HearSat's Frequently Asked Questions (FAQ)
The Expert's View
By Greg Roberts

INTRODUCTION

As a result of John Corby's repeated requests to me for some input from me
on my experiences of tracking articifical earth satellites both optically
and by radio over many years,I have decided to finally get down to the job
and produce something that I hope will be of use to others instead of just
being an interested "lurker"like myself.

Before I start a brief introduction about myself might put things in a better
perspective. I first got interested in Astronomy around 1947 and then space
travel around 1954, so that by the time Sputnik 1 was launched I was an avid
space fan and had constructed my own telescopes etc. I was fortunate enough
to see the rocket carrier of Sputnik 1 and also hear Sputnik 1 on a domestic
shortwave receiver. I still vividly remember seeing a DISCOVERER satellite
pass over early one evening as well as several other fainter satellites from
time but it was only in 1960 that I got bitten with the prediction bug and
decided to learn how to predict satellite passes. This eventually proved
succesful and I became an avid optical satellite tracker for several
organisations.

Eventually I became frustrated with the low success rate I had,primarily as
a result of poor observing conditions which produced clouds most nights and
satellite element information being either non-existant or rather old by
the time it reached me, so I decided radio tracking would be more succesful.
At that time I knew very little electronics or what was involved but I met
a young radio amateur, Arthur Arnold ZS5SU and we teamed together and called
ourselves the Durban Satellite Tracking Station,with myself doing the optical
and prediction work whilst Art did the radio side. I taught him the mechanics
of prediction and in return I learnt something about radio.

This was a very successful combination and lasted till about 1968 when I
discovered the opposite sex and got married and was transferred by the
company I worked for (Mobil Oil Refinery) from Durban to Cape Town. I still
carried on optical tracking but now had to build my own radio equipment-
without much success I must add. After a year in Cape Town I took a position
at the Republic Observatory ,Johannesburg as an astronomer and this was
virtually the end of my visual tracking since I now had to work at night and
sleep during the day. To cut a long story short I finally got going with
radio tracking and started receiving signals etc. After five years at the
Republic Observatory I was transferred to the South African Astronomical
Observatory in Cape Town (still as an astronomer). Shortly after arriving
in Cape Town I passed my amateur radio examinations and was given the
callsign ZS1BI which I still have.Ive also possessed an Experimenters
License for many years as it is illegal in South Africa to possess listening
equipment for frequencies you are not licenced for ,it only being "legal" to
listen to the high frequency bands up to 30 Mhz and the FM band covering 88
to 108 Mhz. (The Experimenters licence specifies what frequency bands you may
possess listening equipment for.) If my memory serves me correctly it is
illegal even to monitor the amateur radio bands unless you have an amateur
licence (!)

One of the first things I did on becoming a radio amateur was to start working
other amateurs through the amateur radio satellites,starting with Oscar 6.I
was very active in this area for many years and one result of this was my
forming S.A.AMSAT in South Africa. Over the years my astronomical activities
changed somewhat and I was able to resume optical tracking on a small scale
as well as carry out my radio work and until I got bitten by the computer bug
I was reasonably active. However since about 1988 I have slackened somewhat,
mainly because Ive become a computer junkie,but with retirement due in three
years time Im now beginning to think about what Im going to do and satellite
tracking features highly,both optically and by radio.I have all the necessary
equipment and electronics expertise to do just about anything I want to and
I joined HEARSAT to get "up-to-date" and rejuvenate my interests.

Since this group is only interested in listening to satellites I wont say much
more about optical tracking and instead concentrate on the radio side.

RECEIVERS

When one first decides to undertake the radio tracking of satellites there are
several factors affecting ones activities,the main one being "do I have the
equipment needed?" or "what receiver do I need?" . It would not serve much
purpose to go out and buy the first impressive receiver/scanner one sees as it
may not necessarily cover any frequency where satellites may be active,so the
first thing to do is find out where the activity is. With the aid of HEARSAT-L
it should not be too difficult to find out on what frequencies some satellites
transmit on and whether they are active or not.

