Ground Antennas for LV2
We have five ground antennas to build:
| No. | Function | Frequency | Gain | Beamwidth | Bandwidth [MHz] | Polarization |
|---|---|---|---|---|---|---|
| 1 | LCM ↔ LT | 2.422GHz 802.11b | high | narrow | 20 | linear |
| 2 | LT ↔ LCM | 2.422GHz 802.11b | high | narrow | 20 | linear |
| 3 | LCM ← LV2 | 1.277GHz ATV | 17dB | 30° | 20 | RHCP |
| 4 | LCM ↔ LV2 | 2.422GHz 802.11b | 17dB | 30° | 20 | RHCP |
| 5 | LCM → LV2 | 144MHz 2m | moderate | very wide | 0.05 | RHCP |
Our antenna choices are:
Antenna 1 and 2 form a close-by, non-moving, terrestrial link between the LCM and the LT. A parabolic reflector antenna with a linear polarization feed would satisfy this need.
Antenna 3 and 4 need to track the moving and tumbling craft. It has been shown by people doing photography at our launches that using an antenna with about 30 degrees of beamwidth, tracking the rocket would be an easy task. A 12 turn helix antenna with RHCP would fit this requirement. The antenna polarization on the craft is linear, thereby a RHCP antenna would have the correct polarization sense no matter what orientation the craft was in.
Variation on this using a remote telemetry site at 45 degree antenna elevation at apogee.
- Antenna 5 is subject to the same dynamic orientation problems found with antenna 3 and 4, with the added problem of being a low VHF frquency requiring a large antenna size. However the link is limited to uplink only, allowing us to just apply a huge ammount of power on the ground to overcome any gain problems. We just need to throw the power in the right direction with the right sense. A Turnstile-Reflector antenna will meet these needs. The T-R antenna has a very wide upward beamwidth, almost a complete hemisphere upward. The polarization sense is horizontal on the ground, and RHCP straight up, with an eliptical polarization variation in between. The horizontal polarization on the ground is not ideal since the rocket ?might? have vertical polarization when on the launch pad, but the distance is small so the large transmit power should make a reliable link.
The link budget of these antenna systems are described on the LinkBudget page.
Helical Antenna Design
Dimensions of interest:
- 2.422GHz wavelength (lambda) = 123.78mm (4.873")
1.277GHz wavelength (lambda) = 234.76mm (9.243")
2.422GHz ground plane for helicals: 0.8lambda - 1.1lambda = 9.9 - 13.6 cm (3.90 - 5.36")
1.277GHz ground plane for helicals: 0.8lambda - 1.1lambda = 18.8 - 25.8 cm (7.39 - 10.17")
This is a Helix Calculator © 2001 Caleb Crome that does the work found in the ARRL Antenna Book. The online link was picked out of the helix reference from Jason Hecker (jhecker) and placed here because it seems real usefull.
Equation for helix Half Power Beam Width (HPBW)
Helix antenna references:
Jason Hecker's Helix Antenna site.
ARRL The ARRL Antenna Book.16th ed. 3rd pr. 1993, p19-22
Antenna Engineering Handbook, Helix
Space Mission Analysis and Design, Communication Architecture
Turnstile-Reflector Antenna Design
- Dipole propagation over reflecting surface
- Dipole radiation resistance over reflecting surface
- Connecting feed system; Impedance match, Phase delay
Turnstile antenna references:
ARRLSatelite Experimenters Handbook
ARRLAntenna Handbook
Antenna Engineering Handbook