... which is going to be the best type of antenna... to receive the 145MHz downlink signals
There are lots of good hemispherical satellite antennas that provide equal gain for all satellites in view. However, these designs ignore the fact that satellites on the horizon (LEO passes spend almost 70% of their time below say 22 degrees) are so far away, most of them are too weak to be decoded until they get closer (and higher) anyway. Given the weak downlink (300mw) of FunCube there is no -omni- antenna that is going to decode that signal at the horizon when the satellite is over 1500km away.
In this light, it is better to simply sacrifice low angle coverage and concentrate the gain higher up to make sure that when the satellite does get to high angles, then there is plenty of gain to decode it.
The simplest and best performance (for LEO satellites) and easiest to construct is a 3/4 wave vertical over a ground plane. It has over 7 dBi gain above 25 or 30 degrees and requires no matching circuit. Just shield to ground plane and center coax to the 3/4 wave whip.
The antenna pattern is ideal for LEO satellites because it concentrates the gain above 30 degrees and does not waste gain down on the horizon where satellites are 6 to 10 dB farther away and not decodable on an omni anyway unless the satellite has decent power (which the fun cube does not).
Background: In the past, a 19" whip is an ideal 3/4 wave gain vertical for UHF (over 7 dBi) and also acts as a 1/4 wave vertical (5 dBi) for VHF. A nice dual band antenna for satellite work with weak UHF downlinks. But then Funcube is not UHF, but VHF. So you want more gain on the VHF downlink, so you would want to make this antenna almost 58 inches long to be 3/4 wave on VHF (and ignore it for UHF where the gain pattern would be too narrow). Also, by having reduced gain on the horizon, the VHF link will have less noise to contend with.
You don’t you see this excellent antenna more often because it is useless for terrestrial operation (negative gain on the horizon), and for satellites, most people think they want omni-gain on the horizon even though for anything other than the 10W transmitter on the ISS, they still won't decode anything on the horizon until the satellite gets higher (and closer) anyway.
So my receommendation is the 3/4 wave vertical for unattended fixed omni operation with high gain above say about 25 degrees and use a gain YAGI and tracking antenna if you need to have coverage down to the horizon.
The over 7 dBi gain of this simple 3/4 wave antenna plus the nearly 5 dB path-loss (and noise) gain of VHF over UHF for omni antennas makes this link over 12 dB better than a comparable UHF downlink to-an-omni experience with cubesats.
My opinion anyway. You can see some of my early writings on this antenna about 80% down this page: http://aprs.org/astars.html
Bob, Wb4APR
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Graham Shirville Sent: Tuesday, April 09, 2013 3:47 PM To: AMSAT BB Subject: [amsat-bb] FUNcube Project - Your advice requested
Hi All,
As part of the FUNcube project we now need to urgently decide which is going to be the best type of antenna to propose for use at schools and colleges to receive the 145MHz downlink signals from both FUNcube-1 and FUNcube-2 on UKube after launch.
Both spacecraft will be transmitting approx 300mW of BPSK 1k2 telemetry with FEC – FC1- will be using a dipole antenna and will have passive magnetic attitude control. FC-2 on UKube will use a single monopole antenna and the spacecraft is intended to have active attitude control. This is primarily to ensure that the deployed solar panels are illuminated and that the face with the S band patch is usually earth pointing.
We believe that the resultant signal should be easily usable using an omnidirectional antenna but the questions is which would be the best type to use. Obviously ease of construction, cost, robustness and safety in a school environment will also be important factors as well as suitable “gain” and circularity from horizon to horizon. We envisage that some operations may be from fixed installations with the antennas mounted permanently but some will be temporary - perhaps hand held or bolted to a step ladder or similar.
We know there are many AMSAT members around the globe with great experience in this field and, we suspect, with some strongly held views. So please let us have some constructive input about which type is likely to be best for this purpose - your comments, ideas and lessons learnt will be very valuable for us.
Thanks in advance for your support
73
Graham G3VZV - for the FUNcube Project team
As part of the FUNcube project we need to decide which is going to be the best antenna to propose for use at schools and colleges to receives the 145MHz downlink signals from both FUNcube-1 and FUNcube-2 on UKube after launch.
Both spacecraft will be transmitting approx 300mW of BPSK 1k2 telemetry with FEC – FC1- will be using a dipole antenna and has passive magnetic attitude control. FC-2 on UKube will use a single monopole antenna and the spacecraft is intended to have active attitude control to ensure that the deployed solar panels are illuminated and that the face with the S band patch is usually earth pointing.
We believe that the resultant signal should be easily usable using an omnidirectional antenna but the questions is which would be the best type to use. Obviously ease of construction, cost, robustness and safety in a school environment will also be important factors as well as suitable “gain” and circularity from horizon to horizon.
We know there are many AMSAT members with experience in this field and, we suspect, some strongly held views but all constructive input would be very welcome.
_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb