What is the astronomy reason for no UHF channel 37?

[swampwiz]

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?

 

[berkeman]

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)

 

[Merlin3189]

I think 608 - 614 MHz

 

[sophiecentaur]

the mysterious reason

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.

 

[Vanadium 50]

@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.

Here 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.

 

[Merlin3189]

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.

 

[sophiecentaur]

I assumed, as the OP, that there might be some reason for that specific frequency.

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.

There were some very important factors when trying to negotiate Analogue UHF TV channel allocations for countries who aim at good coverage (with several channels) of areas with multiple transmitting sites. Groups of channels are interleaved to minimise co and adjacent channel interference and receiving aerial design tends to force a limited total bandwidth. Different countries and areas use(d) different channel arrangements but I imaging that Europe would have driven the 'International' aspect of the model. US was early in the game and was always doing its own thing. This wiki link has a lot of data about frequency allocations and so does this one. Good cures for insomnia if you don't happen to be in the business or when your jobs not on the line.

 

[Keith_McClary]

Significant Radio Astronomy Frequencies
Radio astronomical use of the electromagnetic spectrum at Onsala Space Observatory
Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses (2007)
Chapter: 2 The Scientific Background

 

[swampwiz]

@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.

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.

 

[swampwiz]

@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.

I typically read the article at Wikipedia before posting here.

 

[swampwiz]

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)

https://en.wikipedia.org/wiki/Channel_37

 

[swampwiz]

OK, it looks like the observation of everything between the important 410-1400 MHz range can be done simply by tuning in at the frequency range 608-614 MHz. Perhaps by having no interference in this small range, the noise in the rest of the spectrum can be attenuated? I was hoping that some radio astronomers at this forum would have a good thorough answer.

 

[swampwiz]

Significant Radio Astronomy Frequencies
Radio astronomical use of the electromagnetic spectrum at Onsala Space Observatory
Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses (2007)
Chapter: 2 The Scientific Background

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?

 

[sophiecentaur]

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.

Proposition: "The population of the World would rather know about the Universe than watch TV": Discuss.

The needs of radioastronomy were way down the list amongst the claims on the use of UHF spectrum at the time.

 

[sophiecentaur]

Perhaps by having no interference in this small range, the noise in the rest of the spectrum can be attenuated?

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).
PS It's not 'noise' in the adjacent channels that counts (that is not relevant to what comes into the wanted channel; it's Interference and the specification of Transmitters to limit outgoing adjacent channel interference into the 'protected' channel. There is no 'brick wall' between channels.

 

[Vanadium 50]

So why haven't all the UHF channels that go from 410-1400 MHz been removed?

You mean "why did they not remove all 69 channels if the astronomers only needed one?" Is it not obvious?

There must be something very special going on right about at 611 MHz.

Evidence?

 

[Jansky]

Well swampwiz simple answer, you can always read a scientific paper:

Publications of the Astronomical Society of the Pacific, 115:675–687, 2003 June
2003. A Survey for Transient Astronomical Radio Emission at 611 MHz

http://www1.phys.vt.edu/~jhs/eta/references/KatzEtal2003.pdf

 

[Vanadium 50]

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".

 

[sophiecentaur]

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".

@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.
Light and atmospheric pollution take very little account of the requirements of Astronomers.

 

[swampwiz]

You mean "why did they not remove all 69 channels if the astronomers only needed one?" Is it not obvious?
Evidence?

Uh, because that was the one that was used?

 

[swampwiz]

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.

That would be a valid reason, although I had always thought that arbitrariness in science is to be discouraged.

 

[sophiecentaur]

That would be a valid reason, although I had always thought that arbitrariness in science is to be discouraged.

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.

 

[Jansky]

As some of the comments regarding the radio astronomy reserved UHF ch37 frequency have missed the mark, a very valid reason for it's allocation exists. Regardless of the rather cynical but correct observations by sophiecentaur regarding radio astronomy not being competitive with commercial financial revenue generating uses that utilize the radio spectrum, it is actually a reasonable compromise as to the ch37 600MHz assignment. As if the current insatiable appetite for bandwidth by the cellular companies isn't enough, the TV and C band satellite spectrum is now being cannibalized due to the far greater revenue generating potential of cell phone carriers and the decline in satellite and terrestrial TV viewership.

