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.
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.
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|
|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|
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) ).