Welcome to Issue #4 of the Radio-Sky Journal

March 2001
Copyright 2001 by Radio-Sky Publishing
All rights reserved.

Radio Frequency Interference 
Radio-SkyPipe Update
Amateur Tip #5
Featured Site


Radio Frequency Interference - And What to Do About It

Radio frequency interference, or RFI is perhaps the most
troublesome problem faced by most amateur radio astronomers,
(and no small issue with the professionals either!) Few are lucky
enough to live in areas where the spectrum is not full of un-
wanted noise from transmitters, power lines, neon signs, etc.
I know individuals who have basically given up amateur radio
astronomy because of the hopeless nature of RFI where they live.
When I began my first radio astronomy experiments in the early
80's, I had few problems even though I lived in the middle
of a very urban area. Each year brought new types of RFI to my
antennas, but I managed to stay "on the air" by trying several
strategies which I would like to pass along here.

For our purposes let's broadly categorize RFI into two types; 
narrowband and broadband. Narrowband interference usually arises
from intentional transmissions such as radio and TV stations,
pager transmitters, cell phones, etc. These signals can usually
be avoided provided that they are not so powerful or close to
you that they overwhelm your receiver front end. Use of highly
directional antennas and strong front end filtering as well
as selecting a frequency that is as far removed as possible from
the offending transmitter are the natural solutions. Of course,
we are usually limited in how directional our antennas are by
their size. Under-illuminating a dish antenna or applying skirts
may help in some situations. Compromises can sometimes be made 
in Yagi antenna designs which yield better front-to-back ratios
or better side lobe patterns. 

Front end filtering has the drawback that it can degrade your
noise figure. The degradation depends on the lossiness of the
filter and where it is applied in receiver. The absolute worse
place to apply a filter in terms of noise figure is prior to
the low noise amplifier (LNA), yet this is the best or only 
place to put the filter if that same LNA is being overloaded by
RFI! At lower frequencies, noise figures are not as vital a
consideration, so front end filtering becomes more attractive.
You can also try to applying a bandpass filter between the LNA
and the remainder of the receiver. This can still be very helpful
if the mixer is being overloaded. Speaking of mixers, many people
believe that the best option for mixers being used in high RFI
areas are of the passive diode ring type. Narrowing the overall
bandwidth of your radiotelescope with filtering reduces sensi-
tivity. You can make up for this, at least to some extent, by
increasing your integration time and by averaging multiple
observations, but consider applying a notch filter for a
particular offending transmitter. You may be able to preserve
a relatively wide bandpass this way.

Broadband RFI usually comes from incidental radio frequency 
emitters. These include power lines, electric motors, thermostats,
bug zappers, etc. Anywhere electrical power is being turned off
and on rapidly is a potential source. The spectra of these sources
generally resembles that of synchrotron sources, stronger at low
frequencies and diminishing at higher frequencies, though this
noise is often modulated (varied) by the creating device in some
way. Let's include in this category computers and other digital
equipment as well as televisions. The rich harmonic content of
these devices means that they can interfere over a very broad

Characteristic of broadband RFI is the inability to filter it
effectively once it has entered the receiver chain. The exception
here is pulse type interference which can sometimes be handled by 
noise blankers which turn off the receiver for the duration of the
pulse. For most broadband RFI, the best approach is to try to find
and eliminate or to modify the offending source. This is of course,
easier said than done in many cases. The first place to look is
in your own home. Use a battery powered radio capable of picking
up the interference and begin a walk through, turning off and on
every switch electronic device you can find. Don't forget those
dimmer switches for your lights and ceiling fans, they are common
culprits. Unplug those powerpacks that sit unswitched. Turn off
and on the blower to your furnace/air conditioner. Be methodical
so that you won't overlook anything. If you want to be extra sure
you can cut the power to the house at the breaker box and see if
that has an effect (though some of your equipment like your
computer might not like that approach). If you can't find anything
in your home, then its time to walk around the neighborhood looking
as cool as you can with your headphones on, pausing to ponder the
fronts and backs of every neighbors' houses, If you can muster a
directional antenna for your portable receiver, so much the better.
The AM broadcast band antennas in most portable radios are quite
directional and you can learn to use it to help find the source
with a little practice. 

If the problem is with a transformer or bad insulator on a power
pole, you can sometimes get the utility company to replace it. I
was lucky in that the utility company where I used to live was
quite responsive about this and at one time even had a team of
two workers who did nothing but investigated interference claims.
I doubt if many companies have such resources today, but the law
is still on your side (at least in the USA) and you can remind
the company that they are required to fix these things. If they
simply ignore you, then it is time to get on the phone to your 
local politicians and the FCC and try to get some action.

Electric motors can sometimes be quieted with new brushes or the
application of small value capacitors across their terminals. I
have also used the capacitor trick to quiet thermostats that used
bi-metallic contact switches. Aquarium thermostats are common 
offenders. Dimmer switches are so terrible that I usually just
replace them with standard on/off switches even though this is
not popular with some household members. If you find a brand of
dimmer switch that seems to perform well for years without causing
RFI problems, please let us know about it.

