70cmQuadiDesign
Bold text=Note: This page is a reconstruction to a now defuct Univeristy of Idaho nearspace balloon wiki article I posted in 2008=
70cm Quagi
Project Status
V1 Flight Tested on F08L01
V2 Preconstruction
Problem Statement
Ginger is a bidirectional data routing system equipt with 70cm j-pole antennas. These antennas have a gain of around 2.5dBi and using quarter wave whips the link range is around 37 miles at full power (250kbps) at 10 Watts of output power. To extend the range of operations which might be due to a run away balloon, damaged transmitters, or geographical obstructions, we have built two directional quagi antennas.
The quagi antenna consists of a quad driver (a driven element and a reflector) which already has a higher gain than a dipole fed yagi and reflector. This means that by feeding the same number of directors, a quagi will outperform a yagi at the cost of not being a completely flat design (which is merely a transport concern anyway). The 8 element quagis we have built should have a gain between 12 and 13 dBi and should increase the working distance to 80 miles at 250kbps.
Photos
V1
V2
Dimensions
V1
The dimensions can be found here, in a 1977 article. http://www.geocities.com/garyntricia/quagi.html
V2
The deisgn has been optimized for 437MHz using NEC2. The dimensions are as follows:
RWIRE = .00258826 wire for quads diameter RROD = .003175 yagi director rod diameter RE = 0.755962 Total Length of Reflector Element DE = 0.718693 Total Length of Driven Element D1 = 0.280124 Total length of Director element D2 = 0.28434 Total length of Director element D3 = 0.283594 Total length of Director element D4 = 0.276571 Total length of Director element D5 = 0.244994 Total length of Director element D6 = 0.283213 Total length of Director element RE2DE = 0.177867 Distance from RE to DE DE2D1 = 0.138696 Distance from DE to Director DE2D2 = 0.383653 Distance from DE to Director DE2D3 = 0.547346 Distance from DE to Director DE2D4 = 0.851997 Distance from DE to Director DE2D5 = 0.984569 Distance from DE to Director DE2D6 = 1.092358 Distance from DE to Director
Performance
V1
- At 437MHz the SWR is measured at 1.0 for Quagi Orange and 1.2 for Quagi Green (due to coax losses from crappier cable)
- Raw data showing rough measurements of directionality Excel XLS file
V2
- At 437MHz the gain should be 13.43 DBi best theoretical gain SWR is 1.07
Insights
- Wind is a real problem and keeping both aimed at the same location is not a trivial task. Mount both on a single crossbar and have a lockable central pivot.
- It would be pretty neat to read the GPS data from the CNDH board (already scheduled) and have a QT module to decode them and steer the antennas in the right direction.
- I don't trust ARRL - always simulate those "best anntenna ever" designs first before building!
