Strange CB Radio Hack

"Bill Lucas"

Wanker.

.... Phil

Seems rather unlikely to me.

If I understand what you're proposing here: the two radios would be emitting similar RF signals, but with the polarity of one radio's RF output being 180 degrees out of phase with the other's (assuming identical radio designs, lenghts of coax, etc.).

Assuming completely identical coax / antenna systems, and a completely reflection-free environment (i.e. free space), and an omnidirectional receiver antenna located precisely half-way between the two transmitter antennas (and with all of the antennas being perpendicular to the line connecting them)... what you'd end up with is a receive antenna which detects no signal. It would be receiving two RF signals, on the same frequency, at the same strength, with the same modulation, but 180 degrees out of phase... and they'd cancel.

To sum it up, there would be a deep null at that location.

If you were to move the receive antenna 1/8 wavelength (say, about 4 feet) towards one transmitter or the other, the receive antenna would pick up one transmitter's signal a quarter-cycle earlier, and the other's a quarter-cycle later, and the two signals would be in phase and would thus reinforce... and so you'd hear the signal at full strength. As you move along this line, the signal as seen by the receiver would "picket-fence", changing between full strength and zero and back again.

The very same phenomenon would exist (in slightly different locations) if the two transmitters were producing RF in phase, rather than 180 degrees out of phase... picket-fencing would occur.

Basically, this is the same phenomenon you see whenever you receive identical RF signals through two different pathways of different lengths... "multipath". This occurs constantly, and affects both radio and TV reception.

In order to demodulate *either* RF signal (or both in combination) you'd need some sort of nonlinear phenomenon to take place. There's no apparent nonlinearity of the sort you'd need, in the scenario you suggest.

What would happen if I had two CB radios. Ran both off the same RF oscillator, but inverted one first. The same audio track would then be played into each.

Could this system be used as a form of demodulation to project a "carrierless" message to the point in space equidistant between the two transmitters, that could be audibly perceived by an observer?

Bill Lucas

What have you been smoking?

They would cancel eachother out.

Shaun

Not only are you not paying attention, I do not think you know how.

Hello Bill,

I agree with Dave.

The emitted signal doesn't contain the audio itself (so doesn't contain frequencies that are within the audio band). This will not change when adding two signals from synchronized or non-synchronized sources.

Non-linearity in the transmission path is insufficient to create a low frequency EM component that could be picked up by inductive or capacitive means.

Wim PA3DJS

=A0 =A0 =A0 =A0 =A0 =A0 =A0 AE6EO

Dave's answer is excellent.

Now here's something to keep you up at night:

Also this:

-- Joe

(...)

Now that this is deployed mainstream, there really isn't a reason to present it as an emerging technology.

:)

--Winston

What about the human body which is a non-linear medium?

And/or ... if the frequency of the cancelled carrier was chosen to correspond with the wavelength of the human body?

Bill Lucas

I believe there's a Gilligan's Island episode that addresses this, with a tooth serving the part of a non-linear junction...

:-)

The question would be whether the nonlinearies translated into any significant physical effects, which could result in (a) the signal being demodulated in a physical sense and (b) the demodulation being in a form which is detectable by the human nervous system. There are some nonlinearies in the human body, I suppose, but I don't believe they're large ones - at least not at the power levels we're talking about here. The human body has a lot of (salt)water in it, and salt water is a very good linear conductor.

I've read reports of some such effects, which enabled people to "hear" pulses from high-powered radar transmitters. These instances were said to be due to direct heating of the body (e.g. inside the ears themselves), and occurred only at *very* high levels of RF exposure.

People have reported hearing music from AM radio stations "in their heads", with the culprits being nonlinear detection of the RF due to metal in their mouths (fillings or braces). These situations, also, require high levels of RF - living within a fairly short distance of a many-kilowatt or megawatt-level AM transmission tower.

CB radios are limited to a few watts of power, and even with illegal amplifiers they're usually under 100 watts. Ham radios in the same frequency bands can legally run up to 1500 watts, and although undesired pickup of these high-power transmissions is not uncommon it almost always happens due to nonlinear detection in electronic gear, or corroded metal objects touching one another. I don't think I can recall reading or hearing any reports of "direct body" detection and demodulation of upper-HF frequencies such as CB and 10-meter ham band.

No. The ether is linear.

IF you got up to the point you were ionizing air this might change, but that's Tesla territory, not CB territory :-)