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A few questions about radio and transmitting

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8cd03gro

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Hello all. I am attempting to make an invention of mine a reality, but I have a few snags here:

1. Is it possible to direct radio waves? Basically, is there any possible way to transmit a signal to only a select area? Suppose I have a transmitter with 360* of open air around it, but I only want to transmit or at least focus the signal on a specific 45* area. Is this possible? If so how?

2. If I want to transmit on all frequencies, both AM and FM, what would be the best way to do so?

I realize these questions may seem very strange and/or vague, but I am not an electrical engineer and have almost no experience with radio. Thanks for your help guys.
 
what bands? how you get a 45 degree coverage pattern depends a lot on what frequency you're operating at.

your question is very vague indeed.

what exactly are you trying to accomplish?
 
1. Is it possible to direct radio waves? Basically, is there any possible way to transmit a signal to only a select area?
Not really.

Suppose I have a transmitter with 360* of open air around it, but I only want to transmit or at least focus the signal on a specific 45* area. Is this possible? If so how?
It is possibly to transmit most of the RF energy in one direction, but there will still be significant amounts of RF energy going in most other directions.

2. If I want to transmit on all frequencies, both AM and FM, what would be the best way to do so?
Ugh! this is:
a) a very anti social idea, if I understand correctly, you want a transmitter which will trample all over every other user of the RF spectrum.
b) illegal in all developed countries, and most undeveloped ones as well.

I am sorry to pour cold water on your idea, but I will go easy on you as you are not an electrical/electronic engineer!

On a more positive note, what are you trying to do? There may be a more sensible way of doing it.

JimB
 
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It's hard to describe what I am attempting without talking about the product itself, but I will try. I basically want to make a directional version of the FM transmitters used with mp3 players today, but one that broadcasts on all FM and AM frequencies used on a basic car radio. So I guess 88–108 MHz and 535 kHz - 1705 kHz. It isn't necessary to broadcast both from the same transmitter.

Jim, you are assuming my product is to be used for commercial radio broadcasts, which is not at all the case. This product would be of no use to any business other than the business selling it to the consumer, which would be state and local government. I assure you I have the legalities covered. If I could explain to you the uses of the product, you would understand, but I cannot as I can obviously not trust this entire forum to keep from taking the idea for their own.

You said it is possible to transmit most in a given direction. Can you explain this or link me to somewhere that can?

Thanks for the help so far, anymore input is appreciated.
 
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So what your planning is a device that will allow a police officer or whomever is in an emergency or rescue vehicle to talk over the radios in other private vehicles for what ever reason. Preferably to tell them directly to pull over or get out of the way or take some specific action.

By covering all public radio channels and having a directional antennae system the signal only goes primarily in one direction but has enough power to overwhelm what ever signal is presently coming form the station they are tuned to up to a limited distance.
 
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So what your planning is a device that will allow a police officer or whomever is in an emergency or rescue vehicle to talk over the radios in other private vehicles for what ever reason.
And what happens if the radio is off or in CD/MP3 mode? :D
The COPs have a PA system built into their siren for a reason.

which would be state and local government. I assure you I have the legalities covered.
In the USA it is called the Emergency Alert System or EAS for large scale disasters.
 
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Have you ever looked at the antenna farm of an AM radio station? The kind that transmits a directional beam? Their antenna array is usually three to four 400ft towers, spaced about 300ft apart. That's what it takes to make a directional transmitting antenna on ~1MHz.

On 100MHz, it takes an antenna array that looks sort of like a directional TV antenna for the old analog channels 2-5.

Basically, directional antennas have to occupy a physical space that is roughly one wavelength (λ) by λ/2. At AM broadcast frequencies, λ/2 = 150m = 450ft. At FM broadcast frequencies, λ/2 = 1.5m = 4.5ft.

To get directional antennas with small dimensions you have to get up into the GHz frequencies...
 
getting a directional signal on the AM band takes a very large array of antenna towers. unless you have a few acres to spare, it's not going to work in that band. you need towers 1/4 wave tall (75 meters) and spaced 1/4 wavelength apart, and need at least 3 towers in a straight line or 4 towers in a square and a phasing network to set the phase of each tower.. at FM frequncies, it's easier because you can make a directional antenna of a more manageable size, because a quarter wavelength is about 0.25 meter, but that's still not "portably" small.

as for getting the device to "blanket" the whole AM or FM band, that would actually require many transmitters at once, one on each of the "channelized" frequencies within the band. the only other way to do this would be DDS (direct digital synthesis) using a DSP chip, and SDR (software defined radio) hardware.
 
Jim, you are assuming my product is to be used for commercial radio broadcasts, which is not at all the case.
On the contrary I was not making that assumption at all.

