below the boom. The nylon screw clamps the insulator to the boom; the insulator holds the element by friction through the 3 mm hole, plus a small drop of cyanoacrylate (super glue) applied at the insulator-element junction to prevent the element from sliding laterally.
Build the Hairpin Match and Feedpoint Assembly
Build the hairpin match from 2 mm rigid copper wire as described in the design section. Mount the feedpoint enclosure (small ABS weatherproof box) on the boom adjacent to element 2 (driven element). The hairpin and feedpoint all fit inside or on top of this small enclosure.
Connect the hairpin legs to the two halves of the driven element. The driven element (320 mm total) is split at the center into two 160 mm halves. Each half mounts on its own insulator strip at the center position, with a 5 mm gap between the two halves for the feedpoint. The hairpin bridges this gap from above, with one leg soldered to each element half.
Install a ferrite current choke: wind 3–4 turns of the feedline coax through a type-43 ferrite toroid immediately at the feedpoint enclosure exit. This prevents the coax from carrying common-mode current that would distort the 70cm pattern.
Initial Measurement at the Feedpoint
Connect the NanoVNA directly to the feedpoint N-connector (not at the end of the coax — coax length shifts the apparent resonance significantly at 432 MHz). Hold the antenna horizontally at waist height, pointed away from metal surfaces. Sweep 420–450 MHz:
Mount at Operating Height and Final Verification
Mount the antenna on its mast at a height of at least 3 meters (10 feet) for final measurement. Measure SWR at the feedpoint (not shack end) with the NanoVNA. The antenna at height, away from ground reflections, should show the true operating characteristics:
The 2m/70cm Satellite Station
Combining the 6-element 2m Yagi from the previous guide with this 10-element 70cm Yagi creates a complete satellite operating station covering all commonly used FM and linear satellite modes:
- FM satellites (SO-50, AO-91, AO-92): 2m uplink (145.850–145.980 MHz) and 70cm downlink (435.240–436.795 MHz). Point both antennas at the satellite simultaneously — the 2m Yagi for transmit, the 70cm Yagi for receive.
- Physical mounting: the two Yagis can be mounted on the same boom or mast, one above the other. The coupling between them at these widely separated frequencies (2m and 70cm) is negligible — no isolation hardware is needed. A simple 1-meter vertical separation between the antenna centers prevents any mechanical interference.
- Satellite tracking: for low-Earth orbit (LEO) satellite passes, the satellite moves across the sky in approximately 10 minutes. A simple azimuth/elevation mount (a camera tripod with a home-made elevation adjustment) allows manual satellite tracking by moving the antenna to follow the satellite. No rotator controller is needed for casual FM satellite operation — most operators manually track the satellite during its pass.
- Doppler correction: tune the radio while listening to the satellite downlink. As the satellite approaches, tune down in frequency; as it recedes, tune up. The 70cm Yagi's bandwidth covers the full Doppler shift range (approximately ±10 kHz) without retuning the antenna.
Cross-Polarization and Circular Polarization
Satellites tumble in orbit, which rotates the polarization of their signals continuously. A horizontal or vertical Yagi loses up to 20 dB of signal when the satellite's polarization is perpendicular to the Yagi's polarization — a phenomenon called polarization mismatch:
- Symptom: during a satellite pass, the signal periodically fades by 10–20 dB and recovers — the fade interval corresponds to one full polarization rotation of the tumbling satellite.
- Partial solution — rotate the Yagi: manually rotate the Yagi between horizontal and vertical polarization as the satellite pass progresses. This crude approach reduces the worst-case fade but does not eliminate it.
- Best solution — circular polarization: a circularly polarized antenna (a helix or a crossed Yagi with 90° phase offset between the two polarizations) accepts signals of any linear polarization with only 3 dB of loss (vs 20 dB for worst-case linear polarization mismatch). A crossed Yagi (two Yagis at 90°, fed with a 90° phasing harness) is the standard satellite antenna for operators who want to eliminate polarization fading.
- For casual FM satellite use: the linear Yagi works adequately — the fades are brief and during most of a satellite pass the signal is usable. For linear transponder (SSB) satellite work where QSB from polarization fading disrupts contacts, circular polarization is worth the additional construction effort.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR minimum 10+ MHz away from 432 MHz | Element length error — likely one element grossly wrong length or wrong position | Measure every element with calipers and compare to table; check all element positions on boom | Re-cut incorrect elements; verify all positions measured from reflector end of boom |
| SWR minimum correct but minimum SWR above 3:1 | Hairpin length or spacing incorrect; coax connection fault | Adjust hairpin length in 0.5 mm steps; verify all four solder joints at driven element | Trim hairpin by 0.5 mm; re-solder any cold joints; verify hairpin not contacting boom |
| SWR fine at feedpoint but very high at shack end of coax | Coax connector fault or damaged coax between antenna and shack | Measure SWR at feedpoint (not shack end) — if feedpoint SWR is fine, problem is in the feedline | Check all N-type connectors for damage, corrosion, or loose center pin; replace suspect connectors |
| Antenna appears omnidirectional — no discernible beam | Element lengths all wrong (common length used) or elements severely out of position | Measure all elements — directors should be progressively shorter from D1 (311 mm) to D8 (290 mm) | Verify all 10 element lengths against table; re-cut any elements more than 2 mm from target |
| Performance degrades rapidly after outdoor exposure | Oxidation at element connections or coax connector corrosion at 432 MHz | Inspect all connection points with magnifying glass for green oxide; re-measure SWR | Clean all connections; apply lacquer to element surfaces; replace any corroded N-type connectors |
| SWR varies when antenna is rotated (pattern not symmetrical) | Asymmetric hairpin or one driven element half longer than other | Measure both driven element halves — should be equal ±0.5 mm; measure hairpin leg lengths | File the longer DE half to match the shorter; rebuild hairpin if legs are not equal length |
How much gain advantage does this Yagi have over a vertical quarter-wave on 70cm?
