Safe Speed Forums

what happens to radio waves once they have been transmitted? Some are beamed up to a satellite and beamed back down to earth, so I am guessing that they hit the target, but what about say the waves from a transmitter, would they escape the earth gravitational pull and go travelling indefinitely? and if so could they be watched/ listened to on other planets?

AFAIK they go on forever. Have you seen the film "Contact"? The first 5 minutes is very interesting on this subject.

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Malcolm W.
The views expressed in this post are personal opinions and do not represent the views of Safespeed.

As far as I am aware, It depends on the strength of the transmitter and the amplitude of the waves. just as ripples from a stone thrown into a pond, slowly diminish as they move out from the stones point of entry, so do the radio waves. Bigger stone, bigger ripples, which travel further and so the greater the output from the transmitter the further the radio waves travel.

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.

Radio waves are generally radiated all over the show. Some are absorbed within the ground; others are bounced off the atmosphere back towards ground; other others radiate into space and just like visible light (but on a different frequency/wavelength) and don't stop until they are absorbed by something.

By the time these waves reach a potential listener on another planet (not within our solar system, which means at least 4 light years away [going by the next nearest star]) quite a few years would have passed. Given the inevitable dissipation over that distance, the signal received by them would be extremely weak. To give you some idea, a nuclear bomb going off near Neptune (a mere 4 light hours away) would be practically impossible to detect by our sensors without foreknowledge that it will detonate and where - apparently.

Unless we directed those emitted waves in a non-dispersing (meaning not spreading out so not getting weaker with increasing distance) manner using something like a parabolic dish, their inevitable weakness at the listener's position means they would likely be swamped amongst all the other natural emissions of the universe (if not already absorbed by something along the way). If we did manage to direct them in a collated, non-spreading manner, we could target only a tiny fraction of the universe in our lifetime (even assuming a short duration of emission for each targeted area).

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Judging by the crap signal on my mobile phone, some radio waves don't travel far enough ....

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.

But a nuclear bomb isn't a very efficient generator of radio waves. The EMP requires a local magnetic field to capture the high energy electrons generated from the explosion.

Jodrell Bank and Arecibo, in the SETI program, were able to detect the failing 20 watt transmitter on Pioneer 10 as it left the solar system. But, as you stress, we did know where to look, what frequency to use, and how fast it was travelling (for Doppler correction).

The straightforward answer to Adam's question is that the power of the signal diminishes as the inverse square of the distance and will eventually become less than the background noise. And the pull of gravity doesn't affect electromagnetic waves.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

A few reasons for that -and the light bulb analogy is probably best .

Consider a light bulb hanging from a ceiling - it radiates in alldirections. Make it bigger and light travels further and as the power increases so the intensity increases .Just like a mobile ,except that the signal levels are limited .Put the bulb in a reflector and watch the light output increase ,but only in a restricted area .Change the transmit aerial to a directional one ,and mobles could work a lot further away ( but only in the line of the aerial) .You could make the phone aerial directional -but you'd need to point it at the transmitter .
Look at what happens if the light from a bulb is blocked -shadow areas are created .Radio creates shadow ares, surfaces cause reflection /deflection /absorbtion .

As a generality , frequencies at/above VHF ( 30MHz) travel in a line of sight pattern and suffer from the above problems .
As another generality , the lower the frequency -the further it goes ( missing out groundwaves etc-i.e keeping it simple) .
Then there's the effect of the various layers in the sky ,and the time of day .Long time since I did C&G Radio & Line Trans which lookedat all these phenomena .

Then there's the various technologies for braodcast .
Analogue -signal's always there -till it gets swamped by noise -similar to an AM TRANSMISSION
Digital - you can recieve a signal till threshold is reached - similar to FM braodcasts .

Now -don't get me started on the Digital TV Scam - HMG say it's all about more channels & choice .Won't bore you with things about digital transmission /PCM /TDM theory .
But I VIEW IT AS AKIN TO ROAD SAFETY -IT'S ALL ABOUT CASH.

Stuff all the comercial stuff into a lot less space -then SELL off the rest - HEY HO -IT'S OF TO THE BANK WE GO ,WE'VE SOLD THE AIR ,WITH TIME TO SPARE ,HEY HO HEY HO HEY HO

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lets bring sanity back to speed limits.
Drivers are like donkeys -they respond best to a carrot, not a stick .Road safety experts are like Asses - best kept covered up ,or sat on

You can make your handset as directional as you like...you can even use it next to a base station...it does not mean that it will work better. Sure, the signal into/from the base will be better/stronger...but you will still get dumped when the base reaches its saturation....and other users may have priority anyway...and since the signal has an effective range of only 10km or so you may find that when it comes time to handover to another base....there isn't one available.
Then there is the problem that many people do not want cancer...so new cellphone bases are being turned down fairly often...(never mind that those same people get burned bright red on holiday)
Not to worry....my hf rig works just fine worldwide...who cares if my mobile doesn't ?

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The world runs on oil, period. No other substance can compete when it comes to energy density, flexibility, ease of handling, ease of transportation. If oil didn’t exist we would have to invent it.”

56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing

It does give out a lot of electromagnetic waves in general; we're not limited to detecting just the RF subset of it.


Would I be right in suspecting that transmitter was using a directional antenna to direct those few watts directly to your known position. That, in combination with your knowing which direction to look for it, would result with it being rather easy to pick up. This is of course the opposite of a typical RF terrestrial broadcast where, by comparison, anything goes!


