On Tue, 14 Oct 2014 21:34:52 +0100, "gareth"
wrote:

On antennae that are several half-wavelengths long (beverage, Rhombic, etc)
what does the radiative wavefront look like when close to the antennae, even
though it is presented in the literature as a plane wave further out?

I've never bothered to grind out this calculation, so you've gotten my
attention.

If you find the assumptions insufficiently precise, perhaps some back
of the envelope math might be useful. If you have a point source
radiator, spewing RF at a wavelength = x, where the wavefront peaks
are x distance apart, how large a radius in the form n * x would you
need to have before the wavefronts are essentially a straight line? In
other words, how many wavelengths would you need to be away from the
RF source before the wavefront is essentially a straight line?

Obviously, the wavefront will never be completely straight, but I
would guess(tm) that the wire diameter of a typical 0.5 wave wire
dipole receive antenna should be sufficiently small to be considered
straight. Unfortunately, that dimension makes the calculation
frequency dependent, so I'll just assume 10 MHz and #14 AWG antenna to
make the math easier.
0.5 wave at 10 MHz = 15 meters
Dia of #14 AWG wire is about 1.63 mm.
The calcs are a bit too messy to show in ASCII, so I scanned my
scribbling:
http://802.11junk.com/jeffl/crud/wavefront.jpg
At 10 MHz and a distance of 14.9 km (9.3 miles) the wavefront has
straightened out sufficiently so that the arc is no wider than the
width of a #14 AWG antenna wire. You would be hard pressed to keep
your receive antenna straight within the diameter of a #14 AWG wire,
so this degree of accuracy is massive overkill.

The reason that I ask is the on such longwires, there are parts of the
wire which will be radiating positively, and parts negatively, thereby
suggesting that the outgoing wave, spherical though it might be, has
a +/- modualtion were you to travrse its circumference?

Are you familiar with the term "word salad"? I'm sure you're trying
to convey a question, but it's difficult to find it under all the
misplaced and misused technical terms. A Beverage, Rhombic, etc is
not the same as a longwire. Antennas radiate AC and therefore do not
have the + and - polarity of a DC power source. The radiation pattern
from a long wire is not spherical (isotropic). Nothing in the antenna
affects the modulation. Traversing a circumference results in the
diameter. Kindly spend some time looking up the definitions of the
terms you use so that I don't have to decode your intentions.

Back to hauling firewood up the hill...

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558