No Wi-Fi Around Huge Radio Telescope

JG0LD writes "Students at a tiny Appalachian public school can't use Wi-Fi because any such network can throw the radio equivalent of a monkey wrench into a gigantic super-sensitive radio telescope just up the road. GBT's extraordinary sensitivity means that it's very susceptible to human-generated radio interference, according to site interference protection engineer Carla Beaudet. 'If there was no dirt between us and the transmitter, a typical access point ... would have to be on the order of 1,000,000 km [more than 620,000 miles, or about two and a half times the distance from the Earth to the Moon] distant to not interfere. Fortunately, we have mountains around us which provide lots of attenuation, so we're not seeing everything from everywhere,' she said. A standard Wi-Fi access point would wipe out a significant range of usable frequencies for the observatory. 'It simply ruins the spectrum for observations from 2400-2483.5MHz and from 5725-5875MHz for observational purposes,' wrote Beaudet."

Comments

This is news?

[dtmos]

The National Radio Quiet Zone has been there since 1958. It's not like it was just discovered yesterday. People living in this zone have always had to live without radio transmitters. Not having 802.11 is just another of the services they cannot use, like wireless garage-door openers and cell phones.

Re:This is news?

[camperdave]

Liberty doesn't mean carte blanche for being able to do whatever you want. That region was designated a quiet zone by the people, for the people. If you don't like it, you are free to not go there. You are free to rant and rave about it in public forums. You are free to petition the government to change the rulings. However, you are not free to operate a transmitter there.

Besides, the map shows that the quiet zone is more or less centered in the George Washington National forest, which makes it government owned land.

Re:This is news?

[Obfuscant]

However as you mentioned, these liberties also come with the restriction that the transmission not interfere with other frequencies --

Not entirely accurate. Some radio users are primary licensees. That means they get to interfere with everyone else. Not malicious or deliberate, but if there is interference it is the secondary, tertiary, or unlicensed users who have to put up with it.

FRS, wireless baby monitors, and wifi are all unlicensed devices, and as such any interference to them from licensed users is too bad, so sad, but more important, any interference they CAUSE to licensed users is "shut it off" notice time.

That's why people who have unlicensed garage door openers can't sue anyone when Air Force 1 stops them from working.

There's one catch, though: modern TVs lack an input filter that they're supposed to have by design which would normally reject non-TV frequencies,

If they are lacking the filter, then they were designed that way. Those devices are FCC approved and certificated, and if they were designed and tested for compliance with the filter but are being built without it, they are in violation of federal law (47CFR15) and can be confiscated and destroyed.

In those cases filtering needs to be added back to the TV to isolate it from the Ham transmissions -- it's my understandnig that this filter can be provided by the TV manufacturer upon request.

Since it is not really part of the design, and the manual for the TV clearly states that this device must accept interference (as part of the Part 15 Class B conformance statement), probably not. I think you can find commercial filters to use in this case, but the TV owner is stuck paying for them. And good operating practice says that the ham is not going to touch the TV to try to fix it, otherwise he becomes liable for any perceived failures of that TV. "Hey, the day after you installed your filter to stop your interference, the TV stopped working altogether, and I'm suing you, you basterd."

And, sadly, most of the interference issues would not be solved by installing a filter on the antenna, since a lot of the interference issues comes from modern, cheap ass plastic housings on the low price consumer equipment. You can't stop an interfering signal that is leaking into the electronics through the side of the TV by installing a filter on the antenna lead. You need to install shielding on the TV itself.

Re:This is news?

[Obfuscant]

Sol is one God almighty powerful AM radio station, forces even KGO in San Fran off the air nights in most other markets.

The terms you are looking for are "propagation" and "ionosphere."

In fact, KGO reaches much further at night than in the daytime, not because the Sun is such a good AM radio source, but because of the ultraviolet radiation that comes from it NOT being there at night. This allows the D layer to dissipate, and the D layer is what absorbs the AM radio signal during the day. Without the D layer, and with a weaker E layer, the AM signal can refract off the F (combined F1 and F2) layer and bounce long distances.

And that improved propagation at night is why KGO has (or probably has, I'm not going to research it) lower power limits at night. They were, however, a clear channel station (three letter callsign tells you that) and thus had a protected frequency in the US. Now that clear channels have been done away with, there are smaller competitors, but still only one KGO.

Why don't you hear the Sun during the daytime on your AM radio? You do, a bit. The same ultraviolet radiation that creates the D layer also causes the AM signal from the Sun to be absorbed before it reaches you. The satellite services certainly do "hear the Sun" whenever the Sun is positioned behind the specific satellite that a dish is pointed at, and that is the cause of the solar outages that happen every year.

Re:Low power wifi?

[v1]

Can they not use lower power wifi so that their signal does not extend that far beyond the school? Typically in these cases we have more than 1 AP connected together but all of them with lower transmit power so that the signal does not go far.

If conditions are right, I can have a contact with someone on CW running 5 watts, on the other side of the globe.

Such is the sensitivity of tuned circuits. For untuned interference, like your cell phone trying to interfere with your TV, rejection is great. But when you're specifically tuned to receive a frequency, you've got such a high sensitivity to that specific frequency, (and very high rejection of any other frequencies) that a cricket fart of a signal a long ways away can sound like a lightning strike on your house, if it's on the same frequency you're straining to hear.

They're a little better off than my CW example, being on a high frequency that's mainly line-of-sight, for which surrounding mountains would be a pretty effective shield, but still their receivers are just incredibly sensitive at their design frequencies. They just can't have anything anywhere near them or you will be all they can hear. It'd be like trying to listen to someone talking to you from a table at the other end of the restaurant, while you are seated right next to a table full of loud party animals. You'd have no chance.