Build a 33cm Band Yagi Antenna (902–928 MHz)
At 900 MHz the wavelength is just 33 centimetres, making a high-performance 9-element Yagi small enough to hold in one hand and build from brass rod and a PCB boom in an afternoon. The 33 cm band is one of amateur radio's most versatile allocations — SSB weak-signal work, ATV, narrowband FM, and increasingly SDR-based experimentation. This guide covers a practical 9-element design with an integrated hairpin match and SMA output connector.
The 902–928 MHz band (33 centimetres) sits in a fascinating frequency range — above the primary VHF bands but below the true microwave allocations. Its 26 MHz bandwidth allows high-data-rate digital modes, ATV (amateur television), narrowband FM voice, and weak-signal SSB/CW operation. Propagation at 900 MHz is primarily line-of-sight, with tropospheric ducting occasionally providing long-distance contacts of hundreds of kilometres.
A Yagi at this frequency provides a very useful gain in a tiny package. A 9-element Yagi for 902–928 MHz has a boom of only 370 mm and elements under 170 mm long — smaller than a clipboard. The compact dimensions allow precision construction on a PCB (printed circuit board) boom or from brass rod and a fibreglass boom, with SMA connectors providing a professional low-loss interface. At 900 MHz the construction precision requirements are tighter than at HF — element lengths must be accurate to ±1 mm and the feed system must use matched 50 Ω geometry throughout.
Weak-signal (SSB/CW)
903.1 MHz is the primary weak-signal calling frequency in North America. A 9-element Yagi provides 11 dBi gain — enough to work stations 100–400 km away during tropo openings. For EME the band has a dedicated segment around 902.0 MHz.
ATV and wideband
Amateur television (ATV) on 33 cm uses the band's 26 MHz width for analogue or digital video transmission. A directional Yagi significantly extends ATV range. The same antenna works well for narrowband FM voice repeaters in the 902–928 MHz segment.
SDR and monitoring
A 33 cm Yagi connected to an RTL-SDR or HackRF covers the ISM band (902–928 MHz) where various unlicensed devices operate — useful for RF monitoring and spectrum analysis projects. The antenna's 11 dBi gain and directional pattern help identify signal sources.
This design follows the DL6WU long Yagi optimisation formulas developed by Günter Hoch, which produce a consistently good balance of gain, F/B ratio, and feed point impedance. For a 9-element version at 915 MHz (band centre), the design achieves approximately 11 dBi free-space gain with an F/B ratio of 20–22 dB and a well-behaved 50 Ω feed point impedance amenable to a simple hairpin matching network.
| Element | Position (mm from reflector) | Length (mm) | Function |
|---|---|---|---|
| Reflector | 0 | 165.2 | Passive — longer than DE |
| Driven element | 65 | 161.0 | Fed via hairpin match |
| Director 1 | 130 | 155.6 | Passive — shorter than DE |
| Director 2 | 220 | 153.3 | Progressive shortening |
| Director 3 | 312 | 151.8 | Progressive shortening |
| Director 4 | 404 | 150.5 | Progressive shortening |
| Director 5 | 498 | 149.3 | Progressive shortening |
| Director 6 | 594 | 148.3 | Progressive shortening |
| Director 7 | 692 | 147.4 | Last element |
DL6WU Style Yagi Calculator for 33cm Band
Materials for 9-element 33cm Yagi at 915 MHz
At 900 MHz, construction precision becomes far more important than at HF. A 1 mm error in element length corresponds to approximately 0.3% of a wavelength — small but not negligible when trying to achieve the optimum F/B ratio and feed point match. Use a precision vernier calliper or digital calipers accurate to 0.1 mm for all element length measurements. The boom position of each element can be laid out with a steel rule marked in pencil — accuracy to ±1 mm is sufficient for spacing.
Mark the element positions on the boom from the table above, measuring from the reflector end. At 900 MHz, use a 3.2 mm drill bit to drill completely through the 10 mm boom at each element position — the hole passes through both walls of the square section. Use a drill press if available to ensure the holes are perpendicular to the boom axis. Tap each hole M3 or thread a matching size set screw. Verify positions with a calliper before drilling — a misplaced hole cannot be unmade.
Cut all element rods from 3 mm brass or aluminium rod using a fine-tooth hacksaw or a chop saw with a metal blade. Cut each element 5 mm longer than the table value — you will trim to final length after initial test. Mark each element with its position number using a permanent marker before it leaves the bench — all directors look identical and are easy to mix up. File the cut ends square and deburr.
Insert each parasitic element through its corresponding boom hole so that it protrudes equally on both sides (equal half-length each side). Tighten the M3 set screw to clamp the element in position. Apply a small drop of Loctite 243 (medium-strength threadlock) to each set screw after verifying the element is centred. The parasitic elements make direct electrical contact with the aluminium boom — this is correct and expected, and the boom correction in the element lengths accounts for this contact.
The driven element must be electrically isolated from the boom. Drill the DE hole slightly larger (4 mm) and insert a PTFE or nylon sleeve through the hole before inserting the DE rod. The sleeve insulates the element from the boom at the crossing point. Alternatively, use a split DE — two half-length rods mounted on opposite sides of the boom using isolated mounts, with the SMA connector feeding across the split in the centre. Verify with an ohmmeter that the DE is open-circuit to the boom.
The driven element of a multi-element Yagi at 900 MHz presents approximately 20–30 Ω at its centre — too low for a direct 50 Ω match. The hairpin match (also called a beta match) uses a short inductive stub across the DE feed point to raise the impedance to 50 Ω. Bend two parallel wires of 2 mm copper or brass, each approximately 19 mm long, bent into a U-shape with 20 mm centre-to-centre spacing. Solder each end of the hairpin to the two halves of the split DE. The SMA connector feeds across the open end of the hairpin — inner conductor to one side, outer to the other. Use a NanoVNA to verify the match and adjust the hairpin length (shortening it raises the impedance transformation effect) until SWR is below 1.5:1.
