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
|