[amsat-bb] Re: FUNcube Project - 7 dBi omni

Tue Apr 9 14:08:05 PDT 2013

> ... 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: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] 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 AMSAT-BB at amsat.org. 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