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. For example the very valuable 1,420 MHz 21 cm line from hydrogen gas, when being observed at the first billion years after the Big Bang has now redshifted to radio frequencies of 30-200 MHz.

In addition many information rich astronomical emissions are not narrow band emissions but are observed as continuum sources. As such it would be ideal if radio astronomy observations could occur across simultaneously wide frequencies. This being impossible, the placement of regularly spaced windows of observation have been established to reserve an approximate range of the actual continuum emissions that exist.

Thankfully, in order to coordinate what would be a chaotic mess of the radio spectrum, international treaties have been established to accomplish some degree of order and in this case the ITU is the international regulatory agency to all it's signatories.

"The International Telecommunication Union (ITU) is the United Nations specialized agency for information and communication technologies – ICTs.
Founded in 1865 to facilitate international connectivity in communications networks, we allocate global radio spectrum"
The "ITU Radio Regulations —is the international treaty governing the global use of radio-frequency spectrum"
Every time you make a phone call via the mobile, access the Internet or send an email, you are benefitting from the work of ITU.

RECOMMENDATION ITU-R RA.314-9
Preferred frequency bands for radio astronomical measurements

"certain frequency bands have been allocated for continuum observations, and that the
exact positions of these bands in the spectrum are not of critical importance, but that their centre
frequencies [should be in the ratio not more than two to one], taking the width of relevant
atmospheric windows into account"

As you can see from the table and beginning at shortwave frequencies extending to microwave frequencies the assigned observation windows approximately double in frequency as the recommendations suggest for their assignment although not all are entirely for exclusive use or global locations for astronomical observations.Frequency bands allocated to the radio-astronomy service
that are preferred for continuum observations

Frequency band _ Frequency band
(MHz) _ (GHz)
13.360-13.410 _ 10.6-10.7
25.550-25.670 _ 15.35-15.4
37.5-38.25 (1) _ 22.21-22.50
73-74.6 (2) _ 23.6-24.0
150.05-153 (3) _ 31.3-31.8
322-328.6 _ 42.5-43.5
406.1-410 _ 76-116 (1)
608-614 (4)
1,400-1,427 _ 123-158.5 (1)
1,660-1,670 _ 164-167
2,655-2,700 (1) _ 200-231.5
4,800-5,000 (1) _ 241-275(1)

(1) These bands include secondary allocations.
(2) Allocation (primary) in Region 2, protection recommended in Regions 1 and 3.
(3) Allocation (primary) in Region 1, Australia and India.
(4) Allocation (primary) in Region 2, the African Broadcasting Area (606-614 MHz),
China (606-614 MHz) and India. In Region 1 (except the African Broadcasting
Area) and in Region 3 this band is allocated on a secondary basis.

 

[Jansky]

Due to formatting differences the continuum observation frequencies table is scrambled but you can see it in the official document :

https://www.itu.int/dms_pubrec/itu-r/rec/ra/R-REC-RA.314-9-200202-S!PDF-E.pdf

 

[sophiecentaur]

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.

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'.
It's a nightmare for the `astronomers some times but the use what they've got and, of course, where possible they will choose a quiet site.

 

[Jansky]

