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swampwiz
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I was reading that the mysterious reason why this channel was removed from public use is because it interfered with radio astronomy (the Wikipedia article didn't mention the reason). So what is going on at this frequency?
That's a wee bit melodramatic. There's no mystery. It is just a channel reserved for Radioastronomy. It's hard enough to pick up very weak signals because of the basic signal to noise ratio. Interference levels can be a lot higher than the fundamental random noise that all receivers suffer from. Keeping a channel that's free from interference gives a tremendous increase in ability to detect extra terrestrial RF signals.swampwiz said:the mysterious reason
I think that it's more likely that in the commercial world of Comms and Broadcasting, the Radio astronomers felt lucky to get any quiet spots in the spectrum they could get their hands on. They are well down the pecking list. Frequencies are jealously guarded by the states that use them and there are (or at least were) endlessly haggled over internationally in the International Telecommunications Union.Merlin3189 said:I assumed, as the OP, that there might be some reason for that specific frequency.
So why haven't all the UHF channels that go from 410-1400 MHz been removed? There must be something very special going on right about at 611 MHz.Vanadium 50 said:@swampwizHere is what Wikipedia says: "occupies a band of UHF frequencies from 608 to 614 MHz. This band is particularly important to radio astronomy because it allows observation in a region of the spectrum in between the dedicated frequency allocations near 410 MHz and 1.4 GHz."
There's your answer. If that's not sufficient, you need to tell us why that's not sufficient rather than hope we guess right.
I typically read the article at Wikipedia before posting here.Vanadium 50 said:@swampwiz, you are developing a habit of asking questions without looking up things first. It's a very bad habit indeed. Especially when coupled with the non-source "I read somewhere" or equivalent.
https://en.wikipedia.org/wiki/Channel_37berkeman said:Could you give a pointer to the article you were reading? Have you done any searching to try to find a reputable source for this? What is the frequency used for UHF Ch37 in the US? (I'm too lazy to look it up)
Page 40 shows an interesting graph. 611 MHz seems to be the lowest local peak of atmospheric admittance, so perhaps Channel 37 was taken out so as to allow this little blip of admittance to have no interference, since it is of so low a level that it needs the spectrum to be clear there, whereas the other frequencies have a high admittance, and thus the interference is not a problem?Keith_McClary said:
swampwiz said:So why haven't all the UHF channels that go from 410-1400 MHz been removed? There must be something very special going on right about at 611 MHz.
It's called 'filtering'. A reasonable band pass filter in the feed circuit, over the dedicated range can limit adjacent channel signals admitted into the receivers and it will help in suppressing non linear effects that can mix high level signals in an adjacent channel (TV transmitter down the road, for instance).swampwiz said:Perhaps by having no interference in this small range, the noise in the rest of the spectrum can be attenuated?
swampwiz said:So why haven't all the UHF channels that go from 410-1400 MHz been removed?
swampwiz said:There must be something very special going on right about at 611 MHz.
@Jansky doesn't seem to appreciate that Radio Astronomy (in fact, all astronomy) is way down the list of organisations that want spectrum space. Once he accepts that, it will all become clear.Vanadium 50 said:Except that paper doesn't say "because we want to make this measurement, 611 is protected". It says "because 611 is protected we make the measurement there".
Uh, because that was the one that was used?Vanadium 50 said:You mean "why did they not remove all 69 channels if the astronomers only needed one?" Is it not obvious?
Evidence?
That would be a valid reason, although I had always thought that arbitrariness in science is to be discouraged.Merlin3189 said:I read that too and also found a textbook on RA to look for any mention of specific signals in that range.
I assumed, as the OP, that there might be some reason for that specific frequency. But as far as I can see, there was nothing that couldn't be equally served +/- 100 MHz or +more.
If there are no narrow band sources, nor atmospheric absorption at that frequency and it is at neither the arithmetic nor geometric mean of other RAS allocations, it would seem to be an arbitrary choice.
There is no "arbitrariness" in this at all. For very good reasons, the UHF bands (i.e. ranges of RF frequencies) were allocated in the way we see them now. The UHF broadcast TV bands cover almost an octave. Several different channels are needed in order to provide a good coverage of four of five services on a nationwide basis. The channels in any particular service area cannot be adjacent to each other and the frequencies are allocated in interleaved sets, so as to minimise mutual interference between service areas. Also, there is a limit to the bandwidth that domestic receiving aerials can achieve (cost and weight) so the UHF TV spectrum was split into two bands IV and V, with the changeover in the region of Channel 37. It had been agreed that the radio astronomers needed a quiet patch somewhere at UHF so channel 37 was chosen for them. Totally not 'arbitrary' and they were pleased with what they were allocated.swampwiz said:That would be a valid reason, although I had always thought that arbitrariness in science is to be discouraged.
