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[SOLVED] Radio Receiver Effect/Building a Radio

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ElectroEnthusiast

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Sometimes, some radio works well when a person stands near it. And starts having problems when there's noone nearby the receiver. What is this effect called? Why does it happen?

And i want to build a FM/AM radio(i know it's not worth really), how components different between the 2 of them(AM/FM)? How do they vary in components?
 

mvs sarma

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the person standing by acts like an antenna
it implies that the Radio's antenna is poor and signal in that locality is also weak.
 

thannara123

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yes i agree with sarma .if you connect an anteana to the radio then there will have the above said effecj. In FM/AM only the components used according to their frequency range thats why the componet are diffrent .See the frequncy range of both.
 

chuckey

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FM stands for frequency modulation and AM is for amplitude modulation. So for a given radio band only the detectors need to be changed (and perhaps the AGC/limiting circuits). The problem is that FM transmissions are normally on band 2(88 - 108 MHz) and AM on medium waveband (550 - 1602 KHz)
So you need to make a dual band radio set, this is a complex item and I would just stick to a AM superhetrodyne receiver as your first venture.
Frank
 

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FM stands for frequency modulation and AM is for amplitude modulation. So for a given radio band only the detectors need to be changed (and perhaps the AGC/limiting circuits). The problem is that FM transmissions are normally on band 2(88 - 108 MHz) and AM on medium waveband (550 - 1602 KHz)
So you need to make a dual band radio set, this is a complex item and I would just stick to a AM superhetrodyne receiver as your first venture.
Frank
So i need to even use two different demodulators[1) AM 2)FM]. Okay.
the person standing by acts like an antenna
it implies that the Radio's antenna is poor and signal in that locality is also weak.
But, there were many things that were nearby, why would only a hman being like me would act like antennae. Is there any name for this effect?
yes i agree with sarma .if you connect an anteana to the radio then there will have the above said effecj. In FM/AM only the components used according to their frequency range thats why the componet are diffrent .See the frequncy range of both.
And this happens only in certain FM Stations. So, it might mean that 'those stations are emitting low strength signals' - Am i Right?
 

dragonolj

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I guess that the person acts like a conductor to the ground, which adds a non-ignorable capacitance between the person and antenna of your receiver. And this additional cap makes your antenna match better than without the person standing there. Thus, I think it is due to the man standing at specific position that the antenna matches better. If the person move to another place, the result will change. You can try that.
 

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I guess that the person acts like a conductor to the ground, which adds a non-ignorable capacitance between the person and antenna of your receiver. And this additional cap makes your antenna match better than without the person standing there. Thus, I think it is due to the man standing at specific position that the antenna matches better. If the person move to another place, the result will change. You can try that.

Can you elaborate the concepts?
 

betwixt

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It far more likely that the other objects nearby are:

1. Not moving, so you don't see if they have any effect at anyway.
2. Not conductive or reflective so they have less influence on the radio waves.
3. Not of a size or shape that makes them a good reflector of signals at the frequency you are listening to.

There are many tales of people noticing this effect in the past, before portable radios became popular. Noticing the signal was stronger when they touched the antenna socket, they drew the (wrong) conclusion it was something to do with flesh attracting the signal and they used to hang various dead animals above the receiver! I'm not sure if it made the signal better but it must have been very smelly to say the least!

As @chukey says, don't confuse AM and FM with any particular radio band. People have associated AM with low frequency and FM with 88 - 108MHz simply because it's often marked on the tuning dials of cheap radios. The truth is, ANY frequency can carry a signal in either AM or FM equally well. And yes, you need different circuits to turn an AM signal to audio from one that turns an FM signal to audio. Over-simplifyig it greatly, an AM signal conveys the audio by controlling the signal strength, the amount of power transmitted follows the audio waveform while an FM transmission has constant power but shifts up and down the band very slightly in time with the audio.

Brian.
 

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It far more likely that the other objects nearby are:
.....AM signal conveys the audio by controlling the signal strength, the amount of power transmitted follows the audio waveform while an FM transmission has constant power but shifts up and down the band very slightly in time with the audio.
Brian.

You mean to say that Human Body reflects the frequency(ies) well, so Radio works well at that instant of time.

Now, i pose an another question:
Why AM waves travel to a longer distance than FM Waves(as in the case of Radio FM and AM stations)?
Is AM same as SW/MW? What are they(sw1/sw2/mw., etc.,)?
 

mvs sarma

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LW and MW use ground wave
while Shortwaves use even ionospheric reflection principles

as far as Ground wave is concerned as frequency increases, the attenuation of signal for the same distance increases.
you may have to take a wave propagation related book and have a general study, if you are studying in school level.
if in engg studies, there are good books that have deepr explanations and mathematical treatment regarding wave propegation.
 

betwixt

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LW (Long Wave), MW (Medium Wave) and SW (Short Wave) are just names for the lower frequency (longer wavelength) parts of the radio spectrum, there isn't a discernable boundary between them and their properties overlap.

As stated before, AM and FM have exactly the same range and both are used across the whole radio spectrum. Don't confuse FM with VHF, the name "FM" on the radio is only an indication of the type of transmission used for domestic broadcasting in the 88 - 108MHz band, it should really be called 'VHF' not 'FM'.

The distance travelled by ANY radio transmission is only limited by the path between transmitter and receiver. At extremely high frequencies some signals are naturally absorbed into the gases and moisture in the atmosphere but for practical purposes all signals travel the same distance equally well. The reason why lower frequencies can be heard over longer distances is because of ionospheric reflections. The signals travel outward from the transmitter in straight lines and as the distance increases the curvature of the Earth creates a shadow and blocks it. At low frequencies the signal continues in a straight line until it hits the ionosphere which bends it back again toward the ground. The shadow is filled in with the reflected signal so it can be heard over a greater distance. At higher frequencies the ionosphere is less reflective and the signal continues in a straight line right through it and into space. The exact frequency at which the reflection starts and ceases depends on many factors and may be as low as a few MHz and as high as tens of GHz but is usually in the region of 20 - 30 MHz.

Yes, human bodies reflect some signal and absorb some signal. We are like poor quality mirrors as far as the signal is concerned.

Brian.
 

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LW and MW use ground wave
while Shortwaves use even ionospheric reflection principles
I had forgot to remember that.

LW (Long Wave), MW (Medium Wave) and SW (Short Wave) are just names for the lower
......broadcasting in the 88 - 108MHz band, it should really be called 'VHF' not 'FM'.....
Thanks Brian,
Just to confirm, i saw a old Radio Tuner, wherein there is a provision to switch to SW1/SW2/SW3/MW So were there something more than FM/AM in those days? I had read about an wikipedia article that explained about Diffraction of waves increases the reach(through Horizon). I'm unable to recall it's name, so i was confused with it and AM.
 

mvs sarma

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It has somehow became customary to call Medium Wave as AM, and 88(86) to 108MHz remains FM.
the entire HF up to 30MHz used to broadcast AM signals, barring few select bands, reserved for CB radio, HAM purpose. Of course, one can manage narrow band FM even in HF band.
 

betwixt

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SW1, SW2, SW3 and so on are all the same, if you look at the frequencies you would see they are just one short wave band split into three parts to make tuning easier. The SW band is generally defined as 3MHz to 30MHz so trying to cover it all in one half turn dial would make it almost impossible to even mark the frequency on the dial, let alone find stations on it. SW can carry AM and FM broadcasts equally well although most stations are AM on those frequencies. Incidentally, even MW frequencies are now regularly carrying digital stereo broadcasts to add to the confusion!

Brian.
 

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