70cmQuadiDesign

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.

Dimensions

V1

The dimensions, in a 1977 article by K6YNB in QST April 1977

Element Lengths	144.5 MHz	147 MHz	222 MHz	432 MHz	446 MHz
Reflector (all No 12 TW wire, closed)	2200 mm loop	2159 mm	1432 mm	711 mm	689 mm
Driven Element (No 12 TW, fed at bottom)	2083 mm loop	2032 mm	1359 mm	676 mm	657 mm
Directors	913 mm to 889 mm in 5 mm steps	897 mm to 873 mm in 5 mm steps	594 mm to 568 mm in 3 mm steps	298 mm to 291 mm in 1.5 mm steps	289 mm to 280 mm in 1.5 mm steps
Spacing
R - DE	533 mm	521 mm	346 mm	178 mm	173 mm
DE - D1	400 mm	391 mm	260 mm	133 mm	130 mm
D1 - D2	838 mm	826 mm	546 mm	279 mm	272 mm
D2 - D3	445 mm	435 mm	289 mm	149 mm	144 mm
D3 - D4	663 mm	651 mm	432 mm	222 mm	215 mm
D4 - D5	663 mm	651 mm	432 mm	222 mm	215 mm
D5 - D6	663 mm	651 mm	432 mm	222 mm	215 mm

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. SWR S/B 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!