A Foundation licence holder's first steps into software defined radio — building a 3D printed quadrifilar helix antenna from scratch.
Building a QFH Antenna
Why QFH?
A standard dipole works for satellite reception but it's compromised — it's linearly polarised, while weather satellite signals are circularly polarised. This mismatch causes a polarisation loss of up to 3 dB on average, and the loss varies as the satellite moves across the sky.
Circular polarisation: Radio waves can be polarised in a plane (linear) or rotating (circular). A circularly polarised wave rotates its electric field vector as it propagates — either right-hand or left-hand. Meteor-M transmits right-hand circular polarisation (RHCP). A linearly polarised receiving antenna like a dipole will receive RHCP signals with a theoretical 3 dB polarisation loss averaged over a full rotation. A circularly polarised antenna like a QFH matches the incoming polarisation perfectly, recovering that 3 dB and providing more consistent signal strength throughout the pass.
The Quadrifilar Helix (QFH) is the classic weather satellite receiving antenna. It's circularly polarised, has a hemispherical radiation pattern (ideal for satellites passing overhead at any angle), requires no ground plane, and can be built from cheap materials.
The Design: 3dp-qfh
The design I used is LongHairedHacker's 3dp-qfh from GitHub — a 3D printed version using printed connector parts, PVC pipe, and welding rod elements. The printed parts handle all the awkward geometry automatically.
The original design specifies 15mm pipe and 3mm welding rod. I had 20mm white PVC conduit and 2.4mm TIG rod already in the garage, so I modified the OpenSCAD model parameters to match. Neither change has any electrical effect at 137 MHz — the wavelength is 2.2 metres, so the difference between 2.4mm and 3mm rod is completely invisible to the signal.
Important to note, that I changed the outer diameters of the end caps, but not the central connector, otherwise I'd have to modify the pipe lengths. I avoided any changes to geometry that would affect the placement of the rods.
I test printed some of the original parts first to verify the geometry and fit before committing to the modified dimensions. Once happy, I reprinted everything with the adjusted parameters.
QFH dimensions at 137.5 MHz (from John Coppens' calculator at jcoppens.com): Total helix height: 721 mm The antenna consists of two loops — one large and one small — wound as a helix around a central PVC pipe, offset 90 degrees from each other. The size difference between the loops creates the phase quadrature (90° phase relationship between the two loops) that generates circular polarisation. Electrically, the two loops form a single continuous conductor path from coax centre conductor to coax braid, twisted around the pipe.
Materials used:
- 20mm white PVC electrical conduit (already in the garage — cost: £0)
- 2.4mm copper-coated steel TIG welding rod (OpenSCAD diameter parameter adjusted — electrically insignificant at 137 MHz)
- 3D printed connector parts in PETG (white/light grey — better UV and temperature resistance than PLA for outdoor use)
- RG174 coax pigtail, 1m — running up through the centre of the PVC pipe to the feedpoint at the top ( I couldn't find pigtails, so bought 3m male to male cables, and cut in half)
- Self-amalgamating tape for weatherproofing
Total additional materials cost: approximately £5 — built entirely from garage stock and 3D printed parts, the cable being the only thing needed.
Assembly Notes
I'd planned this as a build-with-the-kids project. My 8 year old has been interested in the radio stuff and I thought assembling the antenna would make a good afternoon activity. Instead I found myself having accidentally built the whole thing in one sitting — just testing each part to make sure it worked, fitting one rod to check the clips, then another, and before I knew it all eight rods were in place and I was holding a finished antenna.
That's the thing about well-designed parts — each one goes in so satisfyingly that stopping feels wrong.
The kids will get their moment when it comes to the first live pass reception. Watching a satellite signal appear on a screen, understanding it's coming from something 800km overhead travelling at 7.45 km/s — that's a better payoff than fitting rods into clips anyway.
The design uses a top cap and bottom cap. The feedpoint — where the coax connects and the loop cross-connections are made — is at the top cap. The coax runs up through the centre of the PVC pipe from the bottom.
Feedpoint wiring: The top cap has two separate connection points (AC and BD). Large loop end A laps and solders to small loop end C; large loop end B laps and solders to small loop end D. AC and BD must not touch each other — the gap between them is the isolation that makes the antenna work. Coax braid connects to one pair, coax centre conductor to the other.
Soldering tips:
- Lap joints (10–15mm overlap) are stronger and easier to solder than butt joints
- Work quickly to avoid transferring heat into the printed parts
- A dab of epoxy over each joint adds mechanical strength
- Weatherproof the top cap with self-amalgamating tape once complete
The Upgrade Path
Once the QFH is built and tested, the next improvement is an LNA (Low Noise Amplifier) at the antenna feedpoint:
Why LNA position matters: Coax attenuates signals — RG174 loses approximately 0.6 dB per metre at 137 MHz. Any signal lost in the coax before amplification is lost forever, raising the system noise figure. By placing the LNA at the antenna feedpoint, the signal is amplified before the coax run, so the coax loss comes after amplification. The RTL-SDR Blog V4's built-in bias-T (4.5V) can power the LNA directly through the coax, requiring no separate power supply.
Recommended: RTL-SDR Blog Wideband LNA from technofix.uk (UK authorised reseller). Noise figure under 1 dB, 18.7 dB gain, 50 MHz–4 GHz coverage, bias-T powered.
The complete station stack:
QFH Antenna (137.5 MHz, circularly polarised)
↓ 20cm RG174 pigtail
RTL-SDR Blog LNA (bias-T powered from V4)
↓ 3m RG58 extension
RTL-SDR Blog V4 dongle
↓ USB
MacBook running SDR++ + SatDump
Next Steps
- Solder the QFH feedpoint and weatherproof
- First pass reception with QFH — even without the LNA it should outperform the dipole
- Add LNA once it arrives — expect significant waterfall improvement
- Catch a high elevation (>40°) southbound Meteor-M pass for clean 4/4 RS decode and first weather image — with the kids watching this time
- Investigate HRPT reception at 1.7 GHz for higher resolution imagery — requires directional helical antenna and tracking
Resources
- 3dp-qfh design: github.com/LongHairedHacker/3dp-qfh
- QFH dimension calculator: jcoppens.com/ant/qfh/calc.en.php
