Jump to content
Electronics-Lab.com Community

FM WAVE PROPAGATION


prateeksikka

Recommended Posts


  • Replies 69
  • Created
  • Last Reply

Top Posters In This Topic

Hi Prateek,
You should find out the rules and regulations of transmitters in your country.
In Canada, anyone can transmit on the FM band with a power up to 100mW as long as they don't cause complaints about interference, profanity and stuff like that.
You will be in big trouble if you transmit on RF frequencies reserved for aircraft communication, a little higher than 108MHz.

Link to comment
Share on other sites

Hi Prateek,
You can assume that the transmitter is 50% efficient, then measure its operating current and voltage, multiply them to calculate total power used, and divide the total power by 2. If your transmitter doesn't have an output tuned circuit tuned to its operating RF frequency, then it will be transmitting many harmonics, each using some power, reducing the power for the main fundamental RF frequency.
Higher power transmitters operate the final RF transistor in class-C (not biased) which is more efficient. I will try it with mine and measure the difference in transmitted strength with a simple RF meter I'll make.

Link to comment
Share on other sites

Hi Prateek,
You shouldn't blindly choose a preamp for the FM transmitter. It should operate properly over the circuit's power supply voltage range. It should have the required amount of gain and have pre-emphasis, low distortion and a wide frequency response to sound right. Look at the preamp in the original circuit, it had everything wrong.

An opamp would work better than a transistor, I'll try it next.

Link to comment
Share on other sites

Audioguru,
When posting a modification of someone else's circuit, we should mention where the original came from and give the original author credit for his work, no matter how you feel about it.
Would you post the original for all to see? Or at least give us a link?

MP

Link to comment
Share on other sites


Audioguru,
When posting a modification of someone else's circuit, we should mention where the original came from and give the original author credit for his work, no matter how you feel about it.
Would you post the original for all to see? Or at least give us a link?

MP


Hi MP,
Sorry, I don't know where the circuit came from. It might be very old and its author might not even still be alive. After all, I built the same kind of circuit 42 years ago.
I was just helping someone who found the circuit and built this monster:

post-1706-14279142128352_thumb.jpg

Link to comment
Share on other sites

hi audioguru!
i have read that the world today is craving for bandwidth. thats why we are constantly moving towards higher end of spectrum just like GHz range in satellite communication. But it comes with a disadvantage that higher frequencaties are more prone to noise.is it true that high frequency means high distortion? if yes then why is it not applicable to high frequency F.M over low frequency A.M?
(ANYBODY MAY HELP ME!!!!)
PRATEEK

Link to comment
Share on other sites

Hi Prateek,
I don't think microwave and UHF satellite TV and cell phones are noisy. My 900MHz wireless phone isn't noisy. Even the 100MHz VHF FM band isn't noisy. They all work perfectly. My cable TV company sends signals 6000km using light frequencies (fiber-optics) and my reception is perfect.
Also, the frequency of the carrier has nothing to do with distortion.

Link to comment
Share on other sites

Hi Prateek,
I don't know about RF noise. I just know that noise doesn't affect the VHF, UHF, microwave and visible-light frequencies that I use.

I have never purchased "bandwidth"except for the high-speed-cable that connects me to the internet. Its bandwidth is about 50 to 100 times as much as dial-up, but costs only twice as much.

Link to comment
Share on other sites

Well a 100Hz signal would need a very large antenna 747.5km would do the job.

Low RF frequencies are noisy as they are effected by noise from thunders storms and ignition systems as these noise together with the signal are reflected back to earth by the ionosphere.

100Hz would be horrible the bandwidth is very low and with power companies transmitting power at 50Hz, and 100Hz is the 2nd harmonic that emitted by rectifiers and other AC power systems would cause lots of interference.

2GHz would be a lot better, the bandwidth is bigger and it's i the low noise region where there's very little sky noise. The only disadvantage is it won't travel beyond the horizen and you're limited to only 100km or so even if you have the transmitter on a tall mast on the top of a mountain. For long distance transmition I would recommend 22MHz a happy medium between distance and bandwidth. Or you could use 2GHz with a satalite link.

Link to comment
Share on other sites

The interference gets higher at even higher frequencies.

At 22GHz resonance of water occures an 66GHz Oxygen resonates.

As we go up in frequency rain and eventually atmospheric moisture becomes a problem. Transmitters become increasingly difficult to build at any decent power level and above radiation above 300GHz the atmosphere is opaque to radaiton only to become transparent again in the mid-infrared region.

Having said that 100GHz is used in very high resolution radar and high speed data comunications.

There is a gap in the electormagnetic spectrum called far-infrared/submilimeter RF, between 300GHz and 30THz that's hard to study because lasers are hard to make wavelenghs longer than 10

Link to comment
Share on other sites


hi audioguru!
very less interference at very high frequencies

No as I've already said the interferance gets worse beyond a certain point in the RF spectrum because the molacules in the air resonate and noise from stars gets worse too.

If you want low noise transmision use the low noise window between 2 and 8GHz, but it gets a lot more noisy above 10GHz.



.microscopic pocket antenna to transmit,what else do we desire?wat is the problem at such high frequencies?


Transistors don't work at reasonable power levels at these frequencies so magnetrons and klystrons are used these are often very big and bulky.



can we expect our radio services to run at few Ghz in near future?

Many already do satalite TV works at around 5GHz.
Link to comment
Share on other sites

More bandwidth means more data per second.

Low THz frequencies aren't used but the higher 300THz near infrared is used in fibre optical comunications. This is way above the frequency that normal transistors work at so LEDs, lasers and photo diodes are used instead.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
  • Create New...