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Old 14 January 2006, 20:29   #31
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Old 15 January 2006, 07:34   #32
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Quote:
Originally Posted by Doug Stormforce
Popular misconception that power effects range. With VHF it does not make a blind bit of difference. It does not matter how strong a signal you send out, if the top of your antenna is line of sight with the top of the receivers antenna then the signal will get there. All that higher power does is make your signal drown out everyone elses.

If of course we were talkng about MF then an increase in power would increase range but with VHF and HF not so.

I have always worked on the formulae being 2.12 sq route height, however it does not make much difference so lets call it 2.2 etc.

Jon The formulae works for either sort of aerial. The difference is effectivly the angles that the signal leaves the antenna from. I can explain easily by diagram so will show you when I next see you if you are still interested.

Now to put the formuae into practice. If I have an antenna height of 16m and a CG has an antenna height of 100m- how far apart could we communicate?

Well sq route of 16 is 4, times by 2.2 equals 8.8. My radio horizon is therfore 8.8 miles. Sq route of 100 is 10, multiply this by 2.2 and the CG has a radio horixon of 22 miles. OUr range from/to each other is therfore the sum of the two=30 miles. I expect that if you have a look at the rising/dipping distance table in the almanack it will give you much the same result.

In reality of course the CG generally has a higher mast than that and most RIBS dont have a 16m A frame.

Now there are of course various conditions that can increase or decrease this range. Sea State, atmospheric pressure, temperature differences, sun spot activity to name but a few. Yacht sof Gibralter have been known to pick up Dover Coastaguard but when it happens it does not happen for long.
Hi Doug
Iím afraid you are not correct in stating that RF power in line of sight propagation has no bearing on the ability of the signal to be received at the far end.

You have failed to consider Free Space loss.

As a radio signal propagates through the air, it experiences a loss in amplitude. If the range between the sender and receiver increases, the signal amplitude declines exponentially. In an open environment, one clear of obstacles, the RF signals experience what engineers call free-space loss, which is a form of attenuation. The atmosphere causes the modulated signal to attenuate exponentially as the signal propagates farther away from the antenna. Therefore, the signal must have enough power to reach the desired distance at a signal level acceptable that the receiver needs.

Free space loss. Geometric spreading happens because the wavefront radiated signal
energy expands like a big column as a function of the distance from the transmitter.
When the distance from the transmitter is measured in units of the signal wavelength

Shaggy
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Old 15 January 2006, 09:42   #33
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Blimey Shaggy!! So you are the man next time me' wireless is playin' up.
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Old 15 January 2006, 15:27   #34
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Old 19 January 2006, 19:58   #35
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Quote:
Originally Posted by Shaggy
Hi Doug
Iím afraid you are not correct in stating that RF power in line of sight propagation has no bearing on the ability of the signal to be received at the far end.

You have failed to consider Free Space loss.

As a radio signal propagates through the air, it experiences a loss in amplitude. If the range between the sender and receiver increases, the signal amplitude declines exponentially. In an open environment, one clear of obstacles, the RF signals experience what engineers call free-space loss, which is a form of attenuation. The atmosphere causes the modulated signal to attenuate exponentially as the signal propagates farther away from the antenna. Therefore, the signal must have enough power to reach the desired distance at a signal level acceptable that the receiver needs.

Free space loss. Geometric spreading happens because the wavefront radiated signal
energy expands like a big column as a function of the distance from the transmitter.
When the distance from the transmitter is measured in units of the signal wavelength

Shaggy
Shaggy, Thank you and you are of course quite correct if we were trying to transmit to the moon, however 1Watt of RF power from a VHF on board a RIB will quite happily reach the radio horizon (line of sight), it will in fact reach the line of sight horixzon if transmitted from a taller antenna height (say a container ship).

Let me clarify my badly worded earlier statment. Using high or low power from a marine VHF on board a RIB will not make a blind bit of difference to the range if you are trying to communicate with another station also on earth.
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