After having chosen one or more frequency bands to monitor your next choice is
what receiver to use. If you have a deep pocket there are some nice commercial
receivers available that will do the job, so the question usually is "what can
I afford, and if I cannot afford to buy, how do I acquire something of use ?".
Unfortunately most of us have limited funds and little electronic knowledge
so invariably its a "make-do" setup one ends up with.

SCANNERS:

I must straight away state that for serious work the common scanner so often
used is of little value.It will help you hear some of the stronger signals but
little else- the reason is that most scanners are either for FM or AM (or both)
transmissions and generally have a fixed bandwidth which is usually too wide
for our purposes. All a wide bandwidth does is produce noise which may swamp a
weak signal.Another problem is that most scanners may only scan in set steps-
say every 5 or 10 Khz.

A scanner is entirely adequate for what it was designed for, namely to monitor
terrestial communications traffic but for many satellites one needs something
better. Another big disadvantage of the scanner is that it usually does not
cover reception of single sideband (SSB) signals. Most satellites do not
transmit single sideband but by using an SSB capable receiver one is able to
detect very weak signals.An SSB receiver also usually has provision to vary
the bandwidth of the receiver and thus reduce noise, and one may also tune the
receiver in very small increments. So what does one now do ? In some countries
scanners were/are illegal or those that are available do not cover the required
frequency bands or are extremely expensive ,so one usually has to look
elsewhere.

CONVERTERS AND SHORTWAVE COMMUNICATION RECEIVERS:

What does help is that two of the most active frequencies ( 137-138 and 150 Mhz)
are very close to the radio amateur 2 metre band (144-148 Mhz).Small units,
known as converters, are sold by several radio amateur supply sources at
reasonable prices and it is usually a simple matter-provided you know what you
are doing - to change these to the frequency of interest. In some cases you may
even specify the frequency coverage you want when ordering and for a small extra
charge the supplier will make the modification necessary.

What is a converter ? Basically its nothing more than the front end of a
receiver and usually contains a radio frequency amplifier (RF amp) followed by
a mixer into which is fed an oscillator and the result of this is that one ends
with two frequencies consisting of the sum and difference of the RF signal
coming in at the antenna and the signal from the oscillator. Suppose you want
to receive 150 Mhz and your oscillator (usually fixed) is running at 120 Mhz,
then you will have 150+120, giving 270 Mhz or 30 Mhz. Whilst it may sometimes
help to go for the 270 Mhz output (you might have a receiver covering 270 Mhz),
it is more usual to go for the 30 Mhz, so usually there is another stage of
RF amplification/selection after the mixer and this is called an intermediate
frequency amplifier or IF amp for short.

One now takes this IF signal of 30 Mhz and feeds it into the antenna input of
a shortwave receiver covering 30 Mhz. Such receivers are very common and
readily available and prices range from a very modest amount to the sky is
your limit. A shortwave receiver is normally described as a communications
receiver and in the cheaper versions will only cover AM and SSB - if you go
back far enough in time you will find SSB described as CW on the dial. Do not
make the mistake of purchasing a receiver that does not have SSB (or CW)
reception capability . Such receivers are reasonably easy to get ahold of-
war surplus stores, amateur radio hamfests or even in your local newspaper
FOR SALE column. If you know nothing about communication receivers try and
find a radio amateur in your area and approach him/her for help. Most amateurs
will be only too pleased to help and may even have a spare communications
receiver that he/she is prepared to part with.(I accept no responsibility if
you find a "crooked" ham!). Ideally you should get a communications covering
"all modes", digital frequency display and a "signal strength meter"- all
useful if you can afford it.

So how does one change the frequency of a converter designed for say 144-146
and an intermediate frequency of 28-30 Mhz Mhz to one covering 150 Mhz ? The
easiest way is to change the frequency of the oscillator and keep your IF
fixed. The amateur converter will probably have an oscillator running at
147 - 29 = 118 Mhz and is usually crystal controlled so all you have to do
is change the oscillator frequency to 120 Mhz and you will have frequency
coverage of 120+28 to 120+30 or 148 to 150 Mhz -ie when you tune your
communications receiver to 28 Mhz you will actually be tuned to 148 Mhz and
similarly 30 Mhz corresponds to 150 Mhz. It should be obvious that all you
have to do is add 120 Mhz to the frequency shown by your communications
receiver dial.

