# Carrier wave?

Discussion in 'Wireless & RF Design' started by Mathematics!, May 11, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Correct me if I am wrong but in theory to produce a carrier wave. All you need is an LC circuit. Once you charged the capacitors and taken the battery away from it. The charge should ossilate back and forth forever between the inductor and capacitor. With a carrier wave having frequency equal to 1/( 2*pi * sqrt(LC) ) and being sinisodal .

If this is correct (in theory) ,which I am pretty sure it is?

Then I know in nature it isn't perfect conditions

Then what can be add to the LC circuit to mantain the same ossilating
carrier wave.
I.E. so it won't die out over time to resistance heat disspation
or stray capacitance ...etc etc

So the main question is how can I correct the damping carrier wave to a carrier wave that is the same and repeats indefinitely in the ideal conditions?

I am looking for the easiest and least amount of components needed.
I have the L and C what do I need to add to this to get the ossilations
to repeat indefinitly at the same frequency forever.

Thanks for any help.

2. ### Wendy Moderator

Mar 24, 2008
21,839
3,051
Those radio waves leaving the LC circuit are energy, so it damps very quickly. Resistance is only a small part of it. You need an oscillator to make up the losses.

Last edited: May 11, 2009
3. ### leftyretro Active Member

Nov 25, 2008
394
5
And even if you were coupling no energy away from the LC tank, there are still losses in any pratical inductors and capacitors, so some form of amplification and feedback is need to keep the tank circuit oscillating.

Lefty

4. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok, but does anybody have a circuit digram to make the LC tank circuit work indefinitely.
(I.E Ossilate indefinitely at the same resonanting frequency )

Something to compensate for the energy losses.

But how do you do that without break the LC tank circuit ?

Note I am looking for a circuit digram if possible.

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,208
427

Oscillators are a very very big subject. I'd check out the AAC E-book entry on them as a starting point.

By the way there WERE radio transmitters called spark transmitters that used damped oscillations..but they were VERY obnoxious signals by today's standards (not to mention totally illegal!) But if you look up some history of spark transmitters, you'll learn a lot of physics.

Eric

6. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Do you have a link for the AAC E book.

And would a spark transmitter just be a LC circuit with a switch that connects and disconnect's the battery. This would provide a damped sine wave. I wondering if the frequency of the sine wave is damped or if the amplitude just gets damped out. If the frequency is gets damped to then won't this spam the whole radio FM frequency band along.

Last edited: May 11, 2009
7. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,208
427

Well, if you know the history of radio, you will realize that spark DID pretty well spam the entire radio spectrum. However, the damping is primarily in amplitude....the actual FM component was quite small. The sidebands of a damped oscillation are described by the sinc function (sin x)/x.

Eric

8. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok, I read thru the AAC E book in the AC book. The chapter on tank circuits gives me everything I already know. It is assuming the ideal conditons. But I want to know how to make the wave go forever in the LC tank under real world conditions? Note I don't want to have modulation I just want to produce a steady carrier wave into the air.

In the chapter they are assuming an AC current/voltage source. But how can I get this from a 9 volt DC battery. And even if I got the DC to be converted into AC how can I make it so the LC circuit doesn't damp out the sine wave.

9. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
The reference material you want is "Elements of Radio Communication" by Ellery W. Stone, Van Nostrand Co., 1926. It has everything there is to know about spark gap transmitters.

You're not going to get results with a 9 volt battery. The spark contains all frequencies, and the LC circuit simply couples that fraction of the spark energy having the correct frequency to the antenna. You may need 1000 watts in the spark to get a couple of watts into an antenna - one of the many reasons why spark gap transmitters are no longer in use.

Last edited: May 11, 2009

Apr 5, 2008
19,929
4,152
11. ### Wendy Moderator

Mar 24, 2008
21,839
3,051
When I was starting out in electronics I would make Colpitts and Armstrong oscillators with a simple transistor and coils wound on toilet roll tubes, all mounted to a block of wood. While I wasn't aiming to build a transmitter, my Dad's standard response was "Turn it off" when I trashed his TV reception.

12. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Yes , I have seen alot of different ossilators on wikipedia . ..coplitts , ...etc

However I have a radio transmitter I built with a 9volt battery.
It's carrier wave is 88.1 FM. I am using frequency modulation as well.
I can hear it on the radio reciever but the range is very short.

Is their any way to increase this?

And How much power would I need to get 1 mile , 2miles , x-miles ..etc

Their must be a formula to calculate how much distance you get from how much power/voltage/current you have in the rf circuit?

I believe I need to add a power amplifier before transmitting the modulated FM carrier wave? But I don't know how to build a power amplifier and I don't know how much power gives how much distances?

Any help would be great in clearing up the distance issue.

13. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Power is a product of voltage and current. The range of a radio transmission will also be affected by antenna design and receiver sensitivity.

Before sticking a 500 watt linear on the output of your little transmitter, you might wish to acquaint yourself with the FCC rules that pertain to broadcasting on commercial wavelengths. There are large penalties for doing so without permission.

Most of the radio spectrum is spoken for. It is common for a license to be applied for before being able to transmit more than a few milliwatts on almost any frequency.

14. ### PRS Well-Known Member

Aug 24, 2008
989
36
Math, I believe you are asking for the answer to perpetual motion. I was taught this is impossible.

15. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I know how to figure out the optimal antenna size for a given carrier frequency but I am looking for a formula for how many watts (P= VI) that I need to transmit a 1 mile , 2 miles ,...x miles etc..?

Is their a formula that takes in the power you are using and then gives you as an approximate distance that the signal will be able to be picked up on average made radio recievers?

Because I have seen simple transmitters using a 9 volt battery claim 400 meters. But how can you claim anything if their is no approx. formula for the distance of the wave given the power/voltage/or current?

You can assume I have the optimal antenna height for the transmitter.

I am not into breaking any laws here I am just wanting to know how you calculate the distances approx.

16. ### Wendy Moderator

Mar 24, 2008
21,839
3,051

My college had several courses on the subject. Antenna theory is where I think you are wanting to go.

Antennas can focus radio signals into beams. You can even make laser type signals (masers), where all the power is concentrated in a few inches. For omnidirctional antennas it is the inverse square ratio that rules.

In this subject, there simply isn't any easy answers.

BTW, there are legal requirements on antennas also, since they also have gain and resonance.

17. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Mostly they are to concentration the waves in a certain direction more then other directions ...etc . This is useful if you want to generate stronger signals in a certain direction and weaker signals in the other directions...etc

Since I am not that picky I am going to reask the question in a better form.

For the optimal omnidirectional antennas (straight up and down antenna)
Which propagates the wave in all directions with the same amount of distance except the top of the antenna. What is the math behind how far it will beable to be recieved from an average radio reciever?

I hear somebody say it is the inverse square law. But of what the power , current , or voltage.

Could you provide the formula mathematically for me.
Is their any proportional constants in it ...etc?

I would like to see if this formula works on my 9 volt battery transmitter.
I know their is going to be differences in locations where their are buildings ,...etc etc. But I just want to calculated it...

18. ### David Bridgen Senior Member

Feb 10, 2005
278
0
While this doesn't answer your question it may be of interest. Something I found in my notes:
*******************************************

This information is the calculation of the attenuation (in dB) between two sites
due to the propagation of the radio signal through free space.
It is calculated as follows:

Free Space Loss = 92.4 + 20 " LOG(D) + 20 " LOG(F)

Where: D = Path Length in km.
F = Frequency in GHz.

*****************************************************************************

By the way, an amplifier for f.m. doesn't have to be a linear one, a class C will do.

19. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Is their anyway you can calculate the strength of the orignal broadcasted signal in dB?

Then all it would be is (strength of the signal - Free Space Loss) = signal strength left over.

Then if their was some way to say that a standard receiver can recieve a signal of so many db or more and not less then a certian db approx...

But I am just guessing.