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... 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)

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. 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...

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. 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

Ari Silversteinn wrote:
I am learning AM broadcast basics in particular the issues regarding the
geo-targeted broadcasting of alert messages from moving vehicles. Would
most appreciate any assistance.

I agree Bob. "Tunnel Radio" systems have been around for over 20
years, and AFAIK all broadcast on dual IF frequencies covering both am
and fm car radios. Here in Boston this type of system is in use in all
our tunnels.

Harry C.

On 2 Oct 2005 18:36:56 -0700, wrote:

I agree Bob. "Tunnel Radio" systems have been around for over 20
years, and AFAIK all broadcast on dual IF frequencies covering both am
and fm car radios. Here in Boston this type of system is in use in all
our tunnels.

Harry C.

Harry, is this a "pickup" system for lost signal?
--
Drop the alphabet for email

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

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:

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

Wonder no longer, because broadcasing at IF frequencies has been for
year the standard in the US. In spite of some rather silly speculative
posts, this mechanism has proved itself for more than 10-years here.

In a confined space such as a tunnel, a transmitter of 10-watts more
than sufficint to get the job done, and using nothing more
sophisticated than a simple wire radiator running the length of the
tunnel.

In Boston, we also retransmit commercial radio broadcasts into our
tunnels, but that requires equipment dedicated to each radio channel
that we re-broadcast, and that becomes very costly after 10 stations or
so, hence there is a limit.

The real challenge is in maintaining emergency communications to the
outside from within our tunnels. All are serviced by the traditional
leaky coax that runs along the top of the tunnel, but considering that
all of these emergency services operate on their own indepdendent
frequency bands, so servicing them simulteously becomes somwhat
problematic. particularly when hand-held, low power devices enter the
big picture.

Harry C.

wrote in message
oups.com...
Wonder no longer, because broadcasing at IF frequencies has been for
year the standard in the US. In spite of some rather silly speculative
posts, this mechanism has proved itself for more than 10-years here.

In a confined space such as a tunnel, a transmitter of 10-watts more
than sufficint to get the job done, and using nothing more
sophisticated than a simple wire radiator running the length of the
tunnel.

In Boston, we also retransmit commercial radio broadcasts into our
tunnels, but that requires equipment dedicated to each radio channel
that we re-broadcast, and that becomes very costly after 10 stations or
so, hence there is a limit.

The real challenge is in maintaining emergency communications to the
outside from within our tunnels. All are serviced by the traditional
leaky coax that runs along the top of the tunnel, but considering that
all of these emergency services operate on their own indepdendent
frequency bands, so servicing them simulteously becomes somwhat
problematic. particularly when hand-held, low power devices enter the
big picture.

Harry C.

All agreed, but that is in a tunnel, which has the advantage of not having
to overcome the still-received commercial stations. The OP wants to do this
out in the open, anywhere, any time. He also wanted to do it while driving
at high speed for some reason, but I think he's quietly shelved that part of
the idea.

Cheers.

Ken

On Tue, 4 Oct 2005 14:57:01 +1300, Ken Taylor wrote:

All agreed, but that is in a tunnel, which has the advantage of not having
to overcome the still-received commercial stations. The OP wants to do this
out in the open, anywhere, any time. He also wanted to do it while driving
at high speed for some reason, but I think he's quietly shelved that part of
the idea.

Cheers.

Ken

MOF, it has reared its ugly hi speed head again.
--
Drop the alphabet for email

"Ari Silversteinn" wrote in message
...
On Tue, 4 Oct 2005 14:57:01 +1300, Ken Taylor wrote:

All agreed, but that is in a tunnel, which has the advantage of not
having
to overcome the still-received commercial stations. The OP wants to do
this
out in the open, anywhere, any time. He also wanted to do it while
driving
at high speed for some reason, but I think he's quietly shelved that part
of
the idea.

Cheers.

Ken

MOF, it has reared its ugly hi speed head again.
--
Sorry, what's 'MOF'?

Ken