There are a few extra things to do to "peak" up the system - eg realign the
front end of the converter for a frequency of 150 Mhz instead of 145 Mhz,
check that the new oscillator is set at 120.0 Mhz (otherwise your read-out
frequency may be off by a few kilohertz) and most radio amateurs can do this
for you if you dont know yourself. One may well ask why the 28-30 Mhz band
is chosen as the intermediate frequency - this is usually where one finds
the "cleanest" frequency range. Dont forget your receiver is basically
receiving on (say) 29 Mhz as well as (120+29=149 Mhz), so if there is a
strong signal on 29 Mhz you will hear a signal that comes from 29 Mhz rather
than 149 Mhz - this is known as a "birdie" and is easily checked for by
switching of your converter and seeing if the signal is still there. With
a good communications receiver and some care - for example using co-axial
antenna cable between the converter and the receiver one can get rid of
most birdies.Another usually strong source of birdies is your PC, so either
make sure the PC is switched off or placed sufficiently far away from the
receiver that you dont hear your PC going through its "grunts and groans"
as it churns away in the background.

It should be obvious from the above that a good communications receiver is
a top priority item since, with suitable converters, you can cover any
frequency range. It is also useful for other purposes so is not a wasted
investment. Besides there are a fair number of amateur satellites that
use the 29 Mhz region and even an occassional Russian satellite. It is
also possible to monitor other space related activities- launches,tracking
stations etc if you know where to look for them.

Whilst I played down the role of scanners earlier, it is possible to make
good use of such if you have the knowledge and a communications receiver.
As an example I have a scanner covering 10 Mhz to 550 Mhz with the usual
AM and FM and scanning in increments as small as 5 Khz. By following the
circuit layout with a circuit diagram supplied by the manufacturer I was
able to determine that one of the IF frequencies inside the scanner was
5.5 Mhz. It was a simple matter to add a decoupling capacitor to a piece
of co-axial cable and attach the other end to the IF stage and feed the
5.5 Mhz IF into a communications receiverset to 5.5 Mhz. I could now receive
SSB signals as well as "detune" off the 5.5 Mhz so cover several Kilohertz
either side and thus "fill in" the 5 Khz gaps of my scanner. I am pretty
certain that most scanners can be used in a similar manner but as already
stated you need to know what you are doing, otherwise you could either blow
up something or even electrocute oneself if you have any dangerous voltages
in the equipment. Even if you are an "expert" it pays to be careful - I
once had a 600 volt shock from a transmitter which literally threw me across
a room and made me feel real sick - so that today I will not work on any
piece of equipment that may have a high voltage .

DEDICATED FREQUENCY RECEIVERS:

One should not ignore dedicated receivers. They may be found from time to
time but are usually expensive. War surplus used to be a good source but
when buying such it is useful to know what you are doing . With a little
knowledge you should be able to retune/align a receiver that is close to a
satellite frequency band. Unfortunately good equipment is rare and one may
have to look hard and long.

USEFUL ACCESSORIES:

A useful accessory is a spectrum analyser. This allows one to visually see
a portion of the radio spectrum at one time and thus keep a lookout for
signals that may pop-up. Some radio amateurs use such units and they usually
cover a few hundred kilohertz at a time- these units are sometimes called
panoramic adapters. It is possible to build such units or acquire such second
hand. A true spectrum analyser is relatively rare but if you come across
one its worth spending some money on it.There are at least two types- those
that have a set RF input range and you can see up to a few Megahertz either
side, or a true spectrum analyser that is virtually a complete radio receiver
but has a visual display of what the receiver hears. Nice but generally very
pricey.

