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On Sun, 02 Oct 2005 19:46:56 -0500, Bob Bob wrote:
Hi Ari Thanks for comments, Bob. I always wonder whether broadcast at the most common IF frequencies would be a viable alternative? (eg 455khz for AM) Legality issues aside of course... I'm missing your point. Please explain. This would make frequency selection a little easier to engineer. You wouldnt have to divide your power budget into many operating frequencies as well. Problem is that you'd need a lot higher ERP especially since car mounted radios tend to be better shielded than portables. (You'll need to experiment some) Agreed. Since part of your engineering exercise will be to determine what ERP you want from the system you are proposing, the size of the antenna only needs to be considered in line with the transmitter output power. Obviously there will be a useful compromise somewhere that is cost effective. You might for example want a 100 watt TX for each frequency with a fairly lossy/terminated and loaded antenna. Your first port of call will be to determine what the likely highest strength of an AM/FM broadcast signal you will need to override, add maybe 20dB to that and work out your needed ERP. Good advice here, will 20db do it? You will find lots of texts about the inefficiencies of MF antennas mounted on motor vehicles so that will give you some numbers to work with. Remember that making the antenna wideband will also introduce substantial loss. Note that you cant design the system to always work as you'll have situations where the radio stations transmitting antenna is right next to then freeqy where the event occurred! Decide on some statistical coverage percentage thing etc etc... Yes, we are shooting for max overbroadcasting but the reality is it is a hit and miss proposition. Even the scenario of geo related obstructions is a possibility. I worked for a concern in Sydney Australia that put in tunnel rebroadcast systems in the major underground roadworks in Sydney, Melbourne and Perth. Being in a tunnel of course there werent really any licensing issues. Is that peculiar to Aus? The system consisted of not only broadcast band (AM & FM) receivers but spot VHF/UHF FM receivers and transmitters for police, fire etc vehicles. Each broadcast frequency had its own transmitter. It was thought to be a better idea than using a wide band TX covering all frequencies. AM broadcast was coupled to a very long wire and the rest used a terminated leaky coax run. Cheers Bob W5/VK2YQA Very nice, congrats on that. Was it Yagi technology or fractal? -- Drop the alphabet for email |
#2
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Hi Ari
I always wonder whether broadcast at the most common IF frequencies would be a viable alternative? (eg 455khz for AM) Legality issues aside of course... I'm missing your point. Please explain. I apologise if you already know the following! Its pretty basic radio.. Almost every radio receiver on the market works off the "superhetrodyne" principal where the incoming signal is mixed with a local oscillator down to an intermediate frequency. This makes receiver design a little cheaper as you can then put your expensive filtering at one frequency rather than have something that has to track the actual transmitted frequency. For example a station on the AM band at 870khz is mixed with a local oscillator of 1325 khz (in the receiver) to get a difference of 455khz. If the station frequency is changed then one only needs to modify the local oscillator frequency. This is what the knob on the radio does! The most common IF frequency for an AM radio happens to be 455khz so transmitting on that will mean you will be heard on all AM radios in close proximity. I dont off hand know the most common broadcast FM IF (maybe 10.7MHz or 7.8Mhz) but that should be easy to find. It is important to make sure that the IF of the fire trucks FM two-way radio isnt the same or you'll never be able to hear it inside the truck! To "broadcast over" you would need to amplitude modulate the AM band 455khz TX and frequency modulate the FM broadcast IF frequency. Good advice here, will 20db do it? FM is easier to do than AM. I'd factor in maybe 6dB for FM. You might be able to get by with 12dB for AM but the original station will make some small amount of noise under it. The important thing is that the person listening wil be able to understand the content. Note that this is of course only relevent when transmitting on the actual station frequency. Yes, we are shooting for max overbroadcasting but the reality is it is a hit and miss proposition. Even the scenario of geo related obstructions is a possibility. Well it shouldnt be that hard to model. Do a AM and FM band coverage prediction over the freeways etc you are doing the experiment on to discover what the receive strength will be in dBm. (It doesnt matter what "gain" you make the RX antenna because it will be the same figure that is used for your overbroadcast) For every say 200 yard square take the mean signal of each, then take the 80th percentile as the signal you have to beat. If that was (say) -80dBm you want -74dBm (for FM) at the maximum operating distance (was it a mile each way?) You then reverse model that