Coastal refraction of radio waves

[honchun]

I'm aware that in oceanic NDB navigation coastal refraction can create an ADF error and make you think you're closer to the shoreline than you really are. But I don't understand why it makes you think so when you're actually tracking the refracted beam. :confused:

[411A]

Well, if you fly the relative bearing, you will eventually arrive at the station. Night and coastal effect have been known for years...all a bit redundant now in todays more "modern" world.

[Self Loading Freight]

I think this is one of those things that's best explained with a whiteboard, but as far as I understand it the problem is that as the wavefront passes over the coast heading out to sea it's bent by refraction so that the angle between the direction of travel of the wavefront and the coast is reduced. In other words, your ADF will think it's following one line going straight to the NDB but it's really following another that's pointing to one side of the beacon and closer to the coast.

Here's a quick and sloppy diagram I knocked up in Paint...

http://www.agricola.demon.co.uk/ndbr.jpg

R

[ 21 November 2001: Message edited by: Self Loading Freight ]

[Zeke]

When I teach this I use the illusion that is done in physics when you put a stick in a fish bowl or shine a torch into water, with the stick or light appearing to bend.

Its based on the same physics, conductivity of the medium to the propagation of electromagnetic radiation (radio or light waves)

The conductivity of the water is better than land, leads to greater reception range over water. The radiated signal from the NDB is not attenuated as much over water.

It is the apparent change in transmission medium that leads to the refraction.

If you were a chopper pilot going from a land base to a rig that has an NDB you encounter the same refraction issues when going from land to over water.

Z

[2daddies]

Zeke et al:
Not so much conductivity of water. More to do with the fact that the mass of water usually has a different temperature than the land adjacent to it. This means that the respective masses of air (over land/ over water) possess different densities and as such propogate the NDB's signal with differing degrees of effectiveness - resulting in a refraction of the signal usually coincident with the coastline.

Otherwise your analogy RE bending of light through water is correct. It all just comes down to the density of the mass.

P.S - 411A. I WISH NDBs were obsolete. Apart from major cities and a few big country towns, Australian airports rely on NDBs for the bulk of their instrument approaches!

[Vmu]

Interesting! As far as I know the diffraction is caused by the propagation speed of the radio waves being different over water and land. 2daddies writes that this is caused by differences in air densities. Can anyone give som more details on the relationship between density and propagation speed? Are there other variables that affects propagation speed? Let's say density was equal over land and sea; would different amounts of water vapour cause refraction? Would the frequency of the radio signals affect the results?