Gravitational red shift (meaning less energetic waves [going out of the well]); or to the extreme: a black hole


The risk of an Überthread is rising...

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Jo - you sound like a ham ,so this might go down well - way back in the old evil days of illegal CB, there was a myth that said that you could hear the enforcers closing in .One click - they were close - two , very near ,and three - ust look outside . Problem we had was an idit close to our factory with lots of ilegals -mine was the only legal car ,with a massive whip .So we hatched a plan - I tuned up one of our rigs to chan 19 - we got about 15w out of it -albeit FM .Idjit broadcast - we stuck out a quick signal - he tried again -so did we - and then we never heard anymore .

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lets bring sanity back to speed limits.
Drivers are like donkeys -they respond best to a carrot, not a stick .Road safety experts are like Asses - best kept covered up ,or sat on

Oh god this takes me back to the early 80s and CB radio, CP100 watt burner, Sigma 4 aerial Had a sidebander myself, Tristar 777 .
The joys of 'Skip' Radio signals bouncing off the ionoshere
CQ CQDX Whiskey Delta Charlie ***, Happy days.
Bl**dy Italians and yanks coming through all the time.
The joy of removing ferrite rings from TV tubes to supress TV antenna noise.
Then came FM and kids who totally ruined the whole thing.

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nigel_bytes

The OP was asking about radio waves



Pioneer had a 2.7m parabolic dish - which has a beam width of about 2 degrees at S band. This would be pointed in the general direction of Earth. The main gain, of course is obtained from the huge size of the receiver antenna (and the extremely sensitive cryogenic receivers)



No sir. It was not easy!



Sorry I forgot what a pedant you are The gravitational field of the earth has a negligible effect on electromasgnetic radiation. The Earth's Schwartchild radius is only a few millimeters



Oh no

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Certainly Not!

I understand that up to a certain distance (cant remember the distance) the space craft was "wiggled" slightly so that the dish pointed towards the Earth.

After that distance it wasnt wiggled anymore since the dispresion of the beam encompassed the entire orbit of the Earth!

With a 20Watt signal!

Now, my calculator doesnt have enough places of decimal to calculate how many W/M^2 this works out at at the Receiving end but suffice to say it is VERY SMALL

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"The road to a police state is paved with public safety legislation"

I know; I was only trying to give a sense of scale.


It is easy in comparison to using a non-directional antenna (easier by a factor of about 5000 in your described case).
Looks like I wasn't pedantic enough

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Ah memories.

Back in the early 80s I used to know Radio Hams who were desperate for 13.8 Volts @ Mega Amps, so I used to design and build power supplies for them. I had the mains transformers built to my own spec by a local small company.

My early PSUs were based around a 723 with the obligatory bank of 2N3055s. It had over-volt and crowbar protection etc. and was my first attempt to make money with a view of expanding, but I invested so much time and effort for such little reward I gave up.

Also, my daughter was born in 83 so I had to start changing nappies and things…

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

My calculation, which I am not sure I trust (but Steve will put me right ) suggests that at last contact the received power was of the order of 10^-22 watts/square meter. Even with the gain given by the 5000 sq meter collecting area of the telescope that takes some detecting. Quasars billions of light years away are rather easier.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Though the deep space network did manage to keep in touch with the Gallileo probe - out to Jupiter - with only the isotropic low gain antenna after the high gain antenna failed to deploy. But the raw data rate was only 16 bits per second.

Never

As an aside. I think that those early deep missions, especially the Pioneer 10 and 11, are some of the most impressive engineering feats of all time - much more so than the Moon landings. The thought of those little craft still operating as they depart the solar system ...

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

You really do know me

At Neptune (4.55E12m from sun; ignoring earth's relatively insignificant distance from sun), the surface area of a circular 2 degree width cone at that distance would cover 9E34m^2. 20W spread over that area would yield 2E-34W/M^2. You were ~1E12 out.
Of course that only serves to reinforce the overall point regarding the weak signal strength and the performance of the receiver (and that’s not accounting for its own directionality). Even with that dish size, working with only 1 micro, micro, micro, micro, micro watt of signal (amongst other noise) is quite something.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

And when you consider that "Bleeding Edge" consumer tech at the time was a car radio!

I was also pretty impressed to hear that as of a few years ago (Im not sure about more recently) Hams were still getting a signal from Britains "Sputnik" Prospero!

On amore recent note, I have always been impressed at how the ground teams seem to be able to overcome the most, seemingly, impossible situations in order to achive the missions objectives.

(EG Galleleos antenna, Cassini/Huygans radio link etc)

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"The road to a police state is paved with public safety legislation"

Gotcha! Check my sums (working to one dp)

Take Neptune to be 4.5e12m from the sun . A 2degree beam would be 2*tan(1)*4.5e12 =~ 1.6e11m diameter, which is an area of ~2e22square meters. 20 watts gives 1e-21 watts/square meter.

Multiply by the ~4000sq meters of the telescope gives a received power of 4e-18 watts. In a 50ohm load that is 14e-9 volts.

Thermal noise in a 50ohm resistor at 2K is about 8e-11 volts per root hertz. For a very narrow band of 100Hz that gives 8e-10 volts of noise - an order of magnitude below the signal level which is why we were still able to detect Pioneer 10 at three times that distance (though we suspect that the transmitter power was reducing)

Sorry folks - this is topic drift with a vengeance

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown