----- Original Message -----
Sent: Thursday, September 02, 2004 12:06
AM
Subject: LF: Static
The theory about loop advantage over a vertical
is not correct...
73 de Mal/G3KEV
Dear Mal, LF Group,
The supposed interference-rejection properties of
loops are based on the fact that a loop responds mostly to the H (magnetic)
field component of the electromagnetic wave. Hopefully, E (electric) fields will
be rejected. But I don't think this would help a loop antenna reject
QRN - the high-voltage lightning discharge will generate an E-field
transient, but the kiloamps of current in the stroke will generate plently
of H-field transient also. I don't know about the relative intensity of E and H
fields close to the strike, but once you get several wavelengths away, the ratio
E/H will be constant at 377ohms, just like every other propagating radio wave.
So I don't think there is a theretical basis to expect any advantage
rejecting QRN or other long-distance interference in using an E or H field
antenna for reception.
Where a loop antenna might be an advantage is in
rejecting local E field noise, which will not neccesarily have a corresponding H
field component. This is an advantage in applications where high voltage, low
current phenomena are at play, such as "precipitation static". But I think most
of the local QRM experienced by us LF amateurs is largely H-field; noise
currents of one sort or another flowing down mains cables, generating primarily
magnetic noise fields. Certainly, I have not found that loop antennas are good
at rejecting mains noise. A tuned vertical antenna might seem to be a solution,
but then again, a magnetic field will be just as effective at inducing currents
in the conductors of a vertical antenna as it is in a loop antenna.
Where loop RX antennas do have an
advantage for noise rejection is their directional properties - if there is
a faily large angle between the QRM/QRN and the signal, the loop can be
positioned to null out the noise. In my case, I can null out the Loran noise
from Lessay, while receiving signals from most of Europe and North America,
which gives about 5 - 10dB improvement in the noise level. Also, because the
loops are fairly small, it is possible to move them around to find the position
where the local QRM is at a minimum - the bottom of the garden in my case. The
end result for me is that loops give substantially better received signals - but
they don't reduce the QRN, unless it is in the nulls of the
antenna.
Cheers, Jim Moritz
73 de M0BMU
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