In the radio spectrum, higher frequencies attenuate faster. Also, in my understanding, higher frequencies travel more along line of path. Lower frequencies can penetrate through solid materials but higher frequencies cannnot (it depends upon the structure since light can travel through glass too)
Why is then we can see starlight. why doesn't it attenuate. Why laser light can travel longer distance?
The energy in an EM wave propagating through a lossless medium ( vacuum of space) is constant when integrated over the total area of radiation at the wave front . In an isotropic radiation( equal radiation in all directions) the wavefront is spherical and like the surface are of an inflating balloon, it becomes greater as the ballon expands - ie the radiation wavefront moves outwards spreading the same energy over a wider area. What does this mean? To receive the em radiation, a receiving aerial requires to capture the EM energy , this means that it has to cross a physical structure comprising of a conductor or cavity that is resonant. perpendicular to either the electric or the magnetic component of the EM field at the wavefront ie to receive information carried by it. The larger the apeture or cross sectional area of this structure presented to the em wavefront the larger the energy ( received signal , voltage or current) So for an aerial of fixed size, the recevied energy becomes less the greater the distance from the transmission as the same transmitted energy is spread ou over a larger area as the distance increases . Most practical transmitting aerials ar focussed to some extent - as the isotropic radiator is a theoretical ideal, this means that the energy loss over distance is less than for the isotropic case .That is why your TV aerial is the shape that it is - it has directionality, sacrificing omni direction performance for distance -these focused arrays improve the signal to noise ratio also because they receive higher energy from the transmitter than from other possible unwanted sources. The extreme case of focusing EM waves is the laser, as mentioned JeneralJames . Laser light wavefront has is very shortwavelength in relation to radio frequencies, and is highly coherent and therefore will spread over a much smaller area as a function of distance because it is relatively easy to accurately focus. Radio transmissions / ( RF) are also highly coherent but because of there much longer wavelength it is difficult to provide the accurate focusing that can be achieved at light frequencies - due to physical parasitic or stray inductance and capacitances in the aerial array. JeneralJames is also correct in that energy is also lost or more correctly absorbed when "lossy" objects are encounted and that loss is dependent on the frequency and the type of object ( eg steel buildings at low frequencies - wet leaves at high freqencies) - plus mosture or or free ions in the atmosphere ( ionoshere)
OK Shimano. It is sometimes difficult to provide an answer that is crystal clear!.Sometimes an analogy can be useful:- Imangine you have a garden hosepipe with an adjustable nozzel. You want to water your garden , by standing in one position only. nStanding still with the nozzel facing one direction, you can turn on the hose and adjust the nozzel to produce a wide spray which covers and area of ground near to you with water ( isotropic case). If you want to reach furher you adjust the nozzel to produce a narrower stream, producing more of a jet - which travels farther ( assuming the same water pressure is available from the tap). By making the nozzel smaller you can reach further into your garden, but covering a smaller area than before ( focused or laser example!) . In all cases the distance covered can be increased by raising the water pressure from the tap ( equivalent to raising the transmitters power). This is only a rough analogy to demonstrate a principle. There are other factors such as frequency which affect the distance reached which are not possible with this annalogy. In the case of light from a star which is assumed to be perfect isotropic radiator - you see it over millions of light years distance - purely because of it immence radiating power compared to a gsm signal . All EM radiation will propagate across a distance depleting in strength below which we can no longer recieve it - as we reach a limit where it become indistingusiable from other sources. I hope that helps
As an addendum regarding the ability of EM radiation to pass through objects and tendency to be line of sight at gsm frequencies. First of all , let us think about wavelength ( or its inverse frequency)All EM radiation in freespace away from any objects , by that I mean many wavelengths distance, propagatesnaturally in a straight line away from its source. This is true for all frequencies, ie highest frequencies are no more straight than lower frequencies. However, the direction of propagation can be modied by the environment (or medium) in which the EM radiation is travelling. We are now no longer talking about "free space". Here on earth we have objects and mediums which can be closer in distance to the source than one wavelength. Typically for what we call "Long wave" radiation less than say 500 Hz (600 metres) very long distances can be achieved - well beyond the horizon as the EM radiation tends to be bent by the curvature of the earth. This is to do with the conductivity of the earth ( ground) surface) as it causes the wavefront to slow slightly a the point it penetrates the ground ( induced current voltage in that medium). The proper term is diffraction - the same principle as a beam of light passing through a prism. This is called the ground wave mode of propagation . At much lower frequencies where the wavelength is say 60 miles the EM radiation can propagate easily right around the world, this is because the Earth and ionosphere create a wave guide - similar to the hollow tubes that used for very high frequencies - typically above a Ghz. This is called ionospheric propagation and is widely used to communicate with submarines for obvious reasons There is also another ionospheric mode which allows "short and medium " wavelength long distance propagation, by multiple reflections of the wavefront between earth and the ionoshere. These "non straight" line modes exist purely because of the presence and characteristics of a medium or mediums and their relationship to the wavelength of the EM radiation in question. In the case of a GSM signal - you are correct - these are pretty much straight line line of sight propagations, except where reflections occur. This is because the wavelength is short enough such that the diffraction effects from the ground and atmoshere are much less significant at these frequecies. The EM energy absorbed by objects is also related to their size / dimensions in terms of the wavelength. Metal lengths and loops of steel framed buildings absorb or reflect more energy the closer they are to the wavelength or sub wavelengths - particularly maximising at 1/2 and 1/4 wavelength. The object's conductivity is also a factor as it readily absorbs energy . You can see why gsm and microwave signals are attenuated more by trees , leaves fog mosture and cannot penetrate so well, concrete buildings with metal rebar mesh etc and how such structures tend to reflect rather than absorb , due to having sub wavelength resonances. Distances are maintained by the use of fucused arrays are explianed before.
