My suspicion is that the cell phone companies and public safety people are sharing channels within the 800 MHz band. In this case, additional filtering won't help as the receiver needs to hear signals in the band of interest.
I am guessing that when a public safety radio is near a cell tower, the radio does not have enough dynamic range to hear the desired weak signal when there is a strong interferer nearby. The AGC limits the gain of the public safety radio based on the undesired cell signal. This would create dead zones near cell towers.
For ham operators bothered by intermod, look at one of these (for 2 M users):
http://www.durhamradio.com/dci-intermod-filters. ht m
I just did a quick google, not an exhaustive study.
This sort of interference/intermod problems is a results of wide range receivers. One of the great things about old Radio Shack 2m equipment is that since they only covered 2m's they are very resistant to out of band interferene. TANSTAAFL or however it is spelled:)
Haven't you heard what happens to the cellular system during a larger crisis? It collapses when everyone tries to call at once. Didn't you see the articles about people seeking out the remaining pay phones to get in touch with friends and family.
Cell phone networks are designed to work most of the time. Cell phones would be outrageously expensive if the network was designed to handle peak capacity during a crisis.
Public safety types need an effective communication system seperate from public systems. During a crisis, they use discipline to prevent swamping their communication network.
Many years ago, I had a dead 400 Meg SCSI drive with a mechanical failure and my friend had the same drive with an electrical failure. Combining my good electronics with his good mechanical section produced a working drive.
Um, spectrum that supports international communication is shared cooperatively. Obviously there would be chaos if this didn't happen.
At higher frequencies (VHF and up), a signal will (typically) not travel beyond line of sight. So at higher frequencies international cooperation is not as much of a factor. It is still useful for different countries to use spectrum in similar ways so that cell phones, wireless network cards, etc can work worldwide. Well, they could if everyone would get their shit together:)
Basically, the government is subsidizing terrestial broadcast TV by allowing a profit oriented industry free use of valuable radio spectrum. What should happen is that the users of the spectrum should compete for the right to use it with other industries. Since most people already pay to receive TV over cable or satellite systems, most people would not see the increased cost. By reducing the number of broadcast TV stations, spectrum becomes available for more interesting and spectrally efficient services. After all the information transmitted in a 6 MHz TV channel can fit in a much smaller amount of spectrum using modern communication technology.
And let's face it, if a terrestial broadcast TV station turned off it's over the air transmitter, what perentage of it's customers notice?
So true. I have a DirectV Tivo receiver. A few years ago, I mounted a disk on a piece of plywood so that I could watch the Tour of Italy on vacation.
A friend of mine just bought a 60" V. Today I will find the portable dish and take the Tivo over there so we can watch the commericials with a real video feed, not the crappy, snowy cable signal. And skip the football:)
I loved the XOR answer. I learned that trick when I was writing 8085 assembly as a coop in the very early 80's. I'm surprised it is regarded as unusual, by now I would have thought every good programmer would have discovered or seen this trick.
Thanks for the memories. Somewhere around here I have an IBM PC technical reference with the assmebly listing for the BIOS.
This is my frustration with experimentation in amateur bands today. I would love to do high speed wireless work in the amateur bands. I live in a rural area without access to any high speed internet service.
In order to produce a technically useful solution, I need to encrypt all my traffic. (Never mind some of my email is business related and I like to look at p0rn as much as the next guy:) But under my current interpretation of the ammateur rules, encryption is not permitted.
Personally, I believe some flexibility is required in order to motivate people, such as myself, with the technical expertise in radio and networking to explore wireless networking work to create network links. Without the use of these links to replace my current dial up internet, I am not motivated to invest the time and money required to do this.
Of course, this opens a real can of worms in the area of more than casual commericial use. For eaxmple, I could sell you a wireless link and the training required to pass the amateur exam. You would be using amateur frequencies legally (under the conditions I outlined above) however, you would not be contributing to advancing wireless networking.
Bad news? As several people have pointed out part 15 devices must not interfere with licensed services and must accept interference from licensed services.
