I often hear comments about observing in the so called "protected frequency assignments"; 408 MHz, 610 MHz, and 1420 MHz being the most commonly referenced. It's worthy of mention just what "protected" means.
"Protected" means that by international convention of governing organizations like the World Administrative Radio Conference (WARC) or the International Telecommunications Union (ITU) these frequencies receive special treatment to attempt and keep them clear of harmful interference. Generally this just means that no transmitters are allowed in the particular band. What isn't widely understood is that interference protection ends well above the levels which might prove useful to us as amateur radio astronomers. And in most instances these governing bodies work like the United Nations, exerting only limited influence and recommendations to the FCC.
The power of money is often a much more influential power than the deterrent of government consortiums. As a result we have several satellite systems which blind radio telescopes as they pass over, particularly in the 1665 MHz frequency band. This, and many other spurious signals contribute to the radio equivalent of "optical sky glow".
Most SETI League members' hydrogen line observations are galactic, meaning in our Milky Way galaxy. This limits the Doppler shift to around +/-2 MHz from the 1420.405 MHz rest frequency, staying well inside the 1420 assignment. But professional observatories look at highly red shifted extragalactic emissions. These can be shifted down into unprotected airport radar and even cell phone frequencies where interference avoidance is nearly impossible.
More importantly, the fact that high power transmitting may be allowed right up to the band edge of a "protected assignment" makes radio astronomy signal corruption inevitable. It's nearly impossible to design a system that's immune to the effects of a +90dBm TV transmitter adjacent to a -200 dBm radio astronomy signal (610 MHz), no matter how good we filter. Even when a TV transmitter is perfectly legal, all it takes is a rusty fence joint a few miles from the transmitter, acting as a diode multiplier, to double and triple signals and noise into the microwave range. Multiple transmitters impinging on these crude "mixers" cause a multitude of new "unregulated" multiplied frequencies to contend with.
Other subtle effects involve the RF on a frequency many hundreds of MHz from your desired frequency still influencing the gain compression of your low noise amplifier, in effect AM modulating it with the out of band signal. Notch filters on the transmitter frequency will help with this problem.
But if the noise or spurious signal is actually on your receive frequency, there is little one can do to separate desired from undesired signals. Phase noise and modulation sidebands extend out to infinity around these transmitters. Sometimes you are far enough away for this to be below your own galactic noise floor, other times not. This is another case where a good antenna pattern can be your best defense against interference.
Now for the good news… much of the spectrum is seemingly unused. Recent frequency surveys show that only perhaps 6.5 % of the RF commercial spectrum is used, even in a large city like Atlanta, at any given location. For most of us, that still provides filtering challenges that are unmanageable. But in suburban or rural settings, the remaining, mostly TV and radio station frequencies, can be avoided with careful engineering.
Picking ones neighbors. You can't always pick your neighbors. But some radio services make good cohabitants. Ham radio has long enjoyed a close relationship with military co-users in the UHF frequency range. The reason is that military use of the frequencies is transient and typically benign. Even high powered radars can be dealt with effectively by noise blanking (similar to the techniques mentioned in my article on the effects of new digital techniques in radio astronomy).
Satellite uplink bands are an underutilized frequency resource. With very few transmitters to avoid, the 5850-6425MHz , 14-14.5GHz, and other uplink bands make good potential receive frequencies for us. Arecibo, NRAO Green Bank, and the VLA have long used the spectrum around 327 MHz as a radio astronomy frequency, not because it's a protected assignment, but because it's a military satellite uplink range relatively far from high power transmitters. 408 MHz on the other hand is much closer to high power transmitters and harder to work in than 327 MHz. At PARI we suffer the effects of a legal 410 MHz transmitter on a nearby mountaintop. Its been there causing problems for decades. So a move down to 327 MHz helped improve the situation, and also brought us into frequency alignment with Green Bank where we could use a borrowed feed off the old 300 foot dish.
SARSAT. I've heard SETI League members complaining about a new satellite rescue service called SARSAT. I must say that for me this falls in the category of good neighbors. The lost hiker or downed aircraft transmitters are likely to be few and far between, causing little interference to 408 MHz operations. And they will be more quickly located and shut down than almost any kind of alternate user for the spectrum, since every signal will be treated as an emergency search and rescue operation.
UltraWideband digital may prove to be one of the more problematic new services to watch. Digital signals generally already spread out to cover wider swaths of spectrum than their basic frequency ranges might imply. But UltraWideband is supposed to cover many GHz of spectrum as a means of avoiding localized interference sources. Thin but wide is the way to visualize this one. It means to cause a small amount of interference to everyone. In the radio services, small enough interference to go unnoticed generally. But for us users of noise, it may be a different situation.
UltraWideband is one of a group of new services called "Cognitive Radio". This implies that if it finds a user of the spectrum, it will try to avoid that user. Conversely, if it sees a blank spot of spectrum, it will try to use it. As receive only users, we are invisible to such systems and will not be avoided. Transmitting to run the UWB system off your frequency is not only illegal, it could potentially cause more interference to other users.
My recommendation, in summary, is to borrow a scanner and do a spectrum survey of your location before you invest a large amount of money in a radio astronomy system that is not flexible enough to move away from interference. Join the professional trend toward finding those unused blocks of spectrum and using them. You may well find that they are much more radio astronomy friendly than so called "protected assignments".
Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.
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this page last updated 4 March 2006
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