" No one uses "fixed station frequencies" Every channel requires a bandwidth" . Although signal bandwidth is a very specific characteristic of communication RF signals, their bandwidths are defined specifically for different modes of modulation by strict technical standards, unfortunately natural radio signals are not restricted by such artificially imposed limitations. Generally radio emission can be composed of two main components the carrier wave(optional) and the modulation sidebands. Assignment of channel frequencies is based in reference to a fixed carrier wave or to the defined total frequencies of the modulation sidebands bandwidth and these are the assigned fixed values defining the channel. The bandwidth of the signal is determined by the types and requirements of the modulation and to the amount of information it encodes.
As an example with the use of CW or continuous wave modulation to transmit Morse code, a slow off-on type of modulation, the bandwidth of the signal with appropriate frequency stability is around 18 Hz for a normal hand keyed CW. As the previous post states, the typical DTV TV digital modulation with vastly greater information content uses up to 6-7 MHz of bandwidth but without a defined carrier frequency and depending on the modulation standard ATSC or DVT it might contain a small pilot and sync signal , and is restricted to fixed bandwidth frequencies to the channel that it is assigned.
Radio astronomy has to deal with naturally generated signals with only natural physical laws to define its characteristics and except for atomic transition lines at rest velocity frequencies, almost any other frequencies can be found that shift, slide, pulse and vary in intensity.
"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.
The recent advent of SDR software defined radio receivers that surmount much of the limitations of older analog technology have made reception of these very wide bandwidth of unpredictable cosmic radio signals much easier to receive and analyze.
One of the hottest areas of current radio astronomy currently is the reception and study of the FRB or fast radio bursts phenomena. These are high energy bursts of a few milliseconds long that otherwise from being short, sharp signals an FRB signal becomes "smeared out" in frequency by its journey through space.
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.
SDR digital receivers can easily simultaneously monitor these smeared out signals that can cover up to hundreds of MHz of millisecond time dependent bandwidths.
For waterfall time frequency plots of bandwidth channels across the 400–800 MHz band including the restricted Ch 37 608-614 MHz bandwidth, one can be seen for a signal from the source SGR 1935+2154 that continuously extends from <400 MHz to > 800 MHz in the following reference.

https://chime-experiment.ca/en

Other plots from other signal sources extending across the same bandwidth:

Nine New Repeating Fast Radio Burst Sources from CHIME/FRB
https://arxiv.org/pdf/2001.03595.pdf

 

[sophiecentaur]

"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.

"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 signals

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.

That's an interesting point.

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. I feel that should be born in mind.

 

[Jansky]

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

Regardless of all the technical factors, this ultimately is mostly correct although in fact the needs of radio astronomy at the time did play somewhat of a role in that the enactment of the channel assignment exclusion came about at all.
For a final answer to the original post, one can read an exhaustive historical chronology of how, when, where, why and by whom the TV UHF Ch 37 gap came about here:

The History of UHF TELEVISION
... those channels above 13 almost no one could receive

Why Is There No Channel 37?

http://www.uhftelevision.com/articles/channel37.html

Interestingly, "In 2000, the FCC began allowing the use of frequencies within channel 37 by medical telemetry equipment at extremely low power."

 

[sophiecentaur]

In 1963, the spectrum planning was already sorted, in a broad sense. There were very few UHF TV stations in operation at the time but the slots had been allocated well in advance. In UK there was a programme of planning and measuring the performance of many possible Transmitter sites (all four channels, co-sited) and the network was aimed at serving a large proportion of the country (even for fairly small populations). The only way to ensure this was to impose the site positions, frequencies and radiated powers.

I think you comment probably tells me you are in the US, where the culture of broadcast spectrum use was very different as it was based on advertising revenue. TV coverage and spectrum use was more ad hoc because the fate of small rural populations was of no consequence to the Advertisers. That strategy may have been more applicable in early days to lower TV channel frequencies and, of course, there was a demand for more than just the four (or later, five) off air channels (chicken and egg). There has never been extensive 'DX' TV reception in UK because we never had a vast number of alternative TV channels to watch if your aerial was big enough, high enough and steerable.

 

[swampwiz]

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.

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]

a more costly antenna in the upper region

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.

Taking propagation, topography and populations into account too makes service planning a serious art, on top of what we've already discussed.

 

[wackedoutscience]

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]

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.

I guess the Aliens published their operating channel in the RSGB Bulletin?

 

[Vanadium 50]

I guess the Aliens published their operating channel in the RSGB Bulletin?

No, they just read TV Guide.

 

[swampwiz]

Here's a nice coincidental article about this that just got published. Basically a team of radio astronomers needed to work within the confines of "engineering" in the construction of the radio telescope. I presume that the corresponding size - and thus natural frequency - was the largest that could be done without making the engineering an order of magnitude more expensive.

(Maybe the author is a lurker here. )

https://www.vice.com/en/article/dy8by7/why-channel-37-doesnt-exist-and-what-it-has-to-do-with-aliens

You see? The target of my dogged inquisitiveness DID have a logical answer underneath the surface. I feel so Einsteinian now!