This product would be of no use to any business other than the business selling it to the consumer, which would be state and local government. I assure you I have the legalities covered.
Legalities covered? I remain very unconvinced. I an taking a guess that you are either in the USA or Canada, if so did you ask the radio regulatory authority (FCC or Industry Canada)?

If I could explain to you the uses of the product, you would understand, but I cannot as I can obviously not trust this entire forum to keep from taking the idea for their own.
Something like what TCMTech said?

You said it is possible to transmit most in a given direction. Can you explain this or link me to somewhere that can?
I will start you off with this link:
Directional antenna - Wikipedia, the free encyclopedia

The problem with diectional antennas is that the more directivity, the bigger that are.
A dipole antenna for 100Mhz (VHF FM) will be 1.5metres long and has a radiation pattern which has deep nulls of the ends of the elements. To get a radiation pattern where the main lobe has 90degree spread between the nulls, you will need 4 or 5 elements, each about 1.5 metres long and the overall length of the antenna will be 3 or so metres.
To try and do this for the MW AM band would be a completely impractical.

JimB
 
So what your planning is a device that will allow a police officer or whomever is in an emergency or rescue vehicle to talk over the radios in other private vehicles for what ever reason. Preferably to tell them directly to pull over or get out of the way or take some specific action.

By covering all public radio channels and having a directional antennae system the signal only goes primarily in one direction but has enough power to overwhelm what ever signal is presently coming form the station they are tuned to up to a limited distance.

You're on the right track and for all intents and purposes in this thread, your explanation is adequate. However, this is not exactly what I am doing. The basic idea is the same, but my idea is less specific and I believe more useful.

I think I have been given the best answers I could have asked for. Thanks for the help everybody, I appreciate it.
 
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Mind you directional antennas still broadcast some power in lobes behind and around the antenna, so at close range you'll still be broadcasting signals that can be picked up nearby.
 
Do you intend to broadcast on all frequencies at the same time, or on any (selectable) frequency?

all frequencies is otherwise known as a jammer, illegal and antisocial.
any (selectable) frequency has been done.
 
Is it any chance that what you intend to implement is talking / transmitting to a selected group only?

If so, see what "trunked radio" is.

Many variations come to mind. Most probably, all of them already implemented.

Locally, Nextel is providing a quite useful service based on that idea. Only the "suscriber" you are calling will be called (the suscriber may be also a group of users, previously defined).
 
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Hello all. I am attempting to make an invention of mine a reality, but I have a few snags here:

1. Is it possible to direct radio waves? Basically, is there any possible way to transmit a signal to only a select area? Suppose I have a transmitter with 360* of open air around it, but I only want to transmit or at least focus the signal on a specific 45* area. Is this possible? If so how?

Not really.


JimB

YES YOU CAN!!!!!

Look up schematics for Yagi or beam antennas. I'm no expert on this, but there are several online resources to help you design beam antennas that will be resonant on the FM Broadcast band and or AM broadcast band.

It is worth mentioning however that it may be tricky to design such an antenna for your purposes as the AM and FM bands are very far apart from each other on the Radio spectrum. AM Broadcast is MF (Medium Frequency) 540 - 1600 KHz and FM Broadcast is VHF (Very High Frequency) 88 MHz - 108 MHz (or 88,000 KHz - 108,000 KHz). I personally would not know how to construct such an antenna, so you might need two.

Most importantly, make sure you are complying with FCC rules and regulations.
 
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Radiation patterns show in what direction power is radiated to.

But when you consider what is called the "near field", the non desired "listeners" located there, could be well in condition to receive the emision with enough power to make good use of it.

Knowing so little about your hard to disclose project makes difficult to imagine actual conditions.
 
that works.... at 2.4Ghz, not at 100Mhz, not unless you want to weld a couple of 55gal drums together... and unless you own an oil refinery, you won't find a circular tank big enough for 1Mhz...

so let's state this again.... for directional antennas, you need multiple radiating elements at least 1/4 wavelength long, spaced at intervals of a minimum of 1/10 wavelength (most directional antenna arrays in the AM band have them spaced 1/4 wavelength apart with phasing arrays between the feedpoints so that the directional signal can be shifted). a waveguide antenna must be a minimum of 1/4 wavelength in diameter with a 1/4 wave (actually slightly less due to the capacitive effect of the walls of the waveguide) element placed 1/4 wave from the rear wall. so at 100Mhz, the waveguide must be at least 3/4 meters in diameter (wavelength is 3 meters for the FM band), and at 1Mhz the critical diameter would be 75 meters (wavelength is 300 meters for the AM band). so for 100Mhz and 1Mhz "cantennas" aren't very portable (or practical for the 1Mhz version)
 
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I could be way off, but from what I've found a 100mhz 'cantenna' would require a can over 6 feet in diameter.
 
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