·
A Repeater in Space?
Well, nearly. The uplink (equivalent to a repeater input, or
where you transmit) and downlink (equivalent to a repeater output, or where
you receive) are on different bands. The point of having the uplink and
downlink on different bands is that it allows you to monitor the downlink
(normally using headphones to stop feedback), and check that you are making it
in. If the uplink and downlink were on the same band, we’d need some expensive
filters so that we could listen to the downlink without our uplink de-sensing
the receiver. It’s possible to operate without listening to the downlink, but
it’s not recommended unless you’re pretty experienced. Being able to hear your
signals coming back from the satellite will save you a lot of frustration!
·
How far can I get?
The coverage area, or footprint, (see figure 1) is huge compared
to a terrestrial repeater, and is typically 3,000 miles in diameter. Depending
on the pass (how the satellite travels relative to the groundstation),
most if not all of Europe is usually available, together with Northern Africa.
It’s often possible to operate across the Atlantic. This means that there are
potentially hundreds of thousands of amateur stations who could simultaneously
operate the satellite. With only one channel, this satellite is inevitably very
busy! QSOs are generally contest style, with an exchange of callsigns, QRA
locators, signal reports, and occasionally names are exchanged.
·
Prediction
The
satellite is continually orbiting the Earth. This means that it is not always
visible to an observer. So how do you know when it will be visible? The answer
is to use prediction software. There are a number of software packages
available, including freeware. Check out http://www.amsat.org and
http://www.amsat-uk.org. Predictions are also online, at http://www.amsat.org/amsat-new/tools/predict/
and http://www.heavens-above.com/.
Having
a prediction listing together with a magnetic compass will allow you to plan
the pass in advance by knowing where to point the antennas. If you’re not
completely confident about the software you’re using yet, having a listing
printed out in advance will save a lot of ‘computer panic’ during the satellite
pass.
·
Doppler
Because
the satellite is moving relative to the ground station, it’s necessary to be
aware of a phenomenon known as Doppler shift. This is similar to the
apparent drop in frequency you hear when a police car screams past with its
siren on. When the satellite is hurtling towards you, the frequency appears higher
than when it is going away from you.
o
As
a rule of thumb, at the beginning of each satellite pass (known as AOS,
or acquisition of signal) on the 435.300MHz downlink, the frequency on the
downlink will appear at the observer to be about 10kHz above the
published frequency. At the end of each satellite pass (known as LOS, or
loss of signal) the downlink will appear to be about 10kHz below the
published frequency. The frequency on the satellite in fact remains constant.
o
On
the other side of the coin, on the uplink on 145.920MHz, in order for this
frequency to be correct at the satellite, you can uplink at about 3kHz below
the published frequency at AOS, and 3kHz above at LOS. In practice, it’s
generally not necessary to adjust the uplink on a 2m FM uplink, but you will
benefit from being able to tune the downlink. On some radios it’s possible to
set the uplink frequency together with the 67Hz PL tone into a memory, which
will help.
o
It’s
possible to automatically correct for Doppler using a suitable computer program
and transceiver.
·
Equipment
A
commonly held misconception is that you need large Yagi arrays and expensive
rotators to operate satellites, not to mention expensive radios in the £1k
bracket. This is not true! To operate AO-51, you need a radio that can receive
FM on 70cm and transmit 5W FM on 2m with a 67Hz PL tone, i.e., a typical
handheld radio these days. You can also use two separate radios, one on each
band, equally well. If you use a single dual band radio, it’s extremely
worthwhile using one 
which allows you to transmit
on one band while simultaneously receiving on another. Ideally the radio should
be able to tune to within 5kHz so that you can correct for Doppler.
Pre-programming the radio’s memories will help here if you’re using an FM only
radio. Be aware that some ‘satellite-ready’ radios have a problem setting PL
tones in satellite mode.
The
old adage, “if you can’t hear them, you can’t work them” is especially true in
satellite operation. So before considering increasing your ERP, concentrate on
your downlink! This can either be with antenna mounted low noise preamps,
better quality coax and/or better antennas, depending on the configuration.
Because the signals on the downlink are not always very strong, and
because the downlink will have fading due to polarisation changes, it’s
recommended that the receiver’s squelch is opened permanently during the pass.
It
can be tricky to receive AO-51 using just the rubber duck antenna supplied with
most hand held radios. Using a small hand-held directional antenna will help enormously.
Even an HB9CV, or alternatively a three element yagi will make quite a
difference on the downlink. For AO-51, you shouldn’t need more than about 10W
ERP to make contacts, but this is assuming there’s no other stations running
QRO and have insensitive receivers. There’s nothing more frustrating than
knowing that you’re making it in (by listening to yourself on the downlink),
only to be trampled on by a station who clearly cannot hear the satellite at
all. These operators are known as alligators – all mouth and no ears.
One antenna, now available in the UK from AMSAT-UK is the Arrow Antenna
(Fig. 3) (http://www.arrowantennas.com/146-437.html)
which is infamous in the amateur satellite community. Alternatives include homebrewing
your own antenna. Two WA5VJB antennas (http://www.clarc.org/Articles/uhf.htm),
one for 70cm and the other for 2m mounted orthogonally on the same boom
(http://www.g6lvb.com/HomebrewArrow.htm), or commercially, the Cushcraft
A270-6S.
The
real benefit of these hand held Yagis over a combined azimuth and elevation
rotator system is that the operator has feedback and can very rapidly correct
for polarisation mismatch by turning the boom along its axis.
·
Operator conveniences
AO-51
passes are short (normally only 15 minutes or less), so preparation will pay
off. Also keep in mind that Wednesdays are Experimenters’ days when the voice
transponder may be switched off – check http://www.amsat.org/amsat-new/echo
for the schedule.
Consider
all the tasks you’ll be doing:
o
Retune
due to Doppler: on FM-only radios, pre-program five memories in 5kHz steps into
the memories for the downlink: 435.310, 435.305, 435.300, 435.295, 435.290. The
start of the pass will be at 435.310 and will decrease in frequency.
o
Key
the PTT: you might want to consider a foot switch. Beware of VOX: the VOX tail
delay can hamper the rapid QSO style on FM satellites. There is a short delay
for the uplink PL tone to be recognised.
o
Hold
the microphone: a headset/microphone combination is a good idea, giving you a
free hand. Use headphones to avoid feedback in full duplex operation!
o
Point
and orient the antenna: although the antennas are light, your arm will start to
ache during the pass!
o
Check
the predicted heading and elevation with the prediction listing and a compass:
it’s worth preparing for the pass by tracking the predicted pass of the
satellite, ensuring that there are no obstructions.
o
Log
the QSOs: a cassette recorder or dictation device helps a lot here.
o
Have
a QSO in a very busy channel: patience is a virtue here. It is likely that you won’t succeed on your first attempt,
and this is where preparation and the learning experience come in.
·
Good luck!
Satellite operation is a challenge, both technically and
from an operating perspective. But you don’t need large or expensive equipment
to operate them. Remember to support AMSAT-UK who represent amateur satellite
operators in the United Kingdom. Finally, welcome to the space age and the
world of amateur satellites! 73,
Howard G6LVB
Operating AO-51
AO-51 is a satellite in the Amateur Satellite
Service, and one of its functions operates very similar to a traditional
terrestrial FM repeater. It’s available for general amateur use, and described
below are the operating techniques and equipment used to access it. As with
many areas of technology, satellite operation includes its fair share of
jargon. Although not exhaustive, a number of these terms are used in this
introduction, with corresponding explanations. Good luck & welcome to the
world of amateur satellites!