I bet everyone has experienced this. A radio station is coming in rather weak. You walk around the room and notice the signal is stronger when you stand it certain spots and has more static when you stand in others.
This morning I experimented and found that waving my arms in front of the radio also made a difference.
Are radio signals not as pervasive as we think? Do they travel in a beam that you can block with your body?
there are pervasive.
with an indoor radio and antenna you can get shielding and interference depending on what it is close to. your body can also enhance the antenna depending on your distance and position.
radio signals especially AM or weak ones can have fading or noise interference so the signal will vary.
I am not quite understanding your second sentence. Can you elaborate how a human body can “shield” radio waves while at the same time act as an antenna?
Bodies have resistance and capacitance, so depending upon how we are grounded we could be an R/C filter. I recall a resistance of about 470K with the voltmeter leads in each hand.
Oh, and don’t admit to waving your hands in front of the radio. 
your body has capacitance. you become part of a more efficient antenna in certain positions and distance from the antenna.
building materials could shield some or reflect radio waves, though not severely with broadcast radio. electrical wires and devices can interfere.
The radio waves scatter off your body. They an tend to add to or tend to cancel with the direct wave depending on their relative phase. Think of diffraction patterns.
They’ll add at various locations around you, and cancel at other locations. Those locations can change when you wave your arms.
So far I have a physical reason, diffraction and an electrical reason, capacitance. What is the wavelength of a radiowave? Isn’t it too large to be affected by a human body? As for capacitance, I don’t understand that at all. Can anyone dumb it down a bit for me?
broadcast radio waves are too large for the body to be much of a reflector for, shorter radio waves like tv certainly will be reflected by your body.
your body has the ability to hold an electrical charge which is capacitance, if you bring that charge near an antenna it can affect the antenna. antennas are designed to be affected by electric fields. some antennas have various elements that are electrically insulated but in physical proximity to the antenna element (driven element) connected to the wire going to your radio and affect the electric field sensed by the antenna. your body due to its position is becoming part of a better antenna.
I can undertand my body adding to the antenna when I am near or touching it but how does it work when I am 10 feet away? I increase the strength of the signal when I stand 10 feet away. I step one foot to the side and I am reducing the strength of the signal.
I’m still lost.
In the US, the FM band is from 88 to 108 megahertz, so the wavelength is around 3 meters. A human body can reflect these signals, so if it is an FM station you are listening to I assume that is what is happening. You might also hear effects when an airplane or a nearby car or truck passes by.
So I might be reflecting signal away or toward the radio depending on where I stand? If I rotate in one spot does the signal get stronger than weaker etc.?
Note that reflections can be constructive or destructive at the antenna. The destructive interference between the direct “line of sight” path from Xmtr to receiver and the path created by reflection from the ground in-between has a sizeable influence on direct TV/FM-band reception (this is one of many factors affecting FM reception, and is not as much of a factor for DX reception).
Only if you are holding large squares of sheet metal under your arms. Otherwise, it’s your distance from the receiver that matters. You are creating a “multipath” situation–the signal bouncing off of you back to the radio receiver is interacting with the main signal from the radio station. Also, there likely are other variables and additional sources of multipath signals, as varied as motor vehicles, airplanes, trees (especially if the branches are blowing in the wind), and even atmospheric conditions, which can cause signal strengthening and weakening.
FM broadcasts are horizontally polarized so effective antenna and reflectors are horizontally orientated.
This is true for old VHF TV antennas (UHF too, but some broadcasters in this band use elliptical/circular patterns). Most FM broadcasters use circular polarization to avoid having to worry about the receiver’s antenna orientation. Here’s a sample of FM antennas; nearly all are circular polarization, and the first few generally capture the elements of any FM broadcast antenna I’ve ever seen.