This reminds me of a conversation I had with the campus admin responsible for wireless networking. He is aware that wireless networking at 2.4 GHz is subject to interference from other services. He feels they can control this by using university policy to control on campus users. For example, if the local amateur radio club decided to broadcast a TV signal over 802.11 frequencies, they could tell them to stop since they would be on university property. However, they know that I could legally build a TV transmitter and a highly directional antenna to point at one of their access point and place this off campus. Of course this would be a waste of my time and money.
What does all this mean? In the 2.4 GHz band part 15 wireless networking devices are at the mercy of the other licensed users of the band.
Everyone (with an IMDB account at least) head to http://us.imdb.com/Title?0159784 and rate the movie "Takedown" a 1. Show people you are tired of bogus reporting of technology stories.
Well I have a Tivo. Sometimes when I FF through the ads, I'll see one that catches my eye. Then I will watch the ad.I won't watch the same ad a million times, which is what happens on alot of of shows.
Let's see:
1) There is a head on one side
2) A hand on the other
3) People want the batteries to last forever
4) The phone should work in the basement of your local bar
These factors make antenna design for cell phones very difficult.
Energy radiated in the direction of the head is wasted, since it doesn't come out the other side. So it makes sense for antenna engineers to design antennas that minimize the amount of energy abssorbed by the body. After all this wastes battery power.
No conspiracy here, just engineers trying to improve the performance of your cell phone.
Philip
I thoroughly enjoyed Hunt the Wumpus. At the time it was a great game to play. I still have found memories of seeing "I smell a Wumpus" appear on the paper.
Younger readers: Find and play it. You'll get bored quickly, but in the process learn where we came from. And just how far we have come.
Slashdot Mirror
It's Jodrell Bank guys. please fix the spelling in the headline.
It's Jodrell Bank guys.
My suspicion is that the cell phone companies and public safety people are sharing channels within the
. ht m
:)
800 MHz band. In this case, additional filtering won't help as the receiver needs to hear signals in the band of interest.
I am guessing that when a public safety radio is near a cell tower, the radio does not have enough dynamic
range to hear the desired weak signal when there is a strong interferer nearby. The AGC limits the gain of the public safety radio based on the undesired cell signal. This would create dead zones near cell towers.
For ham operators bothered by intermod, look at one of these (for 2 M users):
http://www.durhamradio.com/dci-intermod-filters
I just did a quick google, not an exhaustive study.
This sort of interference/intermod problems is a results of wide range receivers. One of the great things about old Radio Shack 2m equipment is that since they only covered 2m's they are very resistant to out of band interferene. TANSTAAFL or however it is spelled
Philip
Haven't you heard what happens to the cellular system during a larger crisis? It collapses when everyone tries to call at once. Didn't you see the articles about people seeking out the remaining pay phones to get in touch with friends and family.
Cell phone networks are designed to work most of the time. Cell phones would be outrageously expensive if the network was designed to handle peak capacity during a crisis.
Public safety types need an effective communication system seperate from public systems. During a crisis, they use discipline to prevent swamping their communication network.
Philip
Many years ago, I had a dead 400 Meg SCSI drive with a mechanical failure and my friend had the same drive with an electrical failure. Combining my good electronics with his good mechanical section produced a working drive.
:)
400 Meg SCSI drive should date this event
Philip
Um, spectrum that supports international communication is shared cooperatively. Obviously there would be chaos if this didn't happen.
:)
At higher frequencies (VHF and up), a signal will (typically) not travel beyond line of sight. So at higher frequencies international cooperation is not as much of a factor. It is still useful for different countries to use spectrum in similar ways so that cell phones, wireless network cards, etc can work worldwide. Well, they could if everyone would get their shit together
Philip
Basically, the government is subsidizing terrestial broadcast TV by allowing a profit oriented industry free use of valuable radio spectrum. What should happen is that the users of the spectrum should compete for the right to use it with other industries. Since most people already pay to receive TV over cable or satellite systems, most people would not see the increased cost. By reducing the number of broadcast TV stations, spectrum becomes available for more interesting and spectrally efficient services. After all the information transmitted in a 6 MHz TV channel can fit in a much smaller amount of spectrum using modern communication technology.
