FM WAVE PROPAGATION

[prateeksikka]

hi chaitanya!
my query was why do high frequency waves have such a property and low frequencies have different properties of reflection?
prateek

 

[trigger]

wavelength and the media reflection/refraction ratio.

 

[prateeksikka]

HEY TRIGGER!
I JUST WANNA KNOW CAN I START MY LOCAL F.M BROADCAST AND TALK TO A PERSON IN NEXT BUILDING?IF YES, HOW SHOULD I PROCEED?
PRATEEK

 

[audioguru2]

Hi Prateek,
Make an FM transmitter!
1) The simple one is just a toy. Its preamp is a thermometer and battery voltage indicator. I tried it.
a) With a new 9V battery and a transistor with a typical hFE (300) or more, the preamp transistor is saturated and doesn't work, but works with a battery voltage between about 7.8V and 8.2V.
b) With a new 9V battery and a transistor with a low hFE (196, I found one), it works but is distorted and the preamp's gain drops and distortion increases when the battery's voltage drops and stops working with the transistor cutoff when the battery is about 7V.
c) When it works with a lucky combination of hFE and battery voltage, the transistor saturates when it is warm (I touched it with my finger) and is cutoff when cold (outside here in winter, it's freezing).
d) The radio frequency changes if you or something conductive gets near it, if its temperature changes or if its battery voltage changes.
e) Its received sound is very muffled with no treble. It sounds worse than an AM radio and is distorted. It might be satisfactory for voice but is lousy for music.
f) Its range isn't very far (about 30m outside to my Walkman).
g) The circuit is typical of poor design found on the internet.

2) I fixed most problems with modifications in the deluxe circuit. It should sound pretty good, not drift its radio frequency too much, work when its battery is between 6V and 9V and go a few hundred metres outside. I'll let you know when I finish it.

EDIT: My new preamp didn't have enough bass so I fixed it. It has very low distortion and works well when the battery is from 6.5V to 9.5V. Here is the changed schematic:

View attachment 36608

 

[prateeksikka]

hi audioguru!
dont u think it will be a good idea to use a band pass filter just before modulator in your modified ckt.if yes pls try it out and let me know the results. can i have the reciever ckt too .how are u testing it?
have u made your own reciever too or r u using conventional f.m reciever..
wat frequency range?
prateek

 

[audioguru2]

Hi Prateek,
Both very simple FM transmitters don't have an FM modulator. Their preamp actually AM modulates the FM oscillator. Since the oscillator transistor has its amplitude changing, its capacitance also changes, creating FM.

Maybe you mean I should cutoff extremely high audio frequencies in the preamp, since the pre-emphasis boosts the highs. But it happens naturally by C3 and C8. I tested the high-end response and it peaks at +7dB between 10KHz and 15KHz then slowly drops to 48KHz where it has the same level as lower audio frequencies.
Maybe if speech or music has a sustained 19KHz sound, then maybe an FM receiver would suddenly switch to stereo. I doubt it.

There are many poor FM radio circuits on the web and as kits. Most don't have enough tuned circuits nor enough gain. They certainly don't even have AGC so overload easily. The poor ones are super-regen and circuits using an obsolete Philips "FM tuner in a single IC". You can buy a whole radio with earphones and battery that uses a newer "scanning" Philips FM tuner IC for only $5.00US. I got one for free and it is junk!

My analog Sony Walkman AM-FM stereo little radio works very well and my digital Kenwood hi-fi stereo receiver works perfectly.

In my country the FM band is from 88MHz to 108MHz. These FM transmitters cover it and are easily changed by squeezing or stretching their coils.

 

[prateeksikka]

hi audioguru!
i exactly meant the way you thought..Did u have to purchase the frequency used by you from the authority or one is free to chose any frequency at which transmission is done.IN short can i make a transmitter that works at 115 Mhz. well beyond f.m range and that too without permission!!!!!!!????????
prateek

 

[audioguru2]

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.

 

[prateeksikka]

audioguru!
as far as i know we in india can transmit upto 100 mW in F.M range(88-108)Mhz. how can i ensure that my transmitted power does not exceed 100mW? for frequencies i think that we have to purchase the bandwidth from government.
prateek

 

[audioguru2]

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.

 

[prateeksikka]

audioguru!
how do you rate the idea of using low level modulation in this circuit?
in that case we can almost blindly chose the amplifier
prateek

 

[audioguru2]

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.

 

[MP1]

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

 

[audioguru2]

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:View attachment 36612

 

[prateeksikka]

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

 

[audioguru2]

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.

 

[prateeksikka]

AUDIOGURU!
IF I HAVE 5mV SIGNAL (AMPLITUDE) AND I TRANSMIT IT AT 2 FREQUENCIES SAY 100Hz and 2Ghz .do u think that the two will be equally affected by noise?if not,which one is affected more?
if noise effect is independent of frequency we could not be fighting for and paying huge bucks for purchasing bandwdth.
prateek sikka

 

[audioguru2]

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.

 

[Alun]

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.

 

[prateeksikka]

hi alun and audioguru!
can i conclude that high frequency waves dont travel far because they mostly travel line of sight but have the advantage of low noise over loe frequency signal??????????plz reply
prateek