Reflection loss can vary from zero to infinity; depending on the
material
and angle of incidence.
====================================

Yes I know. That's obvious.

But what I would like to know is a typical or average value, or a
range of most likely expected values found in practice. Some
experienced person must know!

Needed to crudely ESTIMATE ionospheric multi-hop path loss.

Ground reflection losses are a small proportion of the total path loss
but not small enough to be neglected.

Just a whole number of decibels per reflection will do please.
---
Reg.

"Reg Edwards" wrote in message
news
Reflection loss can vary from zero to infinity; depending on the
material
and angle of incidence.
====================================

Yes I know. That's obvious.

But what I would like to know is a typical or average value, or a
range of most likely expected values found in practice. Some
experienced person must know!

Needed to crudely ESTIMATE ionospheric multi-hop path loss.

Ground reflection losses are a small proportion of the total path loss
but not small enough to be neglected.

Just a whole number of decibels per reflection will do please.
---
Reg.

Some approximate examples at 7 MHz with angles of incidence
from 0 - 40 degrees:

Sea Water 0.2 dB
Rocky Ground 5 dB
Average Gnd. 2 dB

Frank

Thank you Richard, and the others, for digging out and presenting the
information from which I can deduce a ball-park figure.

My original figure of 5 dB per hop for a ground reflection was
slightly too high. It's nearer to 3.5 dB. I'm still thinking about
it. But 4 dB is near enough. Not that it matters very much.

I deduced 5 dB from my experience of signal strengths received from
such places as the antipodes and the Western US states. It's not
surprising hopping across just the North Atlantic results in zero dB
loss.

The excess path loss, over and above spreading loss, must be due to
ground reflection losses. Just calculate backwards from received
signal strength, path distance and the probable number of hops.

It's easy to do this with dedicated computer programs.

For very long distance paths assume that half the ground reflections
are due to sea/ocean water and the other half is due to land masses
and you won't go far wrong. Ground reflection losses at the
transmitting and receiving sites can be ignored. Or such losses can
be lumped together to constitute another complete hop.

I needed the information to include in a simple program which predicts
(approximately) path loss for multi-hop propagation. Program users
cannot be expected to know incidence angles, ground conductivity and
permittivity, Brewster angles etc, for every hop along a route. Some
guesswork is needed on the part of the programmer to make a program
useful.

Despite your continuing abuse of the English language I know you are
quite capable of understanding the foregoing waffle.

I'm on South Eastern Australian Merlot tonight. Hic!

I think the Chinese will be coming along shortly with their own high
quality stuff. They are not just becoming adept with their high
technology. Californian wine growers should look to their laurels.
But no doubt you are all more immediately concerned with the price of
oil, the gas in your tanks, and from where it may be obtained. Iraq
and Iran for example.
----
Reg, G4FGQ

Roy Lewallen wrote:

Reg Edwards wrote:

The amplitude and phase of a field after ground reflection depends

on

the polarization, and is quite different for horizontal and

vertical. It

of course also depends on ground conductivity and permittivity,
frequency, and angle. The equations are simple, and can be found in
Kraus and other references. Those equations are used by NEC for
determination of the far field pattern.

Roy Lewallen, W7EL


===========================================

Yes! But what's the ball park, rule-of-thumb value in dB ?
----
Reg


I dunno. What's the ball park, rule-of-thumb value of a resistor in ohms?

Sumptin tween nuttin and everthin

Roy Lewallen, W7EL

Frank wrote -
Some approximate examples at 7 MHz with angles of incidence
from 0 - 40 degrees:

Sea Water 0.2 dB
Rocky Ground 5 dB
Average Gnd. 2 dB
=========================================
Thanks Frank,

Your figures for 7 MHz are representative of HF.

Nobody is particularly interested in MF, LF or VLF.

They confirm my latest estimate of an average, between sea water and
dry land, of roughly 3.5 dB per ground reflection.
----
Reg.

"Reg Edwards" wrote in message
...
Frank wrote -
Some approximate examples at 7 MHz with angles of incidence
from 0 - 40 degrees:

Sea Water 0.2 dB
Rocky Ground 5 dB
Average Gnd. 2 dB
=========================================
Thanks Frank,

Your figures for 7 MHz are representative of HF.

Nobody is particularly interested in MF, LF or VLF.

They confirm my latest estimate of an average, between sea water and
dry land, of roughly 3.5 dB per ground reflection.
----
Reg.

