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In Asia, specifically in Bangladesh, we often listen to FM radio. But the signal is not clear everywhere, that means it produces some noise.

From my childhood till today, I have observed that, if I keep the radio in a fixed position and I increase distance between me and it, the signal starts to be more and more noisy.

And sometimes, if I touch the antenna, the signal become more clearer. This is an observation from real life. But I don't know the exact reason behind it.

Would you please describe the exact reason behind it?

Note:

This question came in my mind few days before, but I was confused which site best suits for this question. However, I thought this site is the most suitable one to ask the question. If you think it is off-topic here, then don't hesitate to have a migration request to the appropriate site.

I am also sorry for my bad English.

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    \$\begingroup\$ Yeah I know the very effect. When I get out of bed, my bedside radio looses its reception where it was playing just fine before that. I also have to slightly move the radio depending on the time of year, but that's probably a different effect. \$\endgroup\$ – jippie Oct 2 '15 at 7:08
  • \$\begingroup\$ @jippie I think that is another good observation relating to the time of the year. I did not sincerely observe that. \$\endgroup\$ – Enamul Hassan Oct 2 '15 at 7:10
  • \$\begingroup\$ The explanation is similar to the one given here: physics.stackexchange.com/questions/101913/… \$\endgroup\$ – Stefan Gruenwald Oct 2 '15 at 12:32
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    \$\begingroup\$ Is it a battery powered radio using a quarter wave monopole? \$\endgroup\$ – Andy aka Oct 2 '15 at 13:32
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    \$\begingroup\$ @Andyaka I am under the impression that it would help if you gave a brief description of how to recognize a "quarter wave monopole". \$\endgroup\$ – jippie Oct 2 '15 at 15:07
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No one is surprised by the general phenomenon that radio signals get weaker, the further you are from the transmitter. However, most people are surprised at how much the signals will vary within a short distance, locally.

This effect is called multipath fading, and is caused by multiple reflections of radio waves off walls, cars outside, people etc. At the receiver, the signals can combine to be bigger or smaller than the signal would have been with the multiple reflections. Of course, you don't notice a problem when the signals are bigger.

A particular issue with FM radio in the 100MHz band is that a typical human is about half a wavelength tall. This means that moving a human around in the region close to a receiver is particularly effective at changing the multipath reflections.

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    \$\begingroup\$ I think touching the antenna is a different (but related) phenomenon though. You are basically a big antenna, not just a reflector. Apparently the NYT thinks that too: books.google.com/books?id=oRR5K1v86N4C&pg=PT89 \$\endgroup\$ – Fizz Oct 2 '15 at 7:49
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    \$\begingroup\$ This answer could be made immensely better with a bit more detail on why the half-wavelength height of a human is such a significant factor. \$\endgroup\$ – Dan Henderson Oct 2 '15 at 13:01
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Today I found an interesting experimental paper on the issue of touching the antenna (in fact several if you count the related papers cited therein).

Here's a figure with the current picked up via the human body and conducted by hand to a spectrum analyzer. The source of the RF is a "60 cm monopole antenna located 3 m away" and this "antenna is fed by a battery powered RF source tuned to generate 1–200 MHz at 0 dBm."

enter image description here

The "calculated" graph in this figure is based on a model of the human body as a simple cylindrical antenna; the authors of that study also say that the unaccounted for 75Mhz peak might be due to the simplified nature of their model, which doesn't account for the arm itself as a resonant body. Anyway, this experiment shows that the human body is a fairly effective antenna around 40-80Mhz and you get a smaller effect at twice those frequencies (and probably other multiples, but that's not shown here).

And the body can act as a transmitter antenna too. The Top Gear experiment can be replicated in more controlled environments, e.g. using the human arm as transmitting antenna. The arm appears to work as 80 MHz resonant antenna, by assuming that the rest of the body acts like ground plane, the whole body accounting for the boost around 50 MHz. In the experiment below they've also swept from 1 to 200MHz with the "VNA".

enter image description here

Accounting for the effect that the human body has (as a reflector) has been proposed for reducing the size of FM radio antennas needed inside mobile phones, for example.

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