Propogation

[zachtheterrible]

In a totally open space with no obstacles, will a 1GHZ radio signal go as far as a 100MHZ signal at the same power level?

I am not talking about natural phenomena, such as very low frequency signals traveling in the earth's ionesphere (or somethin like that) and going halfway around the world, or VHF and UHF communications being line of sight.

 

[Nigel Goodwin]

Outside the earth's atmosphere (actually in SPACE), I see no reason why not - and microwave frequencies are the ones usually employed for space craft communications. On the earth itself, 100MHz will travel further (1GHz is absorbed more by the atmosphere), and also 'bend' slightly, which 1GHz won't.

It's benifical that the high frequencies allow small aerial systems with very high gains - and you can't really afford massive aerial on space craft.

Another reason of course is that high frequencies can escape the earth, lower frequencies can't (they bounce back, which allows round the world communication) - as I remember 30MHz is about the transistion point?, but higher frequencies are better.

 

[zachtheterrible]

Thanx for the quick reply Nigel :lol:

 

[Pyroandrew]

Assuming no fresnel effect and there is nothing in the way, to simplify things, a 1GHz signal is subject to 20dB more of freespace loss than a 100MHz signal. Two signals identical in every way except frequency will not travel the same distance. A 1W 100MHz signal will travel 10 times further than a 1GHz 1W signal before it disappears into the noise floor. The reason being is that, the higher the frequency the more energy that is needed to produce a signal.

In spacecraft and satellites, huge antenna are employed. You can only achieve low data rates at very long distances because you need to keep the carrier frequency low, so your power gets your data over that longer distance. The Venera missions used antenna and dishes that were over 10m wide or long. The vast majority of earth orbiting satellites use 4GHz for the downlink and 6GHz for the uplink. Note the fact that the downlink carrier is the lower out of the two, this is because the satellite has less power available but still needs to achieve the same distance.

 

[zachtheterrible]

Outside the earth's atmosphere (actually in SPACE), I see no reason why not - and microwave frequencies are the ones usually employed for space craft communications. On the earth itself, 100MHz will travel further (1GHz is absorbed more by the atmosphere), and also 'bend' slightly, which 1GHz won't.

You're telling me the exact opposite of what Nigel said. Who is right???

 

[Pyroandrew]

If you imagine a radiation source (rf etc.), the radiation spreads in all directions. The radiation emitted at a point in time expands like a balloon. A spherical expansion occurs, because of this inverse square law if applicable. If you double your distance you quater the recieved power. But it is not quite that simple in the real world.

The formula to work out Free Space Loss is as follows

FSL = 20Log(ω/2Pi) + 20Log(s) + C

Where C is constant depending on what unit you use for distance. It is 36.6 for miles. It is 32.4 for km. The FSL is expressed in dB.

As you can see, as well as a distance componant, there is frequency componant in the equasion. As explained before this is because a high frequency signal needs more power to produce a signal of the same level.

 

[Nigel Goodwin]

As you can see, as well as a distance componant, there is frequency componant in the equasion. As explained before this is because a high frequency signal needs more power to produce a signal of the same level.

I'm quite happy to be proved wrong, but which is the frequency component in the formula?.

 

[zachtheterrible]

So let's say that I want to have a short whip antenna. Would I be better off doing low frequency, which would mean an antenna that would not be totally efficient, but have better range; or high frequency, which would mean an antenna that would be totally efficient, but have a smaller range? Or is there a happy medium somewhere?

I don't care about data rate.

 

[Nigel Goodwin]

So let's say that I want to have a short whip antenna. Would I be better off doing low frequency, which would mean an antenna that would not be totally efficient, but have better range; or high frequency, which would mean an antenna that would be totally efficient, but have a smaller range? Or is there a happy medium somewhere?

I don't care about data rate.

If your aerial is too short it will perform FAR worse than any problems occuring due to the frequency in use (within reason!). But bear in mind that any VHF signal (or above) is essentially line of sight.

How about mentioning what you have in mind?, there are VERY strict restrictions on what frequencies you can use, and what you can use them for.

 

[zachtheterrible]

How about mentioning what you have in mind?, there are VERY strict restrictions on what frequencies you can use, and what you can use them for.

Not really sure, i was just curious. :lol:

 

[Pyroandrew]

ω/2Pi = f

 

[Nigel Goodwin]

ω/2Pi = f

So why not replace it with 'f' in the formula?.

Another query?, what is 'ω', it looks like a capital omega?.

 

[zachtheterrible]

what is 'ω', it looks like a capital omega?.

Wavelength? Even though its the wrong character.

 

[Nigel Goodwin]

what is 'ω', it looks like a capital omega?.

Wavelength? Even though its the wrong character.

Not a bad try!.

But the formula doesn't work for that:

Frequency = 300,000,000/wavelength

 

[Pyroandrew]

The "ω" is indeed an Omega, it does not stand for wavelengh though.

It is, frequency, in radians per second.

If is used a lot more than f in most calculations as it provides an easier and neater solution with less working out. In this instance a neater way of expressing it is by using f, sorry for any confusion, it's a habit, a bit like always having your calculator set to radians all the time, and then messing up calculations because of not changing it back to degrees.

 

[Nigel Goodwin]

The "ω" is indeed an Omega, it does not stand for wavelengh though.

It is, frequency, in radians per second.

You learn something every day :lol:

I recognised Omega because it was mentioned at technical college (back in about 72/73?), the lecturer described it as 'like a babies bottom' - I can't remember the context though?.