Welcome to Issue #5 of the Radio-Sky Journal
Copyright 2001 by Radio-Sky Publishing
All rights reserved.
Effective Apertures of non-Dish Antennas
Selecting a Frequency
Radio-SkyPipe Update Available
Movie Review: The Dish
Editorial: The Changing Face of Amateur Radio Astronomy
Amateur Tip #6
Effective Apertures of non-Dish Antennas
Many radio astronomy related formulas use the effective
aperture(Ae) as a parameter, Using Ae will, for example, give a
direct theoretical value for the power available (P) at the antenna
terminals from a source of known flux density S by the simple
relation P = S * Ae. For horn antennas and dish antennas, Ae
is typically between 70 and 50 percent of the physical area of the
opening. What do we do when our antenna is not one of these types and
instead is comprised of a collection of wire or tubing elements
such as we have with dipoles and yagi arrays?
You can still assign an effective aperture to these types
of antennas because the aperture is directly related to
the gain, G, of the antenna. The gain of an antenna is the
ratio of the power received from a source in a given direction
to the power received by a fictitious isotropic antenna that
receives power equally from all directions. The gain of an
antenna can be calculated using antenna analysis software such
as EZNEC, taken from a manufacturer's spec sheet, or even
measured directly using a far field transmitter.
For our simple example lets use our dear friend the half wave
dipole antenna. A dipole has a gain of 2.14 dB over the isotropic
antenna. This is equal to a power ratio of 1.63. This is well
known but can be calculated again if you wish using the formulas
from most antenna texts. We plug this into the formula relating
Ae to G.
Ae = (G*L^2)/(4*Pi)
Where L is the wavelength of the frequency of observation in
the same units as Ae. Thus the frequency is important to the
conversion. Thus at 50 MHz, (L = 6m), for example, the dipoles
Ae = (1.63*6^2)/(12.57) = 4.7 square meters
It was pointed out to me that "the effective area of a dipole is
0.13*L^2 which is almost the same as the product of
(L/2)*(L/4) = 0.125*L^2 - so the effective area of a dipole is an
area half a wavelength long (the length of the dipole) times
a quarter of a wavelength wide - more or less."
Doing the calculation for a 144 MHz dipole we get roughly 0.5
square meters. So we see our aperture falling off quickly at higher
frequencies due to the squaring of the wavelength in the formula.
To achieve a constant Ae at higher frequencies we must increase
the gain of the antenna by, for example, adding more elements.
As a last example, assume you have a 432 MHz yagi antenna with
a specified gain of 7 dB over a dipole (an often used way of
specifying antenna gain in the literature). Since the dipole
has 2.14 dB of gain over the isotropic antenna, your antenna
would have 2.13 dB + 9.14 dB = 9.14 dB of gain over the isotropic,
but remember our formula above specifies G as a simple power
ratio, so you must first convert from decibels to this form. To
do this you divide first by 10 to get 0.914 and then take the
antilog (10^0.914).This equals a power ratio of 8.2. The wavelength
(L) = 300/432 = 0.694 meters. Now we can plug into our formula:
Ae = (8.2*0.694^2)/12.57 = .31 square meters.
 This grand simplification assumes 100% antenna efficiency,
and refers to the power from the stated source only. In
practice there will be other contributions to actual power
measured at the antenna terminals such as other sources within
the field of view, side lobe contributions, and the physical
temperature of the antenna.
 Personal correspondence from Richard Flagg.
Selecting a Frequency for Radio Astronomy
It is often tempting for beginning amateur radio astronomers to
jump into the microwave dish world for their first project. Some
do it quite successfully, but I think the majority soon become
frustrated with their results (or lack of them) at 4 GHz and
above. It is important to take a little time to understand the
implications of working in a particular frequency range. There
are many factors to consider and the selection process involves
a series of compromises.
Compromise 1 - Signal Strength vs. Frequency
The two major forms of emission by cosmic radio sources are
thermal emission and synchrotron emission. A few special cases
don't fall into these categories but we need to be general here.
Thermal emissions come from every thing that's out there that can
absorb and re-emit energy. Two important characteristics of
thermal emission are:
1) The energy per unit wavelength is inversely proportional to
the fourth power of the wavelength.
2) The energy per unit frequency range is proportional to the
square of the frequency.
Put more simply, most of the thermal radiation occurs at higher
frequencies. Even the Sun which is pretty darn hot and close to
us puts out very little thermal energy below a few tens of MHz,
(though the "active Sun" has other emissions which at times can
be very strong in that region). The Sun is always detectable
at mid-VHF and above frequencies, however, due to it's
thermal nature. The Moon is detectable as a thermal source
because it is so close to us, and while some more distant thermal
sources such as the Orion Nebula are detectable with small
microwave radio telescopes, in general, the number of sources is
To recap - Thermal sources are best observed at microwave
frequencies but there aren't too many of them you "see" without
the aid of a very large aperture telescope.
