question: Why does radio reception sometimes increase when we stand next to a radio or touch the antenna?
comment: Your body reacts to electromagnetic radiation (which includes radio). FM just happens to have frequencies near the resonant frequency of the human body. So your body acts like a tuned antenna for some stations.
passages: Section::::Electric current.
  
  Electric currents that oscillate at radio frequencies ("RF currents") have special properties not shared by direct current or alternating current of lower frequencies.
  
  BULLET::::- Energy from RF currents in conductors can radiate into space as electromagnetic waves (radio waves). This is the basis of radio technology.
  
  BULLET::::- RF current does not penetrate deeply into electrical conductors but tends to flow along their surfaces; this is known as the skin effect.
   The average person can notice the effects of changes in radio propagation in several ways.
   High-power, extremely-low-frequency RF with electric field levels in the low kV/m range are known to induce perceivable currents within the human body that create an annoying tingling sensation. These currents will typically flow to ground through a body contact surface such as the feet, or arc to ground where the body is well insulated.
  
  Section::::Effects by frequency.:Shortwave.
   BULLET::::- Another property is the ability to appear to flow through paths that contain insulating material, like the dielectric insulator of a capacitor. This is because capacitive reactance in a circuit decreases with frequency.
  
  BULLET::::- In contrast, RF current can be blocked by a coil of wire, or even a single turn or bend in a wire. This is because the inductive reactance of a circuit increases with frequency.
   If unobstructed and in a perfect environment, radio waves will travel in a relatively straight line from the transmitter to the receiver. But if there are reflective surfaces that interact with a stray transmitted wave, such as bodies of water, smooth terrain, roof tops, sides of buildings, etc., the radio waves deflecting off those surfaces may arrive either out-of-phase or in-phase with the signals that travel directly to the receiver. Sometimes this results in the counter-intuitive finding that reducing the height of an antenna increases the signal-to-noise ratio at the receiver. 
   VHF/UHF television and radio signals are normally limited to a maximum "deep fringe" reception service area of approximately in areas where the broadcast spectrum is congested, and about 50 percent farther in the absence of interference. However, providing favourable atmospheric conditions are present, television and radio signals sometimes can be received hundreds or even thousands of miles outside their intended coverage area. These signals are often received using a large outdoor antenna system connected to a sensitive TV or FM receiver, although this may not always be the case. Many times smaller antennas and receivers such as those in vehicles will receive stations farther than normal depending on how favourable conditions are.
   The two "hot" feeds must lie halfway between the "true" ground voltage at the antenna and transmitter: Inside the ATU, the matching circuit's "ground" level is equally different from the actual ground level on either the antenna or the transmitter side as the conventional "hot" wire is. Either the "hot" output wire or the matching circuit "ground" will give you exactly the same shock if you touch it.
   In an ideal communication scenario, there is a line-of-sight path between the transmitter and receiver that represents clear spatial channel characteristics. In urban cellular systems, this is seldom the case as base stations are located on rooftops while many users are located either indoors or at streets far from base stations. Thus, there is a non-line-of-sight multipath propagation channel between base stations and users, describing how the signal is reflected at different obstacles on its way from the transmitter to the receiver. However, the received signal may still have a strong spatial signature in the sense that stronger average signal gains are received from certain spatial directions.
   Section::::Modes.:Tropospheric modes.:Airplane scattering.
  
  Airplane scattering (or most often reflection) is observed on VHF through microwaves and, besides back-scattering, yields momentary propagation up to 500 km even in mountainous terrain. The most common back-scatter applications are air-traffic radar, bistatic forward-scatter guided-missile and airplane-detecting trip-wire radar, and the US space radar.
  
