Reducing Local Noise Interference for LF & MF Reception
purchase of a new PC here at Omega House resulted in a desperate increase in
noise on the MF bands, limiting MW listening and completely precluding DXing
below 7MHz. The noise was present as long as the New PC (NPC) was plugged-in,
whether or not switched 'on'. Indeed, it was clear that the NPC's PSU is always
on, as evidenced by the optical mouse always showing its sensing light. The
resultant noise appears to be broadband and significant up to at least 20MHz.
the first few days, the NPC dominated my attention, but after a week or so it
was obvious that something would have to be done for the radio. The HF Receiver
was set up adjacent to the PC with a remote ærial connected by a co-ax feeder.
The signal levels due to the noise in a 4kHz bandwidth were measured on two
normally quiet frequencies, 1611 and 3925kHz. When the Vatican appeared to be
testing DRM on 1611, 1621 was used instead.
With the NPC off and unplugged (really 'off') the noise was at a reasonably low level. With it plugged in or really on, the noise was 9 + 6dB on 3915 and S6 on 1611. It was found that the CRT monitor does not contribute anything on these frequencies (well done that emc engineer!).
A large professional RF mains filter was to hand, and was
set up in the NPC's mains feed. The peripherals were tried both through and not
through the filter, and it was found that there was a useful reduction in
received noise, the noise being slightly lower with the monitor and other
peripherals NOT fed via the filter.
Various simple filters were then tried, starting with the
mains cable wound round various ferrite toroids, then with the mains supply
taken through various ferrite inductors, with the earth cable connection made
directly or via the inductors, and it was observed that, generally, the more the
L the less the Pn, and a direct earth was better than one via an inductor. The
optimum required a small earth inductance, obtained by winding a few turns on a
Rather than commit the rather fine mains filter to serving
the NPC, I wanted to make one specifically for this use. It then occurred to me
that I had a suitable mains-filter type of common-mode dual choke available on
an old PC monitor board that I'd recently scrapped. Closer inspection showed a
number of suitably rated Y-type filter capacitors arraigned around the choke, so
it was decided to cut out the section of the board which carried the mains
filter. An unbroken earth connection was made through the assembly, which was
fitted in a suitable Maplin plastic box.
The effect was impressive, with the noise levels from a
remote æerial no longer raised from the 'off' condition. I suspect that most
recent CRT monitors include effective mains filtering, so it would be well worth
salvaging the components or even the relevant section of the board from any
scrap monitors you find. With the whole world rushing to upgrade to TFT
monitors, small CRT monitors are ten a penny, even when they are working. Those
made in the past eight years or so should be emc compliant, implying effective
Additional benefit was obtained by winding the power and
audio leads of the receiver through ferrite toroids for 10 – 12 turns to form
toroidal chokes, as in the final picture below.
1611kHz 3925kHz No filtering
S9+6 Belling-Lee unit
S8 4turns 1" toroid
S7.5 2×330µH, earth thro'
S8 3×330µH incl.earth
S9 'Monitor' mains filter
4turns 1" toroid
2×330µH, earth thro'
'Monitor' mains filter S2 S5.5
Having seen some success with the mains filter, it was
felt that it would be opportune to put up a modest vertical ærial in the
garden, suitable for use from LF to HF and most particularly for LF, MF and the
If your living space allows you the room to put a small
vertical aerial in a garden, with a distance of at least 20' from the nearest
occupied rooms in your house and your neighbours', then you may be able to
achieve a reasonable freedom from all those contemporary sources of noise - QRN
- such as TV's, PC's, economy light bulbs, house alarms and the rest. The best
rejection of local noise in an urban environment will probably be achieved only
with a loop antenna. If a loop is to be mounted remote from the receiver, it
really needs to be an untuned loop with a matching amplifier, such as one of the
Wellbrook products, or to a design such as Marco Bruno's:
But that's a whole different project from mine.
My vertical is a vertical rod of height 20ft, using a
basic ½-wave CB antenna. In place of the original resonant tuned matching
transformer in the base, a broadband ferrite transformer is used to give a
compromise match to 50 ohms at LF and MF, and also gives useful performance
throughout the HF spectrum.
transformer being wound
impedance of a short monopole is relatively high and capacitive, and although
the resistance component is low (relative to 50W)
good sensitivity can be obtained with a high resistance load. Car radio
installations now commonly use a very short ærial length with a MOSFET input
stage to provide a high resistance load and achieve good sensitivity. The
approach here is to use a much longer ærial, with a ferrite transformer to give
a moderately high-resistance load, matched to 50W
without any active devices.
Matching Transformer was wound on 13.5 x 7 x 7mm two-hole core, made by Epcos
(formerly Siemens) using their N30 grade ferrite, and available from R-S
Components as stock number 212-0617
Testing was then carried out with a signal generator and a
wideband ac millivoltmeter performing much the same function as an oscilloscope,
but reading in dB relative to a datum. It was determined that 4 turns was
sufficient for a 50ohm winding at 150kHz. A ‘turn’ consists of one turn
around the central core, going in one hole and out of the other.
Testing of the transformer was carried out from LF to HF
most suitable turns ratio for the 18ft vertical ærial was chosen after
measurements of the received signal strength with various ratios. It was
expected that the required load resistance should be in the region of 500 to
1,000 ohms, and in the event a turns ratio of 20:4 was chosen, which implies a
load of 1250 ohms.
Sensitivity down to the noise floor is all that is required from a
receiving ærial, and this is easily attained with this long-ish vertical.
Because the load resistance is relatively high, there is no requirement for a
low-resistance earth connection. However,
where noise from the PC is a problem, it may be
worthwhile driving a standard electrical earthing rod into the ground adjacent to the ærial.
This ærial is quite portable; with a length of stout
steel pipe to drive into the ground, it can be set up in a minute. However,
rejection of indoor interference may benefit from a better earthing arrangement,
so be prepared to experiment with one or two electricians’ earth rods.