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Message-ID: <001801c7a315$80fc2f40$52217ad5@w4o8m9>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <341873.21191.qm@web86408.mail.ird.yahoo.com>
Date: Thu, 31 May 2007 00:42:27 +0100
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Subject: LF: Re: Field Strength Meter
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Dear Chris,

Interesting to see your field strength measurements - would also be =
interested to know how this compares to any calculated value you have =
for the ERP of your station.

A few years back, I made several hundred FS measurements on my 136kHz =
and 73kHz station, looking at the effects of antenna configuration, =
distance etc., using a mobile measuring set-up with the measuring =
antenna mounted on the roof of my car. I found that one could expect =
"random" variations within a range +/-3dB of ERP at different sites. =
These variations were not just random errors; one could return to the =
same site months later and measure the same variation in field strength. =
I came to the conclusion that things in the environment were causing =
some enhancement or reduction in field strength; presumably conductors =
such as wire fences, buried cables, etc. acting as parasitic antenna =
elements. Some sites produced much larger variations up to +/-10dB or =
more. On inspection there was usually a fairly likely-looking cause for =
this, such as overhead power lines, or large metal structures such as =
buildings and bridges, but there was not always anything immediately =
obvious to be seen. One road leading away from my house gave wildly =
varying results along its length - it turned out that the overhead =
telephone wire from my house ran along this road, and high and low FS =
readings were separated by about the right distance to be interpreted as =
a lambda =3D 2200m standing wave on the overhead wire. But provided a =
substantial number of measurements are available, it is easy to spot =
which measurements are suspect and check them, and to get a good average =
value. Something worth checking is that you get a deep directional null =
with a loop or ferrite rod antenna; if not, the measuring site is =
probably dubious. I expect if one were only to use measurement sites =
that were in the middle of open ground, a good distance from any large =
metallic objects, variations between measurements would be reduced =
greatly. Unfortunately, there seems to be a shortage of accessible sites =
like this near me...

Regarding the letter in RadCom, the difficulties of field strength =
measurements depend a lot on the frequency. I have some experience of HF =
FS measurements - in this range, connecting cables and any other fair =
sized bit of metal can become resonant, and can produce huge peaks and =
troughs in the results without careful attention. At VHF and above, =
multipath propagation becomes a headache, with multiple unwanted =
reflections from the ground and any nearby objects. I think LF/MF is =
relatively straightforward in comparison - the main difficulty being =
that you need to be at least several hundred metres away from the TX =
antenna to get good measurements. But once the problems have been =
identified, measurement results are usually very consistent.

Cheers, Jim Moritz
73 de M0BMU
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<DIV><FONT face=3DArial size=3D2>Dear Chris,</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Interesting to see your field strength =
measurements=20
- would also be interested to know how this compares to any calculated =
value you=20
have for&nbsp;the ERP of your station.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>A few years back, I made several =
hundred FS=20
measurements on my 136kHz and 73kHz station, looking at the effects of =
antenna=20
configuration, distance etc., using a mobile measuring set-up with the =
measuring=20
antenna mounted on the roof of my car.&nbsp;I found that one could =
expect=20
"random" variations within&nbsp;a range +/-3dB of ERP at different =
sites. These=20
variations were not just random errors; one could return to the same =
site months=20
later and measure the same&nbsp;variation in field strength. I came to =
the=20
conclusion that things in the environment were causing some enhancement =
or=20
reduction in field strength; presumably conductors such as wire fences, =
buried=20
cables, etc. acting as parasitic antenna elements. Some sites produced =
much=20
larger variations up to +/-10dB or more. On inspection there was usually =
a=20
fairly likely-looking cause for this, such as overhead power lines, or =
large=20
metal structures such as buildings and bridges, but there was not always =

anything immediately obvious to be seen. One road leading away from my =
house=20
gave wildly varying results along its length - it turned out that the =
overhead=20
telephone wire from my house ran along this road, and high and low FS =
readings=20
were separated by about the right distance to be interpreted as a lambda =
=3D 2200m=20
standing wave on the overhead wire. But provided a substantial number of =

measurements are available, it is easy to spot which measurements are =
suspect=20
and check them, and to get a good average value. Something worth =
checking is=20
that you get a&nbsp;deep directional null with a loop or ferrite rod =
antenna; if=20
not,&nbsp;the measuring site is probably dubious.&nbsp;I expect if one =
were only=20
to use measurement sites that were in the middle of open ground, a good =
distance=20
from any large metallic objects, variations between measurements would =
be=20
reduced greatly. Unfortunately, there seems to be a shortage =
of&nbsp;accessible=20
sites like this near me...</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Regarding the letter in RadCom, the =
difficulties of=20
field strength measurements depend a lot on the frequency. I have some=20
experience of HF FS measurements - in this range, connecting cables and =
any=20
other fair sized bit of metal can become&nbsp;resonant, and can produce =
huge=20
peaks and troughs in the results without careful&nbsp;attention. At VHF =
and=20
above, multipath propagation becomes a headache, with multiple unwanted=20
reflections from the ground and any nearby objects. I think LF/MF is =
relatively=20
straightforward in comparison - the main difficulty being that you need =
to be at=20
least several hundred metres away from the TX antenna to get good =
measurements.=20
But once the problems have been identified, measurement results are =
usually very=20
consistent.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Cheers, Jim Moritz</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>73 de M0BMU</FONT></DIV></BODY></HTML>

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