Aerials, GAH!

[Captain_Scooby]

Morning all,

Have recently started to go over my ATPL books again to keep refreshed (its 4 years since I took the exams). Have got to Chapter 1 in Radio Nav and come to a grinding halt with something that catches me every time - aerials, and basic radio theory. I'm fairly good with understanding systems and the way things work, but this really boggles me.

I think I understand the basic theory and then something else throws me completely off course and is making me seriously doubt my understanding of the real basic stuff. One example is now I'm not sure whether a radio wave actually physically moves up and down, or if the wave shown in textbooks is just a graphical representation of a positive-negative-positive transmission in terms of power/strength (can you even get a negative transmission). I thought it was the former, but then I see that the magnetic field of an EM wave is at right angles to the electrostatic field (which is parallel to the aerial), and I question how a dipole aerial can create a magnetic field at right angles to itself, when there's no physical 'material' there to emit anything in that plane.

Or how the parasite element of a Yagi aerial can absorb a wave at one end, and re-radiate it instantly at the opposite end?

It's probably very simple, but I just can't seem to get my head round it, and it's highly annoying, as reading the rest of the book is kind of pointless if I don't understand the basics.

Anyone know of a website with the absolute basics of this stuff... an idiot's guide, if you will?

Cheers

A brain-frazzled Scoobs

[Captain_Scooby]

I think what this boils down to is that I don't understand what an aerial is emitting. Is it a physical wave, where it transmits from the top of the aerial at one point in time, followed by transmitting a little further down at the next point in time, all the way through to transmitting from the bottom of the aerial, and then working its way back up, (i.e. like it's producing the shape of a wave over time T)

OR.... is it transmitting a signal in all directions at a particular strength, which gets stronger, until it's at its strongest (represented by the peak on the wave diagram) and then gets weaker until it's at its weakest (represented by the trough)?

OR... is it something different, or a combination of the above? Every page I look at seems to contradict the previous one I've just read.

[paco]

Imagine the releationship between the earth and the antenna as a large capacitor set for maximum leakage for the tension that would normally exist across its two plates. Maximum leakage means that the plates are as far apart and as small as possible.

More later when I have time

Phil