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prateeksikka

FM WAVE PROPAGATION

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HELLO EVERY BODY
I HAD A THOUGHT IN MY MIND A FEW DAYS BEFORE.I HAD READ TILL NOW THAT F.M WAVES PROPAGATE ALONG LINE OF SIGHT JUST LIKE ANY OTHER VHF SIGNAL.BUT I COULD NOT FIND THAT WHY F.M WAVES CAN NOT TRAVEL ALONG THE GROUND LIKE A.M WAVES?IN OTHER WORDS,WHAT DIFFERENCE DOES IT MAKE WHEN IT COMES TO MODE OF PROPAGATION IN TERMS OF FREQUENCY.PLEASE HELP ME
PRATEEK

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AM waves reflect on the ionosphere and so can travel by bouncing: ionosphere -> earth -> ionosphere etc.
Higher frequencies cannot do that. So you need a line-of-sight with the other end of the link.

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I think that all antennas radiate in all directions. I also believe the reason behind focusing the signal is because of the power that is wasted from trying to generate a signal in all directions. This means that all of the power put into a signal will be used in the transmission. This is ideal but not practical unless you have a line of sight.

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hi autir!
i have gone through the link sent by you.i have found it much more benificial in understanding modes of propagation of various electromagnetics.according to the link sent by u,it is the water vapours ion atmosphere which determine the mode of propagation of f.m waves as line of sight.do u really think that it could be water vapours?i m surprised!any explanations supporting this which u would like to quote?i know its not a hardcore electronics question but still................plz help any other reason (open to all)
prateek

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Hi Shekhar,
If your building has steel floor beams with steel-rods in reinforced concrete, I doubt that a simple FM transmitter will penetrate.
However if it just has wood-sticks and string, it will probably be OK to travel a few stories. Of course, it also depends on the sensitivity of your receiver.

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hi audioguru and shekhar!
any comments on my question of why high frequencies travel line of sight?why cant waves like f.m waves travel along the ground?is it due to the fact that they cant face multiple reflections.again the question why cant they suffer reflections?
prateek

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Hi Prateek,
No reflections at FM frequencies? Don't you have any tall steel buildings in your town?
They sure do reflect alright! It's called multipath distortion, where a reflection or more cancels or corrupts the signal. It's so bad that car and radio manufacturers have cars and radios with 2 antennas and a switching circuit to select the best one at the moment. Singers and priests with wireless mics also use the same arrangement. The circuit is called True Diversity.

Because the FM broadcast band and wireless mics RF frequencies are so high is why they don't reflect off the ionosphere. Similar to light.

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Ok here is my generic analogy wavelengths. Radio waves are similar to sound waves in the sense that high frequencies tend to become excited and erratic when obstructed (treble becomes washed out distorted and tinny reflecting off of glass)Microwaves create heat by the waves bouncing off everything...etc.
Bass from a car can be heard blocks away before high freq.'s become audible , looking at an audio wave resonating at approx. 45hz, 1 cycle is about 32 feet long , now say 1 cycle at 1khz is 1 inch long. now just by happenstance some freq.'s are gonig to be absorbed, reflected and , some will be just right to penetrate certain materials,

I know from experience that UHF paticularly around 450mhz penetrates cement, heavily forested mountainous areas like no other. why? just because! thats the way the wave acts when presented with a certain obstruction...like spaghetti in a strainer< water becaus eif you just mpasses thru , other matter stays trapped unable to penetrate. Ican't explain it much better than that. I know what i mean ;).

By the way, by "fm" you mean vhf between 88-108mhz correct?
I don't beleive the external encoding/processing of the signal can have much effect, if any on the waves physical behavior...


FYI the rf dispersion patterns i've seen from most unity gain antennae looks like a baloon radiating vertically from the tip, and something like say a maple leaf radiating horizontally, loading coils (found on gain antenna's ) flatten the baloon out horizontally which concentrates more of the signal where we are intending it to go...
heres some more
http://www.google.com/search?q=Antenna+rf+radiation+pattern&btnG=Search&hl=en&lr=


so theres my frigging novel on the subject... now lets see who gets it...

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HELLO PRATEEKSHA,
Ur ques. was very intresting, but i suggest u to better go through the book Eletronic communications by KENNEDY consequently u may try Introducy=tion to modern radar system by Merill I Skolnik.
AKSHAY,

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hi audioguru and akki!
i have read the book on"electronic communication systems "by george kennedy and also by "shoenbeck"
all books say the same as audioguru said in his reply to me. that they do reflect(i.e f.m waves do reflect) and it is called multipath distortion.but i again revive to my question then "WHY NOT FROM GROUND AND THUS GROUND WAVE PROPAGATION" OK THEY DO REFLECT FROM STEEL STRUCS AND BUILDINGS BUT WHY NOT FROM GROUND?one book also says that if they reflect off the ground it undergoes destructive interference.can u tell why?
prateek

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FM wave will reflect by ground...... why not? ???
And the destructive interference is mainly caused by the delayed reflected wave (multipath) + original wave received by the receiver. (for example, in some area, TV set will get some "ghost image" due to the delayed signal mixed with the original signal)

I think this book Principle of Mobile Communication by Gordon L. St&#252;ber is a very good reference. It has mentioned some RF propagation modelling (like OKUMURA-HATA AND CCIR MODELS), where can provide you more deeper understanding in RF propagation.

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most books on electronics communication systems clearly write that F.M waves do undergo more distortion in reflection of the ground than low frequency a.m waves.why is it so?in other words why a.m waves dont travel line of sight and why f.m does?
prateek

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hi pratik
fm can not travel in space as in case of AM because am has frequency band (500kh- 22khz) which have properties to reflect from
ionosphere while fm have frequency band (88mhz-108mhz) which have properties that it passes the ionosphere not reflected so we can not get large distance communcation in case of FM

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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:

post-1706-14279142127481_thumb.gif

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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

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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.

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