Synchrotron emissions are responsible for much of the unique
structure of the radio-sky. These emissions derive from
electrons accelerating in a magnetic field. Even the thermally
cool and diffuse gases of the Milky Way appear bright at radio
frequencies due to this emission mechanism. Distant quasars
can be detected with small aperture antennas because they contain
powerful magnetic fields and thus synchrotron processes. This
type of emission predominates at the low end of the radio
spectrum and tapers off in strength rapidly as we go to higher
frequencies. Many synchrotron sources which appear quite strong
below one GHz are difficult to detect with a 3 meter dish at 4 GHz.
Synchrotron wavelengths are where the action is! Pulsars emit
by the synchrotron mechanism, for example.
Compromise 2 - Resolution
Resolution refers to the ability to distinguish or "resolve" two
objects with a given radiotelescope. For a given size single
antenna system, this ability to distinguish between objects
increases with frequency. We often put this in terms of antenna
beam width. A narrower beam width means higher resolution. So
we can get better resolution by either increasing the antenna
aperture or by increasing the frequency at which we operate.
You can also achieve better resolution by using more than one
antenna in an interferometer arrangement. This should probably
be done later after you have had some experience with a single
antenna at your chosen frequency.
Compromise 3 - Available Spectrum
You need a clear frequency to work on. If the interference level
is high at your location, then this may be your primary criteria
for deciding on a frequency at which to operate. Use a wide
range communications receiver to survey the available spectrum
at your observatory. Narrow down some areas of the spectrum that
look "clean", and then run some longer term observations using
simple antennas (like dipoles) at these frequencies to determine
if they stay clean 24 hours a day. In urban locations, it is
sometimes nearly impossible to find a free slice of spectrum
without escaping into the microwave region. If there is no good
frequency range to use, you might consider a frequency which
lends itself well to a portable observatory which you can take
out into the country on occasion. For years I did this at my old
location in Kentucky with many fun excursions using 38 MHz dipole
and 144 MHz yagi interferometers.
There are a host of other factors to think about. You may not
have much electronics background and need to buy off the shelf
equipment. You may be limited on space. Of course, there may be
a certain phenomena that you want to study (such as Jupiter's
decametric emissions) that determines your frequency selection.
The main point I am trying to make is that you should look at
all of the implications of selecting a frequency before jumping
into a project and expending many personal resources only to
find that you have made a compromise without understanding all
that it entails.
Radio-SkyPipe Update 1.1.24
There is a new update available for Radio-SkyPipe. Among the
new Free Standard Edition features you will find:
-New layout allowing better screen positioning when two charts are
in view. Controls have been re-arranged to the side of the chart
and are now hide-able. Choosing between left and right mouse
clicks of the positioning and sizing buttons gives two ways of
controlling chart behavior.
-A Manual Chart View feature allows you to define the area of the
chart by inputting times and Y axis values, so you don't have to
scroll to find a spot.
-You may now set the program to "Publish" your existence automatic-
ally. No more forgetting to push the Publish button.
-You may now input your observatory information text within the
program. A text editor is not required.
-Automatically match Y Axis scale values of a remote SkyPipe Server.
-Multiple ways to define your network connection to work with or
around a firewall and local LAN have been added.
-Friendlier time and date entry for running scheduled observations.
The Pro Edition has some great new additional features:
-SkyPipe Upload Manager allows you to upload jpeg images of charts
to a website automatically.
-A new Find utility will search your observation files for a
given event time and display it.
-The Atomic Time utility will synchronize your computer clock
with one of the internet time servers.
-An Observer Log is available for note taking with time stamps
and easy inclusion of text from your Chat window.
-Remote Administration allows you to start and stop charts at
some remote network-connected site. By uploading a new "Profile",
you change most any option on the remote SkyPipe installation
without being there.
[This is not a complete list of the Pro Features see the website
for more details.]
Of course, bugs have been fixed in both versions. If you
haven't picked up a copy, you may always find a link to the
Special Note: If you recently downloaded Radio-SkyPipe and
receives a "DAX Error" when you tried to run the program,
download it and try again. There was an error in the setup
list for the program that caused this problem.
