thought experiment to understand radio propagation.
If there is a line of sight between them it should work. When you *just* skim the horizon there can be strange effects but radio waves normally travel in straight lines so as long as nothing obstructs the path it will work.
I can pick up the ISS here when it is above the horizon and that is a distance of at least 400Km.
Brian.
so you are saying as long as there are no obstructions between the helicopter and transmitter, the helicopter can detect the radio signal beyond the horizon? just making sure i understand the response.
it also depends on the effective radiated power - a usual receiver can operate on 0.5uV,
The question is explicitly referring to line-of-sight, not considering field strength or wave diffraction.
It doesn't need to. According to your example, the "horizon" for the helicopter is about 143 kilometers (height above ground 1600 m). Why are you talking about "beyond horizon"? It may help to sketch a line of sight diagram.
so if the receiver is higher above the ground, then the receiver can detect signals that farther away?
See https://en.wikipedia.org/wiki/Line-of-sight_propagation
so a radio receiver cannot detect anything beyond it's horizon?
Read carefully!
A radio signal travels outwards from the radiating point (the transmitting antenna) until something obstructs it. That obstruction may be the Earth itself if you have a clear line of sight to the horizon or it could be something nearer. The same applies to the receiving antenna, it picks up signals from anywhere it can see, as far as the horizon or shorter distance if something obstructs the view.
Imagine you have a fence between you and your neighbor and it is high enough that you cant see them. If you climbed up on top of something so your view was from higher up, you would be able to see further over the fence and maybe see them. If they also climbed up higher so both of you could see over the fence, you would them be able to see each other. The same applies to radio waves, the height of the transmitting and receiving antennas matters and in general, the higher they are the more chance of them 'seeing' each other.
This is why you see transmission masts on towers or on hill tops, it's so they can see over local obstructions and maybe to a more distant horizon.
Before every radio expert on the Forum shoots me down, I have to add that there are other ways radio signals can get around. Very low frequencies can travel (technically we call it 'propagation') using ground waves and up to certain frequencies, the highest depends on atmospheric conditions, they can also bounce back from the ionosphere as though it was a mirror and land somewhere over the horizon. Exactly when and how much bounces back is very unpredictable and a topic in its own right but like any mirror, how far away it is will change where the reflection lands. In general, it is uncommon for frequencies higher than about 40MHz to be reflected back so the 'line of sight' rule applies.
Brian.
so if the vhf transmitters was on the ground and some of the radio waves travel at a 45 degree angle they would reach a helicopter that is 10 miles away and 10 miles above ground as long as there are no obstructions? i know the radio waves travel in all directions, but some of them travel at a 45 degree angle.
Yes, that would be true if there were helicopters that could fly 10 miles above ground!
The transmitted waves are not like a string tieing the antennas together, they are more like the light from a light bulb, they travel outwards from the source and gradually get weaker in strength as they spread out over a wider area. Any antenna within the signal area will pick up some of the signal and the closer to the transmitter, the stronger it will be.
Note that another factor becomes involved, called 'directivity', this is a measure of how well an antenna can concentrate it's signal in one direction. Obviously, if you use a highly directive antenna to transmit, it means the signal will be stronger in that direction but weaker in other directions. Most TV stations for example use 'omni-directional' antennas to transmit so their signal reaches the widest possible area but individual houses use directional antennas pointing to it to pick up the strongest possible signal and to exclude interference from other directions.
I used to explain this to students like this:
Think of a rope, say 2 metres (about 6') long tied in a loop. The length represents the power of the transmitter, longer means more power.
Lie it on the ground in a circle and place a stake in the middle of the circle. The stake represents the transmitter antenna.
The rope marks the range the transmission can reach, equal in all directions from the center.
Keeping the circle, pull the rope to one side and you will see the range decreases at one side but increases in the opposite angle.
Keep pulling the rope aside until it hooks around the stake, now there is almost no range on the side touching the stake but much longer range the other way.
Pull even harder, now the circle deforms into an ellipse and eventually it tightly doubles back on itself. The range in the direction you pulled is maxed out.
The outline of the rope represents the directivity of the antenna, the narrower the beam, the further it reaches.
In a real antenna, all this happens in 3D, you have to imagine the rope outline on the ground has height as well as length and width.
Brian.
so in this hypothetical situation if the helicopter is 10 miles above ground, the helicopters radio receiver can detect the radio signal from the vhf transmitter on the ground, beyond the radio horizon of the transmitter, if it has a receiver that is sensitive enough?
Think of it this way - if the hypothetical helicopter pilot could see the transmitter, albeit needing a telescope and space suit, yes it could be picked up.
The 1m above ground transmitter antenna would have a horizon 4.12 miles away,
the 10 miles above ground receiver in the helicopter would have a horizon 523.43 miles away.
If the transmitter is within 527.55 mile radius of directly beneath the helicopter they have a line of sight between them.
As stated previously, I can pick up the transmissions from the ISS from it rising above the horizon to dropping below the horizon, from here on the west coast of the UK, that means I can pick it up whenever it is inside the circle on the map. If you tell me your home town I'll make a similar map showing coverage of your hypothetical helicopter link.
Brian.
Yes, think of a satellite, which covers a large area because it can "see" a large area.
Also think of radio towers for old TV (before satellite TV): they mount VHF/UHF antennas on high towers which are on top of the hills.