Another useful piece of equipment is a signal generator. This can be used
to align your converter etc but unless the unit is accurately calibrated or
highly stable frequency wise, be a little careful. It will also pay to have
a frequency counter to check that the signal generator is actually on the
frequency you think it is. A good signal generator will put out a signal
on diffent modes and one can normally vary the strength of the signal that
is put out. When you start aligning a piece of equipment that is anything
but aligned you may need a whopping signal,but as you peak up the converter/
receiver you want a weaker and weaker signal. Great fun tuning a piece of
equipment. If you dont have a signal generator then its useful to have a
crystal oscillator that puts out a signal that will appear in your converter
coverage - for example a 30 Mhz crystal controlled oscillator will put
out harmonics at 30 Mhz ( checks your communication receiver ) and one also
at 150.0Mhz.( Twist a few strands of plastic covered hook-up wire around the
crystal can if you want to hear the harmonics better ).

ANTENNAS
--------

Whilst receivers are an essential part of any radio satellite tracking setup,
the most important part is the antenna, so it usually pays to spend some time
and effort in setting up a reasonable antenna.

Some satellite signals are so powerful ( Soviet satellites around 150 MHz and
the NOAA/METEOR satellites on 137-138 MHz) that they can be heard with just a
random length of copper wire stuck into the receiver antenna socket. Replace
this with a suitable antenna and one will be amazed at the difference-whereas
the signal before was noisy, suffered fading etc it will now be a solid signal
completely free of any noise and rich with sidetones etc that you never even
guessed at before - its difficult to describe just what a difference a good
antenna makes. It will also enable you to hear the much weaker signals as well
as get noise free signals, for example, from the weather satellites from
horizon to horizon.

Antennas come in all shapes and sizes- some are easy to construct whilst others
need fancy test equipment. The assumption is made that the reader does not
have test equipment nor does he/she understand all the technical jargon that
one discovers when paging through a book on antenna design and it is not the
intention of this series to provide instructions on how to design/build
antennas but rather provide some pointers as to what kind of antennas to build
and where one may find some of this information.

My first recommendation, if you do not yet have the book, is to get "The
Satellite Experimenters Handbook" by Martin Davidoff,K2UBC and published by
the American Radio Relay League,225 Main Street,Newington,CT 06111. My copy
was published in 1990 and cost $20.00. This book, whilst mainly written for
radio amateurs, contains a terrific amount of information on just about
anything connected with satellites.It is suitable for both the beginner and
the more advanced user and should be in every satellite trackers bookshelf.

However,lets get back to antennas. As a consequence of satellite transmissions
occurring over a wide range of frequencies, no single antenna is ideal for all
transmissions. The antennas required for receiving the signals found around
29-30 Mhz are completely different from that required for say 1700 MHz and in
most cases one "designs" an antenna for a specific frequency range. Another
important thing to appreciate is that on the high frequency bands ( less than
30 MHz) one will encounter several modes of signal propogation, one of which
is "sub-horizon" reception,which do not normally occur on the higher frequency
bands.

The following will illustrate "sub horizon" reception:

suppose you want to track a satellite beacon thats operating on 29.50 MHz.You
run a prediction program that you know gives the correct answers and AOS or
acquisition of signal is predicted for say 7.00pm local time. A few minutes
before time you tune the receiver and start listening and sure enough there
the signal is but its too early!. The signal may last a few minutes and then
disappear completely and then at the predicted time it will appear but much
stronger and it will behave normally until LOS or loss of signal when you may
again have a fade out and then a re-appearance of the beacon for a short time.
This "hearing early/late" is a result of ionospheric propagation. During the
daytime it is sometimes possible to hear satellite HF beacons when the
satellite is nowhere near your location and again its due to our ionosphere.
Monitoring for this kind of reception is very interesting, especially if you
can predict where the satellite was at the times you heard it.