No one uses "fixed station frequencies" Every channel requires a bandwidth. The uhf (analogue) TV bands uses a nominal bandwidth of around 7MHz and the practical design of transmitter filters produces very significant spillage into adjacent channels (hence the requirement for interleaving) The bandwidth requirements for a radio telescope receiver will be as narrow as possible, in order to obtain the best Carrier to Noise ratio. Looking for doppler shifted signals would involve looking at a different frequency but the limit to what's receivable will be the edges of the available spare 'band'.Jansky said:The fact is that not all radio astronomy observations can occur at specific frequencies just as commercial users can rely on fixed station channels or frequencies that you can tune into a favorite AM,FM or TV channel. Many fairly narrow astronomical emission lines at a specific rest velocity frequency can be doppler shifted either up or down the spectrum due to cosmic velocities.
"Possible" does not imply a bandwidth of fractions of a Hz; it implies what it says. If you are looking for a 1MHZ wide signal than the "possible" receiver bandwidth will be 1MHz. Having a 'reserved' channel bandwidth of 6MHz will mean that the telescope receiver bandwidth will probably be less than that and no wider than necessary. CNR is important in a receiver and so is Carrier to Interference Ratio; particularly for faint signalsJansky said:"radio telescope receiver will be as narrow as possible, in order to obtain the best Carrier to Noise ratio". The reception of radio astronomy continuum signals by definition preclude narrow bandwidth reception and thus wide simultaneous frequency reception capabilities are required for signal analysis.
That's an interesting point.Jansky said:This smearing of the radio signal, known as dispersion delay, is often used to estimate distance in radio astronomy: the greater the dispersion, the further the object from Earth.
sophiecentaur said:The available free channel bandwidth was not assigned on the basis of the needs of radio astronomy but on the commercial and cultural pressure to lose no more than perhaps 5MHz of TV band spectrum
OK, this sounds like a good reason as it is due to the "technological debt" of UHF TV needing to be split up into 2 regions, itself due to the need to have a more costly antenna in the upper region. The split region is the perfect place to go dead for the astronomers. And along with this, it doesn't matter that astronomers want to tune into the region of 400-1200 MHz as Channel 37 is pick up those signals, albeit not quite as strongly.sophiecentaur said:There is no "arbitrariness" in this at all. For very good reasons, the UHF bands (i.e. ranges of RF frequencies) were allocated in the way we see them now. The UHF broadcast TV bands cover almost an octave. Several different channels are needed in order to provide a good coverage of four of five services on a nationwide basis. The channels in any particular service area cannot be adjacent to each other and the frequencies are allocated in interleaved sets, so as to minimise mutual interference between service areas. Also, there is a limit to the bandwidth that domestic receiving aerials can achieve (cost and weight) so the UHF TV spectrum was split into two bands IV and V, with the changeover in the region of Channel 37. It had been agreed that the radio astronomers needed a quiet patch somewhere at UHF so channel 37 was chosen for them. Totally not 'arbitrary' and they were pleased with what they were allocated.
Are you not aware that hundreds - even thousands - of people would rather watch TV than read what astronomers had found? In real life, we have priorities and mature choices to make. That's what produced Ch37.
I'm not sure that a Band V antenna would necessarily be more costly but UHF was high tech at the time and I'd be surprised if it wasn't desirable to limit high power stations to Band IV, to start with. Early Klystrons may have been only just able to manage Band IV frequencies. And it wouldn't just be receiving antenna that would have bandwidth problems. Combiner networks are probably cheaper if the four channels are all as close together as other considerations will allow.swampwiz said:a more costly antenna in the upper region
I guess the Aliens published their operating channel in the RSGB Bulletin?wackedoutscience said:Aliens! Aliens I tell you. The frequencies used by channel 37 are also used by scientists to send signals into the cosmos to hopefully get a response. Aliens, pure and simple.
sophiecentaur said:I guess the Aliens published their operating channel in the RSGB Bulletin?
UHF channel 37 refers to a specific frequency on the Ultra High Frequency (UHF) band of the electromagnetic spectrum. It is typically used for television broadcasting and falls between 608 and 614 MHz.
The main reason for the absence of UHF channel 37 is to avoid interference with radio astronomy observations. This frequency range is reserved for scientific research and is protected by international agreements.
The use of UHF channel 37 for television broadcasting can cause interference with radio telescopes and other instruments used for astronomy. This interference can disrupt data collection and hinder scientific research.
In addition to protecting radio astronomy, UHF channel 37 is also used for medical and scientific equipment, such as MRI machines and wireless medical telemetry devices. Allowing television broadcasting on this frequency could also cause interference with these important technologies.
One solution that has been proposed is to move television broadcasting to a different frequency range, such as the UHF band from 470 to 698 MHz. However, this would require significant changes to broadcasting equipment and infrastructure, making it a challenging and costly solution.