Dear LF group,
Looking through my files, I found a report on an experiment carried out in
October 2003. This adresses the issue of local noise and gives some
background on the motives behind the mini-whip development. Luckily it is in
plain text and you will find it here after. Maybe some of you will find this
of interest.
Thank you for the bandwidth,
Roelof Bakker, pa0rdt
Report comparing an active whip and an active loop.
In a field test, both antennas received all stations equally well, but, the
directivity of the loop was useful in separating stations on the same
frequency. This test describes the results at the home location.
The active whip is the G4COL design as appeared in RADCOM, modified for LF.
The
coaxial feed line (RG58U) to the operating position is 28 metre long of
which 10 metre outdoor. Before entering the house, the braid of the coax is
connecting to a good ground system. The active loop is 1.4 metre square and
coupled to a balanced amplifier via a RF isolating transformer. The
amplifier uses two 2N5109 transistors. Current consumption is 35 mA at 12V.
The feed line is 20 metre long. Both antennas are located 6 metre from the
house and 5 metre apart.
The receiver used is a Wandel & Goltermann selective level meter SPM-3 with
an add-on unit containing a tuneable pre-amplifier, tuneable down converter,
25 Hz wide LC filter at 500 Hz and an audio amplifier.
In principle both antennas performed equally well in receiving NDB's. If
there was a difference, the active whip gave just a little better result.
In practice, however the active loop was far more sensitive to local noise.
Especially in the evening with home entertaining electronic devices in full
swing. Noise bursts hardly if at all audible on the active whip were very
strong on the active loop. At many times, this also rendered the active loop
useless for phasing purposes.
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.
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.
1.. The SPM-3 was moved to the garden and connected to the feed line of
the active whip. This feedline was 10 metre long. No provision was made for
grounding the shield of the coaxial cable. Received noise was the same as
indoors with the usual feedline and the braid of the coax connected to
ground before entering the house.
Conclusion # 1: no noise is received on the shield of the coax feed line
outside the house.
2.. Using a T-piece the coax feed line inside the house was connected to
the coax feed line outside the house and to the SPM-3. The noise was now 10
dB stronger than in test # 1. The noise disappeared when the shield of the
coax was connected to ground.
Conclusion # 2: the noise is received on the shield of the coax cable
located inside the house.
3.. Repeating test # 1 for the active loop increased the noise by 12 to 22
dB dependant of the orientation of the loop. Grounding the shield of the
coax made no difference at all.
Conclusion # 3: the noise received by the active loop is received by the
antenna and not by the feedline.
During the tests, the measurement equipment was operated independent from
the mains.
A test was performed using battery power inside the house. No discernable
differences were noted compared to using a mains power supply. So effects
from using the mains power supply can safely be excluded. The position of
the antennas has been interchanged to eliminate effects due to the location
in the garden, but no differences were noted either.
Comparing the level of received noise by ear is extremely difficult. The
actual difference in dB is much larger than the perceived difference. With
these low signal levels the signal strength meter of a receiver is of no
use. The agc does not respond to such low-level signals. This is an
application where a selective level meter is indispensable.
General observations.
These tests show that the local noise is contained within the house.
Therefore, the noise received on the active loop cannot be in the electric
field. Otherwise, the active whip should have received it as well.
One might ask where the noise received on the active loop originates. Most
likely, this noise comes from the magnetic field and this field is not
contained within the house. Further tests can prove this assumption true or
false.
These findings are not without any practical implications. There is now hard
evidence that an active whip is much less sensitive to local noise from
inside the house and that the noise is received on the feed line inside the
house. With the right precautions this little antennas can provide
outstanding performance.
Further tests showed that connecting the radio earth to the mains earth, the
noise level increased 7 dB, ruling out any weak signal work
Roelof Bakker, PAoRDT
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