# Question re cell phone being EM transmitter

Discussion in 'Wireless & RF Design' started by robertt, Jun 21, 2009.

1. ### robertt Thread Starter New Member

Jun 21, 2009
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I know EM transmission requires AC voltage. Where does this happen inside the cellphone to enable it to radiate EM waves?? Thanks.

2. ### ELECTRONERD AAC Fanatic!

May 26, 2009
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Robert,

In a cell phone, the devices have a little oscillator that is usually a MEMS (Micro Electromechanical System) or a crystal oscillator this produces the pure AC which then undergoes modulation. Modulation is used to add "intelligence" to your system, which could be voice, data, CW (or morse code). After it undergoes modulation, it goes into a RF amplifier to amplify the signals strength. Then to an antenna. Note that cell phones work in the GHz range this enables them to have short antennas.

3. ### robertt Thread Starter New Member

Jun 21, 2009
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That was a great explanation! I couldn't find an explanation on the web -maybe I didn't search for it correctly. Thank you kindly.

4. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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EM (electromagnetic) waves have many facets that you should understand. You see the higher up you go in frequency, the shorter the wave gets. In amateur radio, we have different bands that we can operate on; we have 2-meters, 6-meters, 10-meters, 20-meters, and a lot more. The measurement of "meters" defines how long the wave is. So for example, every 2-meters the wave repeats itself. Below you'll find a good diagram to show the different wavelengths, and below that picture you'll find the amateur radio bands according to their license privilages. Another example, if i want to know how long my antenna needs to be I can use the following equation - length (in meters) = 300/ frequency in MHz. So if I had a transmitter for 144MHz, 300/144 = about 2 meters just like on the band chart.

Last edited: Jun 21, 2009
5. ### robertt Thread Starter New Member

Jun 21, 2009
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Wow. Thanks for sharing that chart. I have a couple of follow-up questions if you don't mind: (1) At the top of the chart, it says certain wavelengths don't penetrate the earth's atmosphere - both at the high end and low end. Why is that so - that only IR and visible can reach us? (2) Looking at the chart, I now understand the relationship between antenna length and frequency, but I have a question regarding what goes on in the tall radio towers - Do they use the entire structure as an antenna, that is, do they run electricity up and down the whole steel structure with all the cross members or is there just a wire somewhere that runs from the ground to the top (is the purpose of the tower to keep the wire stretched to the required length)?
Thanks again.

6. ### ELECTRONERD AAC Fanatic!

May 26, 2009
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Sure no problem. I'm not quite sure about your first problem, so you might have to research that. As for tall radio towers, they DO NOT use the whole tower. What the tower is for is to get more elevation. The higher up the antenna is, the better. Then they will stick the antenna up onto the tower either vertically or horizontally depending on polarization. If you were to have one station with a verticle antenna and the other with a horizontal you would have a 20dB loss! But If you have both of them as verticle stations, it works MUCH better.