  Section::::Modes.:Tropospheric modes.:Lightning scattering.
   Radio waves decrease rapidly in intensity by the inverse square of distance as they spread out from a transmitting antenna. So the cell phone transmitter, which is held close to the user's face when talking, is a much greater source of human exposure than the cell tower transmitter, which is typically at least hundreds of meters away from the public on a cell tower. A user can reduce their exposure by using a headset and keeping the cell phone itself further away from their body.
   At large relative distance, the near-field components of electric and magnetic fields are approximately quasi-static oscillating dipole fields. These fields decrease with the cube of distance: (D"/D") Since power is proportional to the square of the field strength, the power transferred decreases as (D"/D"). or 60 dB per decade. In other words, if far apart, doubling the distance between the two antennas causes the power received to decrease by a factor of 2 = 64. As a result, inductive and capacitive coupling can only be used for short-range power transfer, within a few times the diameter of the antenna device "D". Unlike in a radiative system where the maximum radiation occurs when the dipole antennas are oriented transverse to the direction of propagation, with dipole fields the maximum coupling occurs when the dipoles are oriented longitudinally.
   TV modulators generally feature analog passthrough, meaning that they take input both from the device and from the usual antenna input, and the antenna input "passes through" to the TV, with minor insertion loss due to the added device. In some cases the antenna input is always passed through, while in other cases the antenna input is turned off when the device is outputting a signal, and only the device signal is sent onward, to reduce interference.
   Also, in the part of the near field closest to the antenna (called the "reactive near field", see below), absorption of electromagnetic power in the region by a second device has effects that feed back to the transmitter, increasing the load on the transmitter that feeds the antenna by decreasing the antenna impedance that the transmitter "sees". Thus, the transmitter can sense when power is being absorbed in the closest near-field zone (by a second antenna or some other object) and is forced to supply extra power to its antenna, and to draw extra power from its own power supply, whereas if no power is being absorbed there, the transmitter does not have to supply extra power.
   Section::::Reception.
   My Paranoid Next Door Neighbor
  
  Section::::Reception.
   BULLET::::- 100 dBµ or 100 mV/m: blanketing interference may occur on some receivers
  
  BULLET::::- 60 dBµ or 1.0 mV/m: frequently considered the edge of a radio station's protected area in North America
  
  BULLET::::- 40 dBµ or 0.1 mV/m: the minimum strength at which a station can be received with acceptable quality on most receivers
  
  Section::::Relationship to average radiated power.
   Unlike some other long distance modes, high power and large antennas are often not required to make contact with distant stations via a sporadic E event. A two-way conversation can take place over a distance of several hundred miles or more, often using low levels of RF power. Sporadic E is a rare and completely random propagation phenomenon lasting anywhere from a matter of minutes to several hours.
  
  Section::::Communications beyond 50 miles.:Satellite communications.
   Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate.
  
  Section::::Modes.:Other effects.:Absorption.
   The U.S. Food and Drug Administration (FDA), and U.S. Federal Communications Commission (FCC) set limits for the amounts of RF energy exposure permitted in a standard work-day.
  
  Section::::History.
   Lower power levels at a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than away or TV more than away; and obstruction between the transmitter and the receiver, leaving no clear path.
  
  NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with using better antennas, but Non Line Of Sight is usually requires alternative paths or multipath propagation methods.
   The RF signal for the wireless data transfer can be picked up with antennas. The distance from which an attacker is able to eavesdrop the RF signal depends on multiple parameters, but is typically less than 10 meters. Also, eavesdropping is highly affected by the communication mode. A passive device that doesn't generate its own RF field is much harder to eavesdrop on than an active device. An attacker can typically eavesdrop within 10 m of an active device and 1 m for passive devices.
   In addition, the weather may affect the strength of a signal, due to the changes in radio propagation caused by clouds (particularly tall and dense thunderclouds which cause signal reflection), precipitation, and temperature inversions. This phenomenon, which is also common in other VHF radio bands including FM broadcasting, may also cause other anomalies, such as a person in San Diego "roaming" on a Mexican tower from just over the border in Tijuana, or someone in Detroit "roaming" on a Canadian tower located within sight across the Detroit River in Windsor, Ontario. These events may cause the user to be billed for "international" usage despite being in their own country, though mobile phone companies can program their billing systems to re-rate these as domestic usage when it occurs on a foreign cell site that is known to frequently cause such issues for their customers.
   Face-to-face conversations, phone calls, and on-line chats are direct interactions. TV shows, radio programs, videos and books are forms of indirect interaction. In other words, interaction occurs when one person perceives the behavior of another, whether or not the other is present.
  
  For dyads in small groups, direct interactions sum to n(n-1)/2, where n is the size of the population. Indirect interactions equal n(n-1). Indirect interactions, then, are roughly twice as likely as direct interactions.
   Section::::Modes.:Tropospheric modes.:Rain scattering.
   Section::::Reception.:Media reception.
  