Hidden Signals

Hidden Signals was Originally Posted on April 6, 2014 by lavarock

When I was active as a Ham Radio Operator, we had DF-ing exercises. DFing, or “direction finding” involved having another ham use a mobile or home transmitter to send a radio transmission. We would then use our own equipment and wiles to find the “rabbit”, much like a fox hunt.

Although some Hams used large antennas (similar to those old TV antennas), some of us would build a small antenna from PVC and wire. Still others would just use a walkie-talkie, hold it at arms length and turn in a circle. When the signal dropped, then your body was between the transmitter and the radio (thus, behind you).

A more professional system was used by government agencies. I understand that the FCC would take the roof off a vehicle, embed wires in there and then cover them. Looking much like a spider web, by using some electronic magic, they could turn on and off certain patters of reception, thus allowing a virtual direction antenna that was electronically steerable.

I once worked with a larger version of this type of antenna which could be “steered” in a 360 degree pattern and shaped. Thus with no moving parts, the people receiving the signals could determined.

In Atlanta I bought a kit which allowed me to tune hidden sub-carriers of local radio stations. Radio stations use these to transmit control signals and/or audio from the studio to a remote location. One station had their studio audio transmitted and you could hear the DJ answering the phone between records. I don’t know why they would leave that on all the time, unless the DJ had no idea that he was being monitored by perhaps staff. Think of this as an audio nanny cam which, with the right equipment, could be heard anywhere the radio station could be heard.

http://www.fcc.gov/encyclopedia/broadcast-radio-subcarriers-or-subsidiary-communications-authority-sca

Some radio and TV stations used these hidden sub-carriers for paging and other services such as reading for the blind.

The black box used in airplanes uses a very low frequency that other ocean critters and noises won’t interfere with. The signal can get through water pretty well, but I wonder why they don’t use some type of “battery” that would last longer and was powered by salt water.

Another use of “hidden” signals is ELF or extra low frequencies. These very low frequencies have a very long wavelength and require very large antennas. They can be used to pierce through the water to send data to submarines.

http://en.wikipedia.org/wiki/Extremely_low_frequency

While not specifically a radio type signal, you can detect some hidden signals with equipment commonly found at home. A TV remote control usually operates with infrared signals. While not viewable by your eyes, many digital movie cameras can detect infrared signals and will show bright flashes if you point a remote control at the lens and push buttons. This is an easy way to determine if a remote control is completely broken or not.

So there is a whole world out there, hidden from you.