antenna-theory.com

[Nayyarv]

Hi everyone,

My name is Varun, and I'm an engineering student working on a undergrad student project to send a satellite into space - visit http://www.bluesat.unsw.edu.au/ for more information.
Essentially our project is very similar to AMSAT, but we're not going into space (it's too expensive and cubesat's are all the rage now), but we have a weather balloon launch scheduled for later this year. A lot of the work has been done, and the team are very competent, except when it comes to the radios which is the team I'm working for.

The main hiccup is that we have only a very limited understanding of RF electronics. We're using TA451 transmitters and R100 receivers from hamtronics as the radios, tuned to 436.5 MHz for transmitters and 145.9 MHz for the receivers. When we use dummy loads, we can transmit easily (across the room) but obviously that is nowhere near the range we need (50km or so).

For the receiving antenna, I have built a quarter wavelength (51.4 cm) monopole whip (the satellite is being treated as a ground plane) out of brass , but haven't tuned it, nor do I know how (build coax cables and stub matching with a VNA? How do I use a VNA?).

For the transmitting antenna , it was suggested to build 4 antennas in a circular polarised topology - but this felt unnecessarily complicated (all the matching networks and hybrid coupler seemed difficult and prone to error), especially for a range of 50 km. Would a simple quarter wavelength monopole whip work? Or should I have two quarter wavelength whips arranged as a dipole antenna? Is there an advantage one way or the other?

One of my colleagues is suggesting we use a fractal antenna - but we definitely can't build them, and I don't think we can find a company willing to build just one (there's a lot of work involved from my understanding of it).

Long story short - I need a lot of advice to point me in the right direction. The staff at my Uni haven't been very helpful, and have only given me part of the picture.

What I need is some advice from someone who knows what he/she is doing on how they'd approach the problem, and/or where I could ready-made solutions/make stuff myself. We have some funding from the Aussie government, but it's not huge. It's been a case of blind leading the blind here and I'd like to move on from here.

TL,DR: undergrads are building an AMSAT like satellite for weather balloon launch and struggling with the RF side of things. We'd like some advice on how to build the antennas for a 145.9MHz uplink and a 436.5 MHz downlink, for a range of no more than 50km. We're using hamtronics R100 for the receiver and hamtronics TA451 for transmission. We have funding, but not a huge amount to buy ready made products. Thanks