As our frequency goes to 137Mhz or higher we essentially receive the signals
by "line of sight" - if you dont have a clear optical path to the satellite
then you will not hear it. (This is not strictly true - for example with your
antenna inside a room you may still be able to hear the satellite, but there
will be a definite improvement if you take your antenna outside and mount it
high enough to clear local obstructions). Another thing to bear in mind is
that radio signals do not like passing through trees or other obstructions
and as one goes higher in frequency one can encounter problems with rain,
mist etc which attenuate or weaken signals . There are other factors which
can affect the strength of received satellite signals eg faraday rotation,
spin modulation,polarization etc which one can think about when one is more
experienced.

Earlier I said one needs a clear optical path to the satellite- ie you need
to be able to "see" the satellite ( even if its too faint to be seen visually).
An example of this will illustrate this point - to the west of my location I
have the very scenic Table Mountain (Cape Town, South Africa) which ,in parts,
goes up to about 8 degrees elevation as seen by my antennas. Sometimes I may
need to track very low elevation passes - eg in orbit determination - and will
attempt such passes to the west as I am able to hear the satellite for brief
spells as the horizon elevation profile changes and the satellite is in the
"clear" for a brief time. Having plotted the mountain chain profile I can then
pinpoint where the satellite is. In the days when I used to work Oscar 7 by
radio I had a 30 second "window" which enabled me to contact South America
radio amateurs via the satellite and I made several contacts this way.

Directional or omnidirectional antennae:

An omnidirectional antenna more or less covers the entire sky and one will
not need to have any kind of tracking mechanism to follow a satellite as it
crosses the sky. The big disadvantage is that it has low gain and its not
very easy to determine where the satellite is in the sky. There are other
disadvantages, such as fading due to lobes or areas where the gain of the
antenna is not uniform, and the orientation of the antenna which may favour
a particular direction more than another.

A directional antenna gives a higher gain and enables one to pinpoint where
the satellite is in the sky but has the disadvantage in that ones needs some
kind of tracking mechanism if one wants to "track" the satellite across the
sky.For any serious kind of work this kind of antenna is a must. I will deal
with directional antennae in a later part.

The simplest antenna, which can be made for most frequencies of interest,
is the horizontal half wave dipole. At 29.5 Mhz the antenna is about 16 feet
long and at 150 Mhz it is only about 39 inches. One can either use copper
wire,or better still aluminium tubing. By mounting the dipole on a rotator
one is able to move the antenna in a horizontal plane and make use of the
gain pattern of the antenna to determine the direction of the satellite. The
gain pattern depends on the height of the dipole above ground and the best
height is about 3/8th wavelength ( 1 wavelength (metres) = 300/frequency where
the frequency is in MHz ), so for 29.5 Mhz mount the antenna (3/8)*(300/29.5)
metres above ground or about 4 metres above ground level.( Ground is here used
in the sense of a perfectly conducting ground ). Whilst it is practical to put
the 29.5 Mhz dipole at 4 metres or so, the 150 Mhz antenna needs to be at a
height of less than 1 metre above ground, but what about local obstructions
such as buildings etc ? . No problem - the clue lies in " a perfectly conducting
ground" - this you can create by using a reflecting screen beneath the dipole,
which can be made from a grid of wires or other conducting material.Finally
the half wave dipole can be mounted vertically if one wants to experiment
but does present problems with the co-axial feed line if one wishes to rotate
the antenna.

Another kind of antenna is known as the groundplane. This consists of a
quarter wave (usually) or 5/8 wavelength vertical rod/wire and fitted with
a groundplane made of three or four horizontal or drooping rods that are at
least a third of a wavelength long. (The small antenna usually supplied with
scanners is a very crude variation of this without the ground plane and is
for most satellite purposes virtually useless). The groundplane antenna has a
low gain and for the polarization minded uses linear polarization . These
antennas are very easy to make and produce acceptable signals for most low
altitude orbiting satellites of interest.

An omnidirectional antenna which has some appeal is the discone antenna. This
has the advantage that it provides some gain over a wide range of frequencies
and whilst it would be unwise to use it with a transmitter - for those that
have a radio licence - it is useful for satellite work.For example one could
construct one that is useful from say 100 to 500 Mhz.I have used one for many
years and been a satisfied user. These are not that easy to construct and one
needs to do a bit of designing.Fortunately they are available commercially at
modest prices.

