Message-Id: <5.2.0.9.2.20081206150445.024b09f8@pop.claranet.pt>
Date: Sat, 06 Dec 2008 15:22:54 +0000
To: rsgb_lf_group@blacksheep.org
From: Brian Rogerson <brian@esoterica.pt>
In-Reply-To: <001501c956db$eecfeb80$4201a8c0@home>
Mime-Version: 1.0
Subject: Re: LF: Re: ERP calculations / measurements
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Dear Jim and LF

Thank you for your reply.

Rik's comments stimulated my interest and curiosity and the high ground to=20
the East
spurred me into including high level (altitude) measurements.  I was extreme=
ly
surprised when I included these in the survey and noticed the correlation=20
between
the calculated ERP and altitude.  I plotted ERP against take off angle from=20
-1=BA to about 6=BA.  This
might(?) even throw doubt on  ""but by keeping the measuring distance=20
reasonably short, you can
reasonably expect the ground wave losses to be negligibly small""  As the=20
plots I attached
did not make it through the reflector filtering system back to me I am also=20
sending this direct
and attaching the XLS file.

73, Brian CT1DRP








At 13:18 05/12/2008, you wrote:
>Dear Rik, Brian, Andy, LF Group,
>
>As Andy says, it is difficult to seperate out the directivity (i.e. gain du=
e
>to the shape of the radiation pattern of the antenna), the gain reduction
>due to antenna losses, and the losses due to propagation over lossy ground
>(which probably isn't really part of the antenna gain, but in practice will
>always be present to some extent). When you make field strength measurement=
s
>to determine ERP, you are effectively measuring a combination of these
>things, but by keeping the measuring distance reasonably short, you can
>reasonably expect the ground wave losses to be negligibly small. This is
>fine as far as ERP is concerned, because ERP is a measure of the overall
>strength of the signal being radiated. In principle, one could determine
>directivity by measuring the radiation pattern of the antenna, but of cours=
e
>this is practically very hard to do at LF and MF at anything other than zer=
o
>elevation angle, unless you happen to have a suitably equipped aircraft
>handy!
>
>An important thing to remember when looking at the results of antenna
>simulation is that what it gives is the far-field radiation pattern. This i=
s
>effectively the radiation pattern measured "as distance tends to infinity".
>Infinite distance means infinitely more ground loss for the ground wave
>signal at zero elevation as compared to the signal propagating away from th=
e
>ground, when anything other than "perfect" ground is simulated. So there is
>always a null in the radiation pattern at zero elevation, and some reductio=
n
>of the lower angle radiation. But the reality is that field strength is not
>measured at infinite distance, and the ground wave signal is not attenuated
>to an infinitesimal level - in fact, over practical distances it is hardly
>attenuated at all. The consensus seems to be that the radiation pattern at
>these relatively short distances is the same as the "perfect ground" case.
>It obviously isn't easy to check this. But in my experiments with LF/MF
>antennas in an "open field" site last year, the FS measurements agreed
>almost perfectly with the values calculated assuming the "perfect ground"
>4.77dBi gain value.
>
>The far-field-with-lossy-ground radiation pattern concept is probably fine
>at HF, because the ground wave is heavily attenuated even at short
>distances. But at MF, and especially LF, we are often communicating over
>distances where the ground wave is the major component of the received
>signal at distances of many hundreds of km. Since the effective parts of th=
e
>ionosphere are only of the order of 100km high for these signals, and the
>curvature of the earth is also a significant factor affecting ground wave
>strength, the situation that is being simulated using NEC consisting if an
>antenna with flat ground plane and space above going off to infinity is not
>at all realistic. Also, mine and other peoples' experiments have shown that
>the trees and other objects around the antenna at short distances often
>dominate the antenna performance, something that is hard to model. So I
>think simulations of LF/MF antennas with ground losses are not very helpful
>at all in predicting antenna performance.
>
>Cheers, Jim Moritz
>73 de M0BMU

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<body>
<font size=3D3>Dear Jim and LF<br><br>
Thank you for your reply.<br><br>
Rik's comments stimulated my interest and curiosity and the high ground
to the East<br>
spurred me into including high level (altitude) measurements.&nbsp; I was
extremely<br>
surprised when I included these in the survey and noticed the correlation
between<br>
the calculated ERP and altitude.&nbsp; I plotted ERP against take off
angle from -1=BA to about 6=BA.&nbsp; This<br>
might(?) even throw doubt on&nbsp; &quot;&quot;but by keeping the
measuring distance reasonably short, you can<br>
reasonably expect the ground wave losses to be negligibly
small&quot;&quot;&nbsp; As the plots I attached<br>
did not make it through the reflector filtering system back to me I am
also sending this direct<br>
and attaching the XLS file.&nbsp; <br><br>
73, Brian CT1DRP<br><br>
<br><br>
<br><br>
<br><br>
<br>
At 13:18 05/12/2008, you wrote:<br>
<blockquote type=3Dcite class=3Dcite cite>Dear Rik, Brian, Andy, LF
Group,<br><br>
As Andy says, it is difficult to seperate out the directivity (i.e. gain
due<br>
to the shape of the radiation pattern of the antenna), the gain
reduction<br>
due to antenna losses, and the losses due to propagation over lossy
ground<br>
(which probably isn't really part of the antenna gain, but in practice
will<br>
always be present to some extent). When you make field strength
measurements<br>
to determine ERP, you are effectively measuring a combination of
these<br>
things, but by keeping the measuring distance reasonably short, you
can<br>
reasonably expect the ground wave losses to be negligibly small. This
is<br>
fine as far as ERP is concerned, because ERP is a measure of the
overall<br>
strength of the signal being radiated. In principle, one could
determine<br>
directivity by measuring the radiation pattern of the antenna, but of
course<br>
this is practically very hard to do at LF and MF at anything other than
zero<br>
elevation angle, unless you happen to have a suitably equipped
aircraft<br>
handy!<br><br>
An important thing to remember when looking at the results of
antenna<br>
simulation is that what it gives is the far-field radiation pattern. This
is<br>
effectively the radiation pattern measured &quot;as distance tends to
infinity&quot;.<br>
Infinite distance means infinitely more ground loss for the ground
wave<br>
signal at zero elevation as compared to the signal propagating away from
the<br>
ground, when anything other than &quot;perfect&quot; ground is simulated.
So there is<br>
always a null in the radiation pattern at zero elevation, and some
reduction<br>
of the lower angle radiation. But the reality is that field strength is
not<br>
measured at infinite distance, and the ground wave signal is not
attenuated<br>
to an infinitesimal level - in fact, over practical distances it is
hardly<br>
attenuated at all. The consensus seems to be that the radiation pattern
at<br>
these relatively short distances is the same as the &quot;perfect
ground&quot; case.<br>
It obviously isn't easy to check this. But in my experiments with
LF/MF<br>
antennas in an &quot;open field&quot; site last year, the FS measurements
agreed<br>
almost perfectly with the values calculated assuming the &quot;perfect
ground&quot;<br>
4.77dBi gain value.<br><br>
The far-field-with-lossy-ground radiation pattern concept is probably
fine<br>
at HF, because the ground wave is heavily attenuated even at short<br>
distances. But at MF, and especially LF, we are often communicating
over<br>
distances where the ground wave is the major component of the
received<br>
signal at distances of many hundreds of km. Since the effective parts of
the<br>
ionosphere are only of the order of 100km high for these signals, and
the<br>
curvature of the earth is also a significant factor affecting ground
wave<br>
strength, the situation that is being simulated using NEC consisting if
an<br>
antenna with flat ground plane and space above going off to infinity is
not<br>
at all realistic. Also, mine and other peoples' experiments have shown
that<br>
the trees and other objects around the antenna at short distances
often<br>
dominate the antenna performance, something that is hard to model. So
I<br>
think simulations of LF/MF antennas with ground losses are not very
helpful<br>
at all in predicting antenna performance.<br>
<br>
Cheers, Jim Moritz<br>
73 de M0BMU</font></blockquote></body>
<br>
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