Movie Review : The Dish
I promise not make this a regular feature, but I must tell
you about a video I just watched. "The Dish" is about the
the involvement of the Australian Parkes radiotelescope as a
relay station in the Apollo 11 moon landing mission. This is not
some dry documentary, nor is it silly science fiction. The Dish
is based on the true story. There is plenty of humor from a
host of wonderful characters that inhabit the small community
of Parkes. What really struck me was how incredibly authentic
everything seems. The equipment is of the right vintage. The
techno-speak was excellent and applicable. With a single viewing
I saw nothing that seemed unrealistic. I didn't recognize any
big name actors, but the NASA representative might have been
Elaine's boyfriend on Seinfeld.
As an incentive to get you to watch this, I am offering a free
Radio-SkyPipe upgrade or Radio-Sky CDROM to the person who
sends me the most complete list of technical inaccuracies in
the movie. Send your list to firstname.lastname@example.org. The winner
will be announced in the next issue of the RS-Journal.
Editorial: The Changing Face of Amateur Radio Astronomy
You may know that it was an amateur observer, Grote Reber,
who pushed radio astronomy from obscurity into a legitimate
scientific pursuit over 60 years ago. After World War II,
a number of technical radar wizards and scientists jumped
onto Reber's ideas and professional (that is, "sponsored")
observatories cropped up around the world. There was little
amateur activity of note again until the 1960's when a couple
of books were produced for amateur observers. Even then, only
a handful of people were involved. Here and there, schools
acquired modest surplus radio equipment and some "shoestring"
observatories produced a number of papers, but the idea that
big antennas and then expensive microwave equipment were
necessary to produce new discoveries was pervasive. The
potential of radio astronomy as an educational tool and as
an enjoyable hobby had been smothered beneath the "Big Science"
My own perception is that a new face for amateur radio astr-
onomy is emerging. The internet is, of course, fundamental in
this transition from obscurity. The number of amateur web
sites has grown rapidly and images of back yard antennas are
no longer rare. It is significant that NASA has through
the Radio Jove Project ( http://radiojove.gsfc.nasa.gov/ )
and MIT via its Small Radio Telecope Project
( http://fourier.haystack.edu/SRT/index.html ) have given a
dose of official recognition. Several small school observatories
are now on the web, and the NRAO teacher summer school has
churned out hundreds of teachers with a new appreciation for
radio astronomy. The recent Small Radiotelescope conference
at the Pisgah Astronomical Research Institute drew great
interest and attracted some impressive speakers.
A major problem faced by amateurs was that they worked without
any real ability to verify their observations with others. The
internet has, in essence, allowed connections between distant
observers which make correlations in real time possible. Solar,
Jupiter, and other event based observations can now be done
without the uncertainty that this or that spike was due to
a neighbors washing machine being turned on. The Radio Jove
Project has held numerous coordinated Solar and Jupiter obser-
vations, and as I participant I found them quite exciting. The
isolated amateur need no longer work alone. Whether or not any
new discoveries will come of this remains to be seen.
The Society of Amateur Radio Astronomers, SARA, has been around
for 20 years. I have been a member much of that time. When the
group was formed, there was no public internet. Electronics
were simpler, and of course, less powerful. In those days,
it wasn't very hard to find folks who enjoyed building radios
in their basement. The world of amateur radio that fed the SARA
membership (to a large degree), has floundered with the times.
SARA has struggled in the last few years to catch up, but the
inertia of an aging membership (me included!) has been hard to
overcome. New energy and vision is required, and it is my hope
that the more global and public face of amateur radio astronomy
will supply us with the right individuals to provide these.
(visit SARA at: http://www.bambi.net/sara.html )
Overall, I believe things are looking up for amateur radio
astronomy, (so to speak). We have new tools, new connections,
and the same huge wonderful cosmos to investigate. With sixty
plus years behind us we should be happy to be receiving a face
Amateur Tip #6
You can sometimes cut down rf interference from a computer monitor
by using dark colors for all of your backgrounds! For example, in
your favorite text editor choose light gray print on a black
background if it is an option. Sure its annoying, but it might make
the dB difference you need. Don't use screen savers or bright
backgrounds. Reducing contrast and brightness may also help.
You may also try adding a split ferrite choke on the video cable as
it exits the back of the monitor. Of course, it is best if the
offending monitor is located as far away from the radio equipment
Radio Astronomy at the University of Indianapolis
Malcolm Mallette and his associates have done a wonderful job with
their website and on line radiotelescope. The telescope has recently
been changed from 4 GHz to 1.4 GHz so a new look at the sky is now
available. There are suggestions on this site for those wishing to
build their own microwave system.
What is about Australian's that make them such wonderful radio
astronomers? Hans Michlmayr site will inspire you and inform you.
I found James Petrait's webpage regarding the reception of muons quite
interesting. I realize its not radio, but James is interested in that