FM WAVE PROPAGATION

[prateeksikka]

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

 

[autir]

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.

 

[Kevin Weddle]

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.

 

[trigger]

Use directional antenna..... you can improve the distance by 30 to 50%

 

[prateeksikka]

for kevin particularly::::

but i guess that will also be the case with a.m.because after all a.m waves are also radiated by antennas.so why not go for line of sight in a.m?

 

[prateeksikka]

hi autir!
i understand your views but i am actually intrsted in finding the reasons why high freq waves cant do that i.e reflect from ionosphere or ground?

 

[autir]

It is a matter of physics. I believe that you are looking for this?
http://www.wonderquest.com/radio-waves.htm

 

[prateeksikka]

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

 

[shekhar_dandya]

Hi Everyone,
Does LOS mean that the t'mitter and the r'ver should see each other?
what if I want to send a signal a few stories up/down my building?In that case how will the x'mission be, considering omnidirectional antennas?
-Shekhar

 

[audioguru2]

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.

 

[prateeksikka]

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

 

[audioguru2]

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.

 

[rf_tower_monkey]

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...

 

[akki]

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,

 

[prateeksikka]

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

 

[trigger]

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. Stüber 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.

 

[prateeksikka]

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

 

[chaitanya1]

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

 

[chaitanya1]

hi
i need circuite for mobile jammer. have you any idea?