And let's face it, if a terrestial broadcast TV station turned off it's over the air transmitter, what perentage of it's customers notice?
Philip
Well, only if you have a dual LNB hooked up. DirectTivo has two tuners so you can record two programs at the same time, this is a very useful fature!
Philip
So true. I have a DirectV Tivo receiver. A few years ago, I mounted a disk on a piece of plywood so that I could watch the Tour of Italy on vacation.
:)
A friend of mine just bought a 60" V. Today I will find the portable dish and take the Tivo over there so we can watch the commericials with a real video feed, not the crappy, snowy cable signal. And skip the football
Philip
I loved the XOR answer. I learned that trick when I was writing 8085 assembly as a coop in the very early 80's. I'm surprised it is regarded as unusual, by now I would have thought every good programmer would have discovered or seen this trick.
Thanks for the memories. Somewhere around here I have an IBM PC technical reference with the assmebly listing for the BIOS.
Philip
From Part 97.1:
(b) Continuation and extension of the amateur's proven ability to contribute to the advancement of the radio art.
My argument is that other parts of part 97 are reducing our ability to advance the radio art as amateur radio operators.
Philip, K4EP
This is my frustration with experimentation in amateur bands today. I would love to do high speed wireless work in the amateur bands. I live in a rural area without access to any high speed internet service.
:) But under my current interpretation of the ammateur rules, encryption is not permitted.
In order to produce a technically useful solution, I need to encrypt all my traffic. (Never mind some of my email is business related and I like to look at p0rn as much as the next guy
Personally, I believe some flexibility is required in order to motivate people, such as myself, with the technical expertise in radio and networking to explore wireless networking work to create network links. Without the use of these links to replace my current dial up internet, I am not motivated to invest the time and money required to do this.
Of course, this opens a real can of worms in the area of more than casual commericial use. For eaxmple, I could sell you a wireless link and the training required to pass the amateur exam. You would be using amateur frequencies legally (under the conditions I outlined above) however, you would not be contributing to advancing wireless networking.
Philip, K4EP
Bad news? As several people have pointed out part 15 devices must not interfere with licensed services and must accept interference from licensed services.
This reminds me of a conversation I had with the campus admin responsible for wireless networking. He is aware that wireless networking at 2.4 GHz is subject to interference from other services. He feels they can control this by using university policy to control on campus users. For example, if the local amateur radio club decided to broadcast a TV signal over 802.11 frequencies, they could tell them to stop since they would be on university property. However, they know that I could legally build a TV transmitter and a highly directional antenna to point at one of their access point and place this off campus. Of course this would be a waste of my time and money.
What does all this mean? In the 2.4 GHz band part 15 wireless networking devices are at the mercy of the other licensed users of the band.
Philip
Everyone (with an IMDB account at least) head to http://us.imdb.com/Title?0159784 and rate the movie "Takedown" a 1. Show people you are tired of bogus reporting of technology stories.
Me too. Anyone who needs a gaming fix, especially during something as cool as a fencing tournament, needs to be thinking about 12 step programs.
Philip
Well I have a Tivo. Sometimes when I FF through the ads, I'll see one that catches my eye. Then I will watch the ad.I won't watch the same ad a million times, which is what happens on alot of of shows.
Just make ads we want to watch.
Philip
1) There is a head on one side
2) A hand on the other
3) People want the batteries to last forever
4) The phone should work in the basement of your local bar
These factors make antenna design for cell phones very difficult.
Energy radiated in the direction of the head is wasted, since it doesn't come out the other side. So it makes sense for antenna engineers to design antennas that minimize the amount of energy abssorbed by the body. After all this wastes battery power. No conspiracy here, just engineers trying to improve the performance of your cell phone. Philip
I thoroughly enjoyed Hunt the Wumpus. At the time it was a great game to play. I still have found memories of seeing "I smell a Wumpus" appear on the paper.
Younger readers: Find and play it. You'll get bored quickly, but in the process learn where we came from. And just how far we have come.
Philip