Dear Roleof, LF Group,
Most radio amateur textbooks state that qrm from a local noise source is
due
to the electric part of the electro-magnetic field and has a vertical
polarisation. An active whip is a vertical antenna and responds mainly to
the electric field. A loop antenna responds to the magnetic field. The
question arises why an active whip is apparently insensitive to vertical
polarised local noise and an active loop is not.
I have often heard this statement, but is there any justification for it at
LF? Assuming no resonances occur near the receive frequency, a predominantly
electric field QRM source implies a relatively high voltage, low current and
high impedance, while a predominantly magnetic field QRM source implies high
current, low voltage and low impedance. Most of the QRM experienced at LF
seems to be mains related, and most mains wiring must have a fairly low
impedance to ground due to the capacitance between wires in cables, so
relatively large noise currents and small noise voltages would seem likely.
So one would expect predominantly "magnetic" noise from mains wiring.
Before entering the house, the braid of the coaxial feed line of the
active
whip is connected to ground. This makes a large difference in received
local noise. An explanation could be the assumption that the shield of the
coaxial feed line is picking up additional local noise, which flows into
the
earth when the braid is connected to a ground stake. Trying to confirm
this,
the following tests have been performed...
I have found this is often true. I had an amazing level of noise on the old
73kHz band if I grounded the antenna end of the RX antenna feeder, which was
largely eliminated by leaving the loop antenna "floating", except where it
connected to the RX ground. Something similar happened when I operated with
G3GRO from Porthcurno a few years back - we set up a loop well away from
sources of QRM with a long feeder, but still got a high noise level.
eventually I found the noise was eliminated by raising the loop clear of the
long grass and brambles; apparently the vegetation touching the loop antenna
element gave enough of a conductive path to complete a ground loop. With a
loop antenna, even if the screening of the feeder is perfect, a noise
current flowing down the braid will produce a magnetic field that can be
picked up by the loop, so really it is neccessary to prevent such noise
currents flowing, rather than trying to apply better screening. In general,
I find multiple grounding points on feeders are a bad thing from the noise
viewpoint. My current RX vertical antenna has an isolating transformer at
the antenna to reduce this problem. My RX loops are currently grounded only
at the receiver end of the feeders. Also, I have tried to isolate the LF rig
from the mains ground by connecting the mains through a filter which
includes a choke in the ground line, and providing a seperate RF ground
connection. Another source of noise is the TX vertical antenna; when it is
resonant at the receiving frequency, it can introduce a lot of noise in the
RX antennas, which are practically underneath it, and also "suck out" the
wanted signal. This was overcome by adding a relay in series with the "cold"
end of the loading coil, which open-circuits the TX antenna on receive, and
so de-tunes it.
The best tool for investigating these problems is a battery-operated
receiver - if there is no noise when the RX is connected directly at the
antenna feed point with the normal feeder disconnected, you can be sure that
the feeder is the source of the problem. It is also very useful when trying
to find a low-noise location to position a loop or whip antenna..
Cheers, Jim Moritz
73 de M0BMU
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