Radiation RF/Audio

Get Down, Taylor! :)

--Winston

Do not know the use,but there was one on the SF Bay area, morse code "SRF".

LORAN-C and WWVB were (are?) VLF, but not audio. GPS will eventually kill off both.

Fun: make a big loop antenna, a yard square maybe, lots of turns of fine wire. Connect to an audio amp and listen. All sorts of cool and strange atmospherics.

John

Read all about it

Yes!! Ages ago, a small and crazy record company (Cook) did that and made a Binaural* recording of "whistlers" on the east coast. The antennas were placed on a direct E-W line to be perpendicular to the poles. One would hear a crash (at times one after another), the sound of lightning, then a delay, then an eerie down-going in tone (i think that is the way it was) and you could hear it travel back and forth N..S..N etc as it grew weaker.

They also did a full disk (an hour) with uncompensated bass needle travel of an earthquake; if the compliance of your needle was not good, it would throw it off the record(!).

Another record was an hour's worth of clucking chickens.

They also made a number of audio test and compliance records: horizontal velocity bands at different frequencies with constant velocity, constant level etc; had some with vertical cut (this was BEFORE stereo); maybe 5-6 records of different types that way.

I think they were on the cutting edge, introducing the use of the hot needle, introducing vinyl records, introducing wider frequency range, etc.

• Binaural - done years BEFORE stereo, disk had two tracks - outer was Right and inner was Left (or maybe it was the other way - do not remember) and sold a clip-on arm for the inner groove; the pitch of the groves was adjusted for perfect tracking.

Receiving VLF/LF signals is quite easy, due to the very high atmospherics noise levels at those bands. You only need to collect enough RF power, so that the band noise is slightly stronger than the receiver's own thermal noise.

Just put up a few meters of wire in the air, far away from any man made sources. Of course, such antennas are very much smaller than the wavelength and hence very capacitive. To match this to the 50/75 resistive receiver input, typically a low capacitance source follower is used. Alternatively, a high impedance resonance circuit (high L/low C) is used and the inductor is tapped at some low point followed by some emitter followers.

For communication purposes, the problem is the transmitter antenna. For small antennas relatively to the wavelength, the radiation resistance is inversely proportional to the square of the wavelength. The loss resistances (including any loading coil, antenna element and grounding resistances) are in series with the radiation resistance.

At those frequencies with radiation resistances in the order of milliohms and loss resistances of several ohms, the antenna efficiency (and hence gain) is extremely low.

Some ballpark figures: A 90 m radiator in the 300 kHz aviation and maritime beacon bands has an efficiency of about 1 %. In Europe, in the 135 kHz amateur radio band, the maximum allowed radiated power is

-30 dB.

At still lower frequencies, the antenna structures need to be really big and the power demand is huge.

Of course, with extremely low data rates (e.g. 1 bit/minute) information can still be exchanged. One hypothetical use for such low data rate communication for a submarine might be "Submarine XYZ, move closer to surface for further instructions at higher data rates". This could be effectively sent with only one bit, so it does not matter, if this takes a long time.

Thank you to all of you . Very useful information.

Taylor

I remember my Knight Star roamer had a VLF band -- you know you're a true geek when you spend an hour or 2 tuning it up and down -- it was the higher end of VLF but could be pretty weird.

Will it? A WWVB reciever is a lot simpler for an 'Atomic Clock', and they not only raised the transmitter power last decade, but put up new towers to inprove the east coast & South East US coverage.

Larkin == AlwaysWrong in drag ;-) ...Jim Thompson