Quadrifilar Helix

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This is a project to create a UHF quadrifilar helicoidal antenna for use with a ham radio to transmit APRS data from the high altitude balloon payload.

Final result of the QFH build.

Summary

A quadrifilar helix, or QFH for short, is a near-omnidirectional antenna comprised of two square loops usually 1 wavelength long each, twisted a half turn and offset 90 degrees from each other. It is a balanced antenna, so it requires a balun when fed by an unbalanced input (such as coax). In this build, I used a current balun, made of 7 turns of coax around the 0.5" plastic tube. The sizes for the antenna were found using the online calculator at http://www.jcoppens.com/ant/qfh/calc.en.php

The design parameters were as follows:

  • Design frequency: 445.925MHz
  • Number of turns: 0.5
  • Length of one turn: 1 wavelength
  • Bending radius: 1.75mm
  • Conductor diameter: 1mm
  • Width/height ratio: 0.44

The feed point is configured as is shown in W6NBC's article.

QFH feed point configuration.

The total build time for this antenna was approximately 4.5 hours.

Bill of Materials

Part Quantity Unit Cost Total Cost Source
SMA Male Connector 1/5 $2.51 $0.50 Dealextreme
RG-174 Cable 1.5m/3m $1.59 $0.80 eBay
8" Bamboo Skewers 2/100 ~$4 $0.08 Grocery Store
1'x0.5" Plastic Tube 1 ~$1 ~$1.00 Craft Store
18ga. Solid Core Wire 4.66'/100' $6.89 $0.32 Radio Shack
PCB 0.25in2/54in2 $8.69 $0.04 Mar Vac
Total Price ~$2.74

Construction Photos

Results

Impedance of the QFH over the frequency range of 410MHz-460MHz.
File:SWR.png
SWR of the QFH over the frequency range of 410MHz-460MHz.

The antenna is fairly well matched, with an impedance of 55.3 + j18.4 ohms at the design frequency of 445.925MHz. The SWR is 1.43 at the design frequency and stays below 2.0 for the 430MHz-450MHz US Amateur band, corresponding with an effective radiated power of 96.9% at the design frequency, and 88.9% over the Amateur band (calculated by the formula Power % = 100 * (1 - ((SWR-1)/(SWR+1))2) ).