Dear Andy, LF Group,
I did a lot of experiments with measuring
loops a few years back for a project at work. You can get results that are
within a few dB quite easily, provided the loading effect of the receiver/level
meter on the loop is taken into account. However, you do have to be very
careful about nearby conductors, particularly the cables connecting the antenna
and measuring gear. If one of these is anywhere near resonance, gross errors
can be caused, particularly with a small loop. In the upper part of the HF
spectrum, say above about 10MHz, this type of effect can be quite difficult to
avoid. The best bet is to use a battery powered receiver, with a short lead to
the antenna, in the middle of some open ground, in order to minimize the possible
parasitic antenna effects. Using the clamp-on “EMC” ferrite cores at
several points on the cables can also help, but at least a few turns through
the core is needed to get usefully high impedance. Because of the much longer
wavelengths, this sort of thing is not so much of a problem at LF – although
overhead power or telephone lines can produce large errors. A typical tell tale
sign is the signal null is not in the expected direction, or there isn’t
a clear null at all.
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On
Behalf Of Andy
Sent: 15 March 2005 12:31
To: [email protected]
Subject: Re: LF: Untuned loops
Thanks to all
respondees - I knew it was a simple formula and was (theroretically) an exact
way of determining field strength. Just need to make sure the input
impedance of the voltmeter is significantly higher than the loop impedance to
avoid any loading effects.
(Someone here wants to
measure the field strength of a transmission in the HF band)
U = omega * µo * H *
area
with µo = 4 * pi * 10^-7 Vs/A/m,
omega = 2 * pi * f .
Can
anyone recall this equation, I know it was mentioned on this reflector
before...
What
is the output voltage for an untuned small loop into an arbitrarily high
impedance? It is proportional to H, and presumably F^2, and I seem to
recall it is an absolute value, virtually un-influenced by losses, conductor
diameter etc. Therefore can be used for accurate H field (and hence far
field radiation) signal strength measurements.
