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

On 10/14/2014 7:32 PM, Jeff Liebermann wrote:

Back to hauling firewood up the hill...

You are doing it wrong. You will find it much easier to haul the
firewood down the hill.

--

Rick

On Wed, 15 Oct 2014 18:25:14 -0400, rickman wrote:

On 10/14/2014 7:32 PM, Jeff Liebermann wrote:

Back to hauling firewood up the hill...

You are doing it wrong. You will find it much easier to haul the
firewood down the hill.

My cardiologist demands that I get more exercise. Schlepping firewood
up about 50 step is quite good a producing the desired effect. That
which doesn't kill me makes me stronger. I could easily install a
cable lift, bucket hoist, or conveyor belt to move the firewood up the
hill with less exertion. However, I won't do that. Other than
occasional bicycling and fast walks, firewood schlepping is my major
form of exercise.

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

On 10/15/2014 8:36 PM, Jeff Liebermann wrote:
On Wed, 15 Oct 2014 18:25:14 -0400, rickman wrote:

On 10/14/2014 7:32 PM, Jeff Liebermann wrote:

Back to hauling firewood up the hill...

You are doing it wrong. You will find it much easier to haul the
firewood down the hill.

My cardiologist demands that I get more exercise. Schlepping firewood
up about 50 step is quite good a producing the desired effect. That
which doesn't kill me makes me stronger. I could easily install a
cable lift, bucket hoist, or conveyor belt to move the firewood up the
hill with less exertion. However, I won't do that. Other than
occasional bicycling and fast walks, firewood schlepping is my major
form of exercise.

Yes, "that which doesn't kill me". I doubt hauling firewood uphill is
what your doctor would recommend. You should find the time to bicycle
more.

--

Rick

rickman wrote in :

Yes, "that which doesn't kill me". I doubt hauling firewood uphill is
what your doctor would recommend. You should find the time to bicycle
more.


I find that cross country running helps with sciatica, so long as there is no
heavy jarring of my spine. So I have to be careful of that, but the result
proves the old 'use it or lose it' strategy.

On 10/16/2014 3:33 AM, Lostgallifreyan wrote:
rickman wrote in :

Yes, "that which doesn't kill me". I doubt hauling firewood uphill is
what your doctor would recommend. You should find the time to bicycle
more.


I find that cross country running helps with sciatica, so long as there is no
heavy jarring of my spine. So I have to be careful of that, but the result
proves the old 'use it or lose it' strategy.

I know running has some issues which may or may not be a problem for any
given person. I discovered kayaking a few years ago and found it is a
fantastic way to exercise. It really builds the core muscles... you
know, the ones everyone says you need to strengthen. It helped me so
much that my back pain I've had off and on since I was 30 is pretty much
all gone now. It is true what they say about strengthening those
muscles. Too bad I can't do it year 'round here. I need a summer place
in Florida.

--

Rick

rickman wrote in :

I know running has some issues which may or may not be a problem for any
given person. I discovered kayaking a few years ago and found it is a
fantastic way to exercise. It really builds the core muscles... you
know, the ones everyone says you need to strengthen. It helped me so
much that my back pain I've had off and on since I was 30 is pretty much
all gone now. It is true what they say about strengthening those
muscles. Too bad I can't do it year 'round here. I need a summer place
in Florida.



Nice. I've kayaked for only a short time on a couple of occasions (on calm
sunlit sea, and in a grey force 7, off the Pembrokeshire coast) but I liked
it. What I really miss is sailing... Not so arduous, but the balance of
intensity and space to just sit and be aware of it all makes me know why it
is some people's whole life. I'd like to do that again.