On Sat, 01 Jul 2006 11:09:22 -0400, jawod wrote:

Tried to make the subject grab a bit.

I just fininshed the ARRL Antenna Book Chapter on Effects of Ground.
There are HFTA graphs showing elevation response for various antenna
configurations (mostly Yagis).

Most arrays show good response up to 12 degrees above the horizon, then
many show a null and then, a second peak around 25 to 30 degrees above
the horizon.

Here's my question:
At 25 to 30 degrees elevation response, aren't these waves leaving the
ionosphere (i.e., refracted instead of reflected)?

Am I right to consider this component of propagation to have left the earth?

This would indicate a substantial fraction of each amateur transmission
is sent into space.

I always thought Broadcast transmissions were most likely to emanate
from Earth. Are hams more or less likely to transmit into space than
Broadcast?

John
(who wishes to remain a student and never an expert)


To: "Richard Clark"
Subject: Please Post in rraa
From: "Walter Maxwell"
Date: Sat, 1 Jul 2006 17:31:29 -0400

Richard, I've been trying to post the msg below to the HFTA-ARRL-Space
thread, but after three attempts to send it it doesn't get posted.
Would you please post it for me in the spot following Mac's?

Walt, W2DU

Hello John,

I believe your other respondents missed one of your points concerning
reflection and refraction, and therefore didn't respond completely to
it.

Whether reflection, refraction, or total penetration of the ionosphere
occurs depends on the ionospheric layer, the time of day that
determines the sun angle on the layer, the resulting level of
ionization, the angle the ray makes on incidence with the layer, and
the frequency of the energy in the arriving ray.

Consequently, the answer is complex. As we know, when the frequency is
high enough (VHF and above) the result is total penetration--no
reflection or refraction--line of sight reception only.

On the other hand, at HF the ratio between reflection and refraction
varies. There are times when both occur. During those times the
portion of the incident ray that is reflected returns to earth, while
the portion that is refracted continues on through the ionosphere into
space and never returns. I'm not sufficiently knowledgeable on the
subject to go into further detail, but now that they've been nudged,
either Richard C or Reggie can. Or someone else more knowledgeable
than I.

Walt, W2DU

On Sat, 01 Jul 2006 16:11:42 -0700, Richard Clark
wrote:


On the other hand, at HF the ratio between reflection and refraction
varies. There are times when both occur. During those times the
portion of the incident ray that is reflected returns to earth, while
the portion that is refracted continues on through the ionosphere into
space and never returns. I'm not sufficiently knowledgeable on the

But is it actually reflection?

Owen
--

"Owen Duffy" wrote in message
...
On Sat, 01 Jul 2006 16:11:42 -0700, Richard Clark
wrote:


On the other hand, at HF the ratio between reflection and refraction
varies. There are times when both occur. During those times the
portion of the incident ray that is reflected returns to earth, while
the portion that is refracted continues on through the ionosphere into
space and never returns. I'm not sufficiently knowledgeable on the

But is it actually reflection?

no, it is actually a refraction. but it is useful sometimes to model it as
a reflection from a slightly higher level. that makes computation of angles
of incidence and height a bit easier.

But is it actually reflection?

no, it is actually a refraction. but it is useful sometimes to
model it as
a reflection from a slightly higher level. that makes computation
of angles
of incidence and height a bit easier.

============================================
The trigonometry is quite simple. Things become complicated when the
reflecting layer is not horizontal, ie., the layer is tilted.

The angle and direction of tilt are very difficult to predict.
Consequently, where on the surface of the Earth a ray returns is
anybody's guess.

This makes the vertical take-off angle, reported by antenna modelling
programs, even less useful.
----
Reg.

"Reg Edwards" wrote in message
...

But is it actually reflection?

no, it is actually a refraction. but it is useful sometimes to
model it as
a reflection from a slightly higher level. that makes computation
of angles
of incidence and height a bit easier.

============================================
The trigonometry is quite simple. Things become complicated when the
reflecting layer is not horizontal, ie., the layer is tilted.

The angle and direction of tilt are very difficult to predict.
Consequently, where on the surface of the Earth a ray returns is
anybody's guess.

This makes the vertical take-off angle, reported by antenna modelling
programs, even less useful.

no, it doesn't make it less useful. as a statistic it is still good, but
you have to remember that it is nothing more than a statistic. and everyone
knows 'you can prove anything with statistics'. the fact that the
ionosphere is more complicated than a horizontal reflection layer model
represents doesn't mean that its usefulness is reduced, just that there are
some cases when it won't be completely accurate... those are the fun things
that happen with propagation that keep it interesting.

Dear Reg:

You have articulated one of the many reasons why HF propagation is
described in stochastic terms. As you know very well, measurements or
predictions comprise at least two numbers: the best estimate of the number
and an estimate of the uncertainty of the first number. Present models of
HF propagation, which include antenna characteristics, provide both numbers.
Early models of HF propagation tended to be somewhat deficient in providing
the second number.

However, I remember using the early models to predict (extrapolate) in
real-time how much longer a certain frequency was likely to remain usable
from noting the drop-out of a higher frequency. The physics involved has
been understood for many years. It takes a long period of data gathering to
be able to do a good job with the second number.

In the early days (post WW2) of radio astronomy, the uncertainties of
some important measurements were greater than the estimate. That did not
last.

73 Mac N8TT

P.S. Nice to know that W2DU is back.
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Reg Edwards" wrote in message
...

But is it actually reflection?

no, it is actually a refraction. but it is useful sometimes to
model it as
a reflection from a slightly higher level. that makes computation
of angles
of incidence and height a bit easier.

============================================
The trigonometry is quite simple. Things become complicated when the
reflecting layer is not horizontal, ie., the layer is tilted.

The angle and direction of tilt are very difficult to predict.
Consequently, where on the surface of the Earth a ray returns is
anybody's guess.

This makes the vertical take-off angle, reported by antenna modelling
programs, even less useful.
----
Reg.

Dave wrote:
"Owen Duffy" wrote in message
...

On Sat, 01 Jul 2006 16:11:42 -0700, Richard Clark
wrote:



On the other hand, at HF the ratio between reflection and refraction
varies. There are times when both occur. During those times the
portion of the incident ray that is reflected returns to earth, while
the portion that is refracted continues on through the ionosphere into
space and never returns. I'm not sufficiently knowledgeable on the

But is it actually reflection?


no, it is actually a refraction. but it is useful sometimes to model it as
a reflection from a slightly higher level. that makes computation of angles
of incidence and height a bit easier.


If the end result is that the wave returns back to earth, why is this
not termed reflection? Even if it is the result of several and/or
continuous refractions that result in a return of the wave from the 2nd
medium to the 1st, i.e., they sum to result in a reflected angle, seems
to me reflection is a good term.

I understand that a curved surface is more complex but if the result is
the same, ...?