Computers and monitors can produce horrendous RFI but cannot be
simply turned off in many cases because they are integral parts
of our observatories. You can apply ferrite chokes to most of the
wires exiting the computer. Make sure the case of the computer 
has plenty if screws holding the cover on the case. Locate the
computer as far from the radio as is practical. At the NRAO in
Green Bank, they built a huge copper screened room to house 
equipment which could interfere with the new GBT about a mile
away. I'm not suggesting you do this, but I have built screened
boxes to house computers. Sometimes earth grounding helps, and
at other times it seems to even make things worse! Teaching your
computer to co-exist peacefully with your radiotelescope can
be quite a challenge. When it can't seem to be done, you can always
try burying your radiotelescope! What I mean by that is not sending
it off to be with Father Jansky, but actually locating the receiver
in a water-tight container 5 or more feet below the surface near
your antenna. The detected output can be sent back as a DC voltage
or translated to a serial digital format and then sent back to
the observatory. Besides the obvious RFI shielding benefit, my
experiments showed that thermal problems could be significantly
reduced. At my old location in Kentucky,the temperature just 6 feet
down changed less than two degrees Centigrade on any given
day throughout the year. 

See this nice page on suggestions for dealing with computer RFI:

On the Radio-Sky website you will find a page which discusses a
noise nulling technique which can sometimes help eliminate or
reduce noise arriving from a fixed direction. The technique
uses a separate antenna and combines the noise arriving at this
"sense" antenna 180 degrees out of phase with the noise arriving
at your main antenna. The technique unfortuneately is frequency
selective and does no good if the source is propogated along a
powerline or electric fence.
See: http://www.radiosky.com/cancelit.html

When things finally became so bad in Kentucky that I was unable
to carry on experiments at home, I devised a portable radio-
telescope. Using a weather-proof box, a motorcycle battery, and
simple antennas that could be erected quickly in the field, I
struck out to invade the farms of co-operative friends. I was
able to perform some interferometer experiments that were quite
exciting and educational for me. I also learned that large bulls
don't like me very much. Now I am in rural Hawaii and guess what!
There still are sources of interference. 

Finally there are a number of resources on the web.

Look at this page on the ARRL site:

You can peek in on some of the RFI discussions at the NRAO at:

In particular see: http://info.aoc.nrao.edu/doc/vla/html/rfi.shtml

In Australia: http://www.atnf.csiro.au/mnrf/rfi_details.html

An interesting RA handbook from the Netherlands has RFI issues:

Here are some book and document references I have dealing with
the subject. Some may be out of print or generally unavailable,
but look on Alibris or some other used book store service and
you may get lucky.

Digital design for interference specifications / R. Kenneth Keenan.
Vienna, Va. (421 Mill St., S.E., Vienna 22180) Keenan Corp. c1983. 

Interference handbook / William R. Nelson ; editor, William I. Orr.
2nd ed. Lake Bluff, Ill. Radio Publications 1988, c1981. 

Controlling radiated emissions by design / Michel Mardiguian.
New York : Van Nostrand Reinhold, c1992. 

Interference mitigation : theory and application / Rabindra N. Ghose.
New York : IEEE Press, c1996. 

Interference problems in radio astronomy :
report of the ... meeting of the ESF Committee on Radio Astronomy
Frequencies / by T.A.Th. Spoelstra and R.J. Cohen. Dwingeloo,
The Netherlands : Netherlands Foundation for Research Astronomy,

Radio frequency interference / editors, Charles L. Hutchinson,
Michael B. Kaczynski ; contributors, Doug DeMaw ... [et al.].
4th ed. Newington, CT American Radio Relay League c1987. 

Radio frequency interference handbook. Compiled and edited by
Ralph E. Taylor. Washington Scientific and Technical Information
Office, National Aeronautics and Space Administration;
[was for sale by the National Technical Information Service,
Springfield, Va.] 1971. 

Radio frequency interference : how to find it, and fix it
/ editors, Ed Hare and Robert Schetgen. Newington, CT
American Radio Relay League, 1991. 

The suppression of electromagnetic interference from personal
computers at DRAO / W. Wyslouzil. Penticton, Canada :
Dominion Radio Astrophysical Observatory, 1995. 


Radio-SkyPipe Update

Numerous changes have been made to the Radio-SkyPipe program
since its release last year. Everyone's copy expired on Feb 1 2001.
The new download does not have an expiration and you can use it
free of charge for as long as you wish. The most current version
as of the time of this writing is 1.0.80. With it, you can view
multiple channel streams. (You have to have the upgrade to serve
multiple channels). Real-time disk back-up has been implemented.
You can now log in UT without changing your computer clock. Many
bugs have been fixed and stability has been greatly enhanced.
You no longer have an excuse for not putting your observatory
on line! So, please try it out!



Amateur Tip #5

Use feedthrough capacitors when possible on power supply lines
to individual modules. Even though your project box may be rf
"tight", you have to get power to it. The un-filtered power
connection is a common route for external interference to enter
your signal train. Feedthrough capacitors are probably your
best bet to counter this. They come in many forms. Some are
solder types, which do you little good on aluminum boxes but
are excellent for boxes you construct from PC board. The threaded
type may be universally used but are more expensive. Where do
find them? Good question. I constantly seek them out at hamfests
where they can sometimes be purchased for as little a $.25 each.

Downeast Microwave has some:

Debco Electronics has some cheapies (kind unknown):

Featured Site

NASA's Radio Jove Project

I can't emphasize enough what a great program the Radio Jove Project
has become, especially for newcomers to radio astronomy. The Jupiter
observing season draws to a close as Jupiter moves closer to the
Sun from our perspective in the solar system. However, now would be
a great time to begin assembling your Jove receiver for next September
when things begin anew. There also promises to be some good solar
observing through the summer, for which the Jove receiver has proven
quite useful. So if you are looking for a great first project or you
are tired of the same old drift scans visit:


{ I suggest you get Dick Flagg's "Listening to Jupiter" book for a great
reference on Jupiter observing. See the full table of contents at:
http://radiosky.com/newLJTOC.html }

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