A 70cm quarter-wave vertical has approximately 0 dBd gain (essentially the same as a reference dipole). This 10-element Yagi has approximately 14–16 dBd gain. The difference is 14–16 dB — which in practical terms means signals that are inaudible with the vertical are copyable with the Yagi, and signals that are barely copyable with the vertical are S9 with the Yagi. At 432 MHz where propagation is line-of-sight and signals are often marginal, this gain difference frequently determines whether a contact is possible at all. The gain advantage of the Yagi on 70cm is far more practically significant than the equivalent gain at HF where propagation provides natural signal enhancement.
What is the 70cm Yagi beamwidth and how precisely must I point it?
A 10-element 70cm Yagi has a −3 dB beamwidth of approximately 30–35° in the horizontal plane. This means you need to be within about 15–17° of the exact bearing to the station you are working without losing more than 3 dB (half an S-unit). For terrestrial fixed or slow-moving targets like repeaters and distant stations, pointing by compass bearing to ±10° is usually adequate. For satellite tracking, the satellite moves approximately 2–3°/second during the middle of a low-elevation pass — a manually tracked antenna needs to be nudged every few seconds to maintain pointing. For EME, the moon moves slowly enough (~0.5°/min) that manual tracking with periodic adjustment is feasible.
Can I use this Yagi for the 70cm FM repeater band at 440–450 MHz?
The antenna is tuned for 432.1 MHz and will show increasing SWR above 435 MHz. At 440 MHz the SWR will typically be 4–6:1 — too high for efficient FM transmit operation without a tuner. For receive-only use on the 440 MHz repeater band, the SWR mismatch does not affect reception significantly. For a Yagi that covers both 432 MHz weak-signal and 440 MHz FM, either retune this design to 436 MHz as a compromise (SWR below 2:1 from 432 to 440 MHz) or build a separate simple 3-element 440 MHz Yagi for FM repeater operation.
What is the minimum coax run length that requires LMR-600 instead of LMR-400?
At 432 MHz, LMR-400 loses approximately 2.1 dB per 100 feet. LMR-600 loses approximately 1.4 dB per 100 feet. The difference is 0.7 dB per 100 feet — measurable but not critical for most applications. The practical break-even point where the cost premium of LMR-600 is justified by the performance gain depends on your priorities: for a casual satellite station, LMR-400 for runs up to 100 feet is perfectly acceptable. For a weak-signal contest or EME station where every 0.1 dB matters, LMR-600 is worthwhile for any run over 25 feet. For runs over 150 feet on 70cm, consider hardline (7/8-inch or 1-5/8-inch) — the loss savings over LMR-400 become substantial enough to justify the higher cost and more complex installation.
How do I build a crossed Yagi for circular polarization on satellites?
A crossed Yagi pairs two identical Yagis at 90° to each other on the same boom, fed with a 90° phase offset between them. The phase offset is typically provided by a λ/4 length of 75 Ω coax (which transforms between the two Yagi feedpoints and introduces the 90° phase shift). Both Yagis connect to a power splitter/combiner at the feedpoint, with the λ/4 coax between one Yagi and the combiner. For the 70cm crossed Yagi, the λ/4 section at 432 MHz in 75 Ω coax (VF 0.66) is 75/432 × 0.66 × 984 = 113 mm of coax. The two Yagis are mounted perpendicular to each other on a common boom hub — one horizontal and one vertical. The handedness (left-circular vs right-circular) is set by which Yagi gets the λ/4 delay — most satellites use right-hand circular polarization (RHCP) for their downlink signals.
Can this antenna be extended to more elements for EME?
Yes — the DL6WU design scales linearly. Each additional director adds approximately 1 dBd per 2 additional elements and extends the boom by approximately 155 mm per additional element. Practical EME on 70cm requires approximately 20–25 dBd of antenna gain in each direction (transmit and receive). A single 20-element Yagi reaches approximately 19–20 dBd — within range of the most powerful EME stations using WSJT digital modes. A pair of stacked 20-element Yagis (separated by approximately 2.3 meters for maximum gain) produces approximately 22–23 dBd — a competitive single-operator EME antenna on 70cm. The additional directors are simply added at the front of the existing boom, following the same DL6WU progression. No changes to the reflector, driven element, or feedpoint are needed.