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


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On Mon, 03 Oct 2005 12:44:20 -0400, J. Teske wrote:

On Mon, 3 Oct 2005 11:52:49 -0400, Ari Silversteinn


I think the problem here is not the technology, but a fundamental
misconception of what it is you are trying to prove and who is likely
to be your audience. At this point you do not need an engineer, you
need a sociologist.

W3JT

A sociologist?

Yeah, to figure out what has to be done to get the audience to listen
to you. A psychologist might also work.

Gee, are you trying to hurt my feelings?
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On Mon, 3 Oct 2005 11:57:50 -0400, Ari Silversteinn
wrote:
You don`t need a radio tire truck. You need a SOUND TRUCK with an AIR
HORN to get the audience`s attention to listen to the message.
I have seen these systems and they have failed to do two things.
Be heard in a modern car with loud radio and great insulation and 2) pass
the populace acceptance test.

On Mon, 03 Oct 2005 09:56:25 -0700, Richard Clark wrote:

And you've offered this is during an obvious emergency? :

The acoustic testing has been done, yes, and the PA is obviously intrusive
to the nth degree and is not a panacea for all emergency commo.

1 Why would they play music loudly in an insulated car when doom is
on their heels?

Because they haven't yet been informed, perhaps.

2 Why would neighbors complain of noise telling them they were about
to be die?

It's not always a "death or flee" scenario, it could be hazardous but not
deadly.

Richard's description adequately suits:
Local - that would reach out approximately 1 mile and "overbroadcast"
to those immediately in danger within a stationary site.

Your objections are more tailored to suit your solution than the
problem.

73's
Richard Clark, KB7QHC

I have a client who is much more knowledgeable in these situations than I
am, maybe even you. It's a custom build, not an Ari build.
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On Sun, 2 Oct 2005 18:29:13 -0400, Ari Silversteinn
wrote:

Reading about LPAM, this looks technically possible but one concern I have
is antennae size. A fire truck, for instance, could have an antenna mounted
on its front, and up to 4 feet over the top of its roof, so we might look
at as much as 20 feet of length. I realize this places us over the 3 meter
max so one of the ???? is whether LFAM is realistic.

On Mon, 03 Oct 2005 10:05:07 -0700, Richard Clark wrote:

Hi Ari,

As you have described it, and have seen the discussion in regard to
the need for ERP - you are so deep in the debit column with AM power
EQUAL to a local broadcaster, that to climb out of that hole would be
prohibitive. READ: no fire truck has a power plant sufficient to
cover both the antenna system losses AND "overbroadcast" as you
desire. And this is for ONE station only. Imagine your broader
mandate to "overbroadcast" all local stations and that hole just gets
deeper.

OK, so we supply the amplification.

Am I way off base here, can any antennae, fractal or other, or any AM
antennae technology, be utilized to design an antenna and propagate this
type of signal?

With such a plea, to the savvy it reveals you are in over your head.

73's
Richard Clark, KB7QHC

Doesn't matter if I'm in over my head, the technical side is one I only
need to be able to commo on, there will be others who will be cast with
this task.
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On Mon, 03 Oct 2005 13:37:49 -0500, Ben Jackson wrote:

On 2005-10-02, Ari Silversteinn wrote:
This would be an "overbroadcast" (my term)
in that it would override local AM radio broadcasting to reach into cars,

My experience with AM (on the aircraft band) is that two transmitters
at the same time yields a horrible squeal and nothing else. The louder
station (usually on the ground with hundreds of watts) has no particular
advantage over the other station (usually in the air with 5-10 watts).
In other words, there is no effect like FM capture.

Are you saying that if I am attempting an overbroadcast on, say, 680 AM and
the station is doing the same, and I am 12 db + greater, that all we will
get is squeal?
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Ari Silversteinn wrote:
Are you saying that if I am attempting an overbroadcast on, say, 680 AM and
the station is doing the same, and I am 12 db + greater, that all we will
get is squeal?

One signal is the Beat Frequency Oscillator. I am amazed
that no one has thought of this perfect solution before.

If Morse code were required for all driver's licenses, then
all you have to do is furnish a CW carrier wave. The two
signals would mix and the squeal would be Morse code which
would carry the emergency information.
--
73, Cecil, http://www.qsl.net/w5dxp

On Mon, 03 Oct 2005 20:53:17 GMT, Cecil Moore wrote:

Ari Silversteinn wrote:
Are you saying that if I am attempting an overbroadcast on, say, 680 AM and
the station is doing the same, and I am 12 db + greater, that all we will
get is squeal?

One signal is the Beat Frequency Oscillator. I am amazed
that no one has thought of this perfect solution before.

If Morse code were required for all driver's licenses, then
all you have to do is furnish a CW carrier wave. The two
signals would mix and the squeal would be Morse code which
would carry the emergency information.

What if your fat as a whale and your DL is wedged up your crack in your
back pocket?
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On Mon, 3 Oct 2005 16:33:52 -0400, Ari Silversteinn
wrote:
OK, so we supply the amplification.

OK, this is just a re-run of previous speculations. You don't know
the depth of your energy budget crisis, do you?

Doesn't matter if I'm in over my head, the technical side is one I only
need to be able to commo on, there will be others who will be cast with
this task.

How many leaps of imagination will it take to land on the gravy train?

73's
Richard Clark, KB7QHC

"Ari Silversteinn" wrote in message
...
On Mon, 3 Oct 2005 12:01:14 -0400, Fred W4JLE wrote:

In that case, simply modify the sign to "Tune to 560, it could save your
life" and use a discrete frequency low power transmitter.

I assume you mean to take up an unused local channel? Or to bargain for
time on a used one?

Both ideas make sense.
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if this is for real time notification you don't have time to bargain or to
find an unused channel... and who would listen to you anyway if it was
unused? how often do you turn the knob on your car radio??

what about the emergency broadcast system? that would seem to be the
'proper' method of emergency notification, its already set up for all sorts
of local, regional, and national notifications.

There are a couple of frequiencies for unlicensed low power am stations. You
can see them in use by real estate folks selling houses. There are no
commercial stations on the frequency.

"Ari Silversteinn" wrote in message
...
On Mon, 3 Oct 2005 12:01:14 -0400, Fred W4JLE wrote:

In that case, simply modify the sign to "Tune to 560, it could save your
life" and use a discrete frequency low power transmitter.

I assume you mean to take up an unused local channel? Or to bargain for
time on a used one?

Both ideas make sense.
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