With the antenna assembled and connected to a NanoVNA via a short SMA-to-SMA cable, sweep 880–950 MHz. Find the frequency of minimum SWR. If above 915 MHz, the elements are too short — add 1–2 mm to the DE and directors by re-cutting. If below 915 MHz, shorten the DE by 1–2 mm per side. When the SWR minimum is at 915 MHz, verify F/B by pointing the antenna at a known signal source (a handheld radio, an SDR noise source, or a 900 MHz signal generator) and rotating 180° — the signal should drop 15–20 dB off the rear.
For permanent outdoor use, apply a thin coat of clear lacquer or Plastik 70 (conformal coating) over all element-to-boom connections and the hairpin solder joints. This prevents oxidation at the aluminium joints which can introduce resistance and detune the antenna. The SMA connector should be the weatherproof panel-mount type with an O-ring seal. Wrap the connector and first 50 mm of feedline with self-amalgamating tape. Mount to a mast using two small U-bolt clamps around the boom — the antenna is light enough (under 100 g) that even thin mast-mounting hardware is adequate.
At 900 MHz, feedline loss becomes a critical concern. The following table shows matched-line loss for common coaxial cables at 900 MHz — the numbers are substantially higher than at HF and demand careful selection:
| Cable | Loss dB/10m @ 900 MHz | Power at antenna (100W TX, 10m run) | Suitability |
|---|---|---|---|
| RG-58 | 5.5 dB | 28 W | Poor — avoid for runs over 3 m |
| RG-8X | 3.8 dB | 42 W | Marginal — keep runs short |
| LMR-240 | 2.4 dB | 57 W | Acceptable for moderate runs |
| LMR-400 | 1.3 dB | 74 W | Good |
| LMR-600 | 0.85 dB | 82 W | Excellent for long runs |
| 7/8" Heliax | 0.4 dB | 91 W | Best — for permanent installations |
Use SMA connectors throughout the system — PL-259/SO-239 connectors are rated to approximately 300 MHz maximum and perform poorly at 900 MHz due to their geometry. N-type connectors are an excellent alternative to SMA for runs requiring more robust mechanical connections — they are rated to 11 GHz and handle the power levels typical at 33 cm well. BNC connectors are rated only to 4 GHz and are acceptable for receive-only and low-power use at 900 MHz.
Performance Expectations| Parameter | 9-element version | 5-element version | 11-element version |
|---|---|---|---|
| Gain (free space) | ~11.0 dBi | ~8.5 dBi | ~12.2 dBi |
| Gain (dBd) | ~8.9 | ~6.4 | ~10.1 |
| F/B ratio | ~20–22 dB | ~15 dB | ~22–25 dB |
| SWR 2:1 bandwidth | ~50 MHz | ~65 MHz | ~40 MHz |
| Boom length | ~730 mm | ~400 mm | ~1,000 mm |
| 3 dB beamwidth H-plane | ~40° | ~55° | ~32° |
Is the 33cm band available worldwide?
No — the 902–928 MHz allocation is primarily a North American amateur band (ITU Region 2). In Europe, most of this spectrum is used by cellular networks (GSM-900, UMTS-900) and is not available for amateur use. Some European countries have a small secondary amateur allocation around 933–935 MHz. Check your national telecommunications authority for your specific allocation. In the UK, the 33 cm band is not currently allocated to amateurs.
Why use 3mm rod for elements rather than thicker stock?
At 900 MHz, element diameter has a noticeable effect on bandwidth. A 3 mm element at 900 MHz (0.009λ diameter) provides a good balance of bandwidth and mechanical rigidity. Thinner elements (1.5–2 mm) have higher Q and narrower bandwidth. Thicker elements (5–6 mm) lower Q and increase bandwidth but require heavier boom hardware. For a general-purpose design, 3 mm rod is the practical optimum.
How accurate must element lengths be at 900 MHz?
At 900 MHz, one wavelength is 327 mm. A 1 mm error in element length is 0.3% of wavelength — roughly equivalent to a 3 mm error on 20 m. This level of error has small but measurable effects on F/B ratio and feed point match. Aim for ±0.5 mm accuracy on the driven element and ±1 mm on parasitic elements. Use digital calipers to verify every element length before installation.
Can I use this antenna for LoRa or cellular monitoring?
The 902–928 MHz frequency range overlaps with LoRa networks (US 915 MHz band), ISM devices, and older cellular frequencies. As a receive-only antenna for monitoring (not transmitting), the Yagi is perfectly legal for this use. The directional pattern is actually an advantage for signal monitoring — pointing the Yagi toward a specific tower or gateway isolates its signal from background noise. Always check local laws regarding RF monitoring before use.
What connector should I use for the feedline?
SMA is the standard choice for 900 MHz amateur radio. N-type is an excellent alternative that is more robust mechanically and equally low-loss. Avoid PL-259/SO-239 (UHF connectors) — their geometry produces significant SWR degradation at 900 MHz. BNC connectors work for receive-only and low-power use but are rated to only 4 GHz and start to behave inconsistently at the upper end of the 33 cm band.
What is the hairpin match and why is it needed?
The driven element of a multi-element Yagi presents approximately 20–28 Ω at its feed point due to mutual coupling with the reflector and directors. A hairpin match (a short inductive stub across the DE centre) cancels the capacitive reactance introduced by slightly shortening the DE and simultaneously raises the feed resistance to approximately 50 Ω. The result is a direct 50 Ω match without a separate balun transformer, keeping the feed system simple and low-loss at 900 MHz.