Another antenna that I have used with great success is the turnstile reflector
antenna ( or T-R for short ). This consists of two dipoles mounted at right
angles to one another and mounted above a reflector screen.The two dipoles are
connected together by means of a 90 degree delay line a quarter wavelength long
and made from 90 ohm co-axial cable. The radiation pattern is nearly omni-
directional in the horizontal plane and the vertical radiation pattern depends
on the spacing to the groundplane reflector. Like the horizontal dipole the
"best" spacing is around 3/8th wavelength. It is relatively easy to make and
produces excellant results on the navigation,weather and other satellites.

Two other antennas that are popular for low altitude satellites are the
Lindenblad and quadrifilar helix antennae. Since I have no experience with
these antennas ( they appeared on the amateur scene after I had "retired" from
active satellite tracking ), I do not feel qualified to write about them.They
are described in the book mentioned earlier, along with the antennas I have
already briefly discussed. From all accounts they are ideal broad-beamwidth
antennas with the Lindeblad easy to construct and the quadrifilar helix
being somewhat more difficult.

Coaxial Cable and connectors:

All the antennas described make use of co-axial cable to convey the signal
from the antenna to the receiver. Since this conveys the signal it should be
apparent that this should be of good quality. Co-axial cable is sold in
difference impedances,measured in ohms, and one should select the appropiate
impedance for the antenna used - usually 50 or 75 ohm. What is more important
is the attenuation of the signal in the cable and this is usually specified
as so many dB per 100 foot length at a particular frequency. These figures
apply to new cable and could be worse for old or cheap cable,so beware. The
signal attenuation is dependent on the length of the co-ax so try and avoid
long runs of co-axial cable. The best way to ensure your signal does not end
up at the other end of the cable is to use cheap high loss cable.

Unfortunately ones problems do not end here.The co-ax cable has to be attached
to the receiver. Beware the so called UHF connector which uses a PL259 plug
and SO-239 receptacle - the UHF does NOT mean good for UHF and try and avoid
using them on any RF equipment operating above 30 MHz. The N type connectors
are relatively safe to use as well as the BNC connectors.

Preamplifiers:

Up till now I have not mentioned preamplifiers. These are very important and
there will be very few systems that will not benefit from using one. As the
name implies they appear before the first stage of RF amplification in a
receiver ( - a receiver may have 1 or two stages of RF amplification before
the mixer and this may be regarded as a preamp, but in the sense I use it,
it appears between the antenna and the receiver unit.

What is important in any RF receiving system is the noise level and it is
easily shown that the so called noise figure of the first RF amplifier in any
system has a major effect on the noise level of the entire system, so the
lower the noise generated in the preamplifier, the quieter your overall system
and the better the ability to detect weak signals.

At frequencies of around 130 to 150 Mhz the noise level of the preamplifier
is not too important as most RF solid state devices today produce less noise
than the noise one receives from space. It is only when one starts to go
higher in frequency that the preamp noise level plays an ever increasing
factor. Despite this is does no harm to have a good preamp at 150 Mhz and the
best place to put this is at the antenna itself. Some people make the mistake
of putting it at the end of the co-ax run just before input into the receiver
and all they are really doing is amplifying the noise mostly generated in the
cable. With weak signal reception one is always battling to increase the
signal to noise ratio (S/N). This is at its best at the antenna,so putting
the preamplifier here will improve matters . As pointed out one loses some
of the signal in the co-axial cable - no matter how good the co-ax - so the
end result is a lower S/N ratio at the co-ax end so it should be obvious that
it does not pay to place the preamp after the co-ax run. A good preamplifier
will produce 20-25 dB of extra gain and this will make a dramatic difference
to any signal.

Finally it will pay to waterproof all connections to your antennae and co-ax
connectors. Water has a nasty habit of getting into funny places - I once
opened a preamplifier that sound pretty dead and found it full of water, so
check all connections. Water in co-axial cable also will eventually cause
problems and I used to regularly replace all my coaxial cable about once a
year until it became too expensive.

-- End --