No problem Reg. I did the computations recently trying to estimate the path
loss, on 7 MHz,
from the west coast to Europe via the long path. The various ground losses
from all kinds of ground, including polar ice, and fresh water, are all
fairly low. The real problem was to estimate the losses due to the
incidence with the diffuse, conductive, ionospheric plasmas, and also to
consider
the possibility that some inonospheric ducting occurs. So far I have not
attempted to estimate these losses, but they must be very high, since only
considering ground incidence produces unrealistically
low path losses.

Frank

Dear Group:
The use of one number is crude and suitable only if one has imbibed
enough grain alcohol or one does not care about Lord K's admonition or both.

The preferred scheme is to model the expected virtual heights at each
ionospheric reflection (note that the apparent TOA (angles) involved are not
likely to be the same at each of these reflections) and then to use the
angles involved to note the expected incidence angle of the intermediate
"ground" reflection. The coordinates of the expected "ground" reflection
areas are calculated and note is taken as to whether those areas are
predominately salt-water, ice, or neither. A heuristic algorithm is applied
that depends on type of "ground" and that weakly depends on the expected
incidence angle. The result is a statistically significant reflection loss.
[Some iteration is needed, which computers are very good at doing.]

It is important to note that the apparent TOA of the major mode between
two points is unlikely to be the same at both ends of the path because the
virtual height is unlikely to be the same near both ends.

For HF paths greater than something like 12 or 14 Mm predictions become
more complex. In all cases of multiple hops (real DX), the TOA at which the
gain of the antennas is effective varies between about 12 degrees and 2
degrees.

If one wishes a simple predictor, one could find it in a paper published
in the UK in the transactions on a conference on HF propagation in the 80s
(as I recall) by a BBC engineer (IEE was the publisher). The entire
computer code is given in the paper. It uses a simple heuristic model that
was found by the BBC to give satisfactory results over the paths that their
World Service used. Note that the BBC used paths that did not include much
of the polar regions.

To digress: The worst path to a DXCC entity from the Upper Midwest is
over the magnetic pole to VU4. 100 watt transmitters, ice reflections,
heavy absorption, and roughly 10 db more noise at the far end make the need
for power and high antennas almost necessary.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Richard Harrison" wrote in message
...

snip

It could be right for some reflections.

E.A. Laport was Chief Engineer of RCA International when he wrote "Radio
Antenna Engineering". On page 236 Fig. 3.17 shows the effect of ground
conductivity on maximum field strength from a horizontal dipole antenna
versus its height in vavelengths.

Optimum height would be about 0,50 wavelength to most concentrate energy
at a certain vertical angle, 30-degrees according to the RAF Signal
Manual quoted by Laport. 30-degrees might hop 1000 kilometers.

At the antenna earth reflection point, frequencies between 2 and 16 MHz
are reduced to 95% of their prereflection field strengths by ordinary
soil from a dipole at 1/2-wavelength height. A reduction to 70.7% of
prereflection strength would represent a 3 dB power loss. So no harm
done yet by the reflection from an antenna over good soil. Lower antenna
height and poorer soil would attenuate more.

The angle at which rhe signal strikes the earth in subsequent
reflections should be the same as the first reflection from the antenna.
Conductivity and dielectric constant at subsequent earth reflection
points are what they are.

Shortwave broadcasters use vertically stacked horizontal elements to
concentrate the vertical beam to avoid multipath interference. They also
prefer targets reached on the first reflection from the ionosphere..

Best regards, Richard Harrison, KB5WZI

Mac, N8TT wrote:
"Note that the BBC used paths that did not include much of the polar
regions."

That`s an indication that they knew something about broadcasting. There
must be a better way somewhere that doesn`t involve a path near a polar
region.

A broadcaster must advertise his programs well in advance. He can`t find
a band that`s open and tailor his emissions to it. He must fire up on
schedule with the right program at the designated time and direction
regardless of conditions. All is planned long ahead of time.

One of the most usless toys we had was a backscatter ionospheric
sounder. It worked well but we had no immediate need for the information
it provided. We were locked in our schedules for months in advance and
today`s ionospheric conditions were short-term indeed.

Best regards, Richard Harrison, KB5WZI

ORIGINAL MESSAGE:

On Tue, 2 May 2006 20:16:04 -0500, (Richard
Harrison) wrote:

A broadcaster must advertise his programs well in advance. He can`t find
a band that`s open and tailor his emissions to it. He must fire up on
schedule with the right program at the designated time and direction
regardless of conditions. All is planned long ahead of time.

*********** REPLY SEPARATOR ***********

A good argument for moving international broadcast to the VHF/UHF or
higher regions. HF was good when there were no alternatives, but the
time has come to retire it.

Bill, W6WRT