to determine what the TX EIRP of the thing on the truck has to be. There are bound to be contract houses that will do this prediction for you. I use to work for one that had the software and I have some GPL versions myself. I cant do MF (ie AM broadcast) though. Being in a tunnel of course there werent really any licensing issues. Is that peculiar to Aus? The licensing issue or the tunnel? Tunnel rebroadcast is pretty common around the world. The company I worked for also did one in Indonesia. Its also used in underground mines. Very nice, congrats on that. Was it Yagi technology or fractal? Err, the leaky coax was just a 2km run of an Andrews product that has leaky holes in the shield. It was fed in the middle with a wide band splitter and terminated with 50 ohmss at each end. Its quite a common thing to do in underground environments. (Buildings too) Handy for two way radio as well as cell phones. The AM thing was just a very long terminated (600r) wire. None of the above are yagi or fractal Cheers Bob Ari Silversteinn wrote: |
#3
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Hi Ari I always wonder whether broadcast at the most common IF frequencies would be a viable alternative? (eg 455khz for AM) Legality issues aside of course... I'm missing your point. Please explain. On Mon, 03 Oct 2005 11:50:35 -0500, Bob Bob wrote: I apologise if you already know the following! Its pretty basic radio.. Man, don't apologize, I have this semi-advanced understanding that missed a lot of the basics, if you get the drift. Thanks. Almost every radio receiver on the market works off the "superhetrodyne" principal where the incoming signal is mixed with a local oscillator down to an intermediate frequency. This makes receiver design a little cheaper as you can then put your expensive filtering at one frequency rather than have something that has to track the actual transmitted frequency. Got that. For example a station on the AM band at 870khz is mixed with a local oscillator of 1325 khz (in the receiver) to get a difference of 455khz. If the station frequency is changed then one only needs to modify the local oscillator frequency. This is what the knob on the radio does! lol ok The most common IF frequency for an AM radio happens to be 455khz so transmitting on that will mean you will be heard on all AM radios in close proximity. I dont know off hand know the most common broadcast FM IF (maybe 10.7MHz or 7.8Mhz) but that should be easy to find. It is important to make sure that the IF of the fire trucks FM two-way radio isnt the same or you'll never be able to hear it inside the truck! Now this I did not know! To "broadcast over" you would need to amplitude modulate the AM band 455khz TX and frequency modulate the FM broadcast IF frequency. Good advice here, will 20db do it? FM is easier to do than AM. I'd factor in maybe 6dB for FM. You might be able to get by with 12dB for AM but the original station will make some small amount of noise under it. The important thing is that the person listening wil be able to understand the content. Note that this is of course only relevent when transmitting on the actual station frequency. Got it. Yes, we are shooting for max overbroadcasting but the reality is it is a hit and miss proposition. Even the scenario of geo related obstructions is a possibility. On Mon, 03 Oct 2005 11:50:35 -0500, Bob Bob wrote: Well it shouldnt be that hard to model. Do a AM and FM band coverage prediction over the freeways etc you are doing the experiment on to discover what the receive strength will be in dBm. (It doesnt matter what "gain" you make the RX antenna because it will be the same figure that is used for your overbroadcast) For every say 200 yard square take the mean signal of each, then take the 80th percentile as the signal you have to beat. If that was (say) -80dBm you want -74dBm (for FM) at the maximum operating distance (was it a mile each way?) You then reverse model that to determine what the TX EIRP of the thing on the truck has to be. There are bound to be contract houses that will do this prediction for you. I use to work for one that had the software and I have some GPL versions myself. I cant do MF (ie AM broadcast) though. Found the expertise for that and there is a chunk of open source stuff out there, freeware. Being in a tunnel of course there werent really any licensing issues. Is that peculiar to Aus? On Mon, 03 Oct 2005 11:50:35 -0500, Bob Bob wrote: The licensing issue or the tunnel? Tunnel rebroadcast is pretty common around the world. The company I worked for also did one in Indonesia. Its also used in underground mines. Very nice, congrats on that. Was it Yagi technology or fractal? Err, the leaky coax was just a 2km run of an Andrews product that has leaky holes in the shield. It was fed in the middle with a wide band splitter and terminated with 50 ohmss at each end. Its quite a common thing to do in underground environments. (Buildings too) Handy for two way radio as well as cell phones. The AM thing was just a very long terminated (600r) wire. None of the above are yagi or fractal Cheers Bob Thanks, Bob, I'm all overYagied and underfractalled. lol -- Drop the alphabet for email |
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