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HB9ABX > TECHNIK 10.06.20 10:34l 140 Lines 5990 Bytes #999 (999) @ WW
BID : A6UDB0FHN037
Read: GUEST
Subj: Local QRM - QRM-Remedy
Path: IZ3LSV<DB0ERF<DB0RBS<DB0FHN
Sent: 200610/0929z @:DB0FHN.#BAY.DEU.EU [JN59NK Nuernberg] obcm1.07b12 LT:999
From: HB9ABX @ DB0FHN.#BAY.DEU.EU (Felix)
To: TECHNIK @ WW
Reply-To: HB9ABX @ HB9EAS.CHE.EU
X-Info: Sent with login password
Local QRM - QRM-Remedy
Frequently one hears strong QRM on the bands, which is not received from the
air but is created locally by any device in the local vicinity.
Then, we first have to find the device that produces this interference, and
then we have to remove the cause of this interference.
Here follows the story how I solved this kind of problem:
During my recent antenna tests I had a lot of problems with strong local QRM.
After installation of the antenna in Santiago I had a QRM level of S9
throughout the HF band. Only stations with signals over S9 could be heared.
So I had to localize the source of this interference.
I got help through my transistor radio which has 7 SW bands. With the extended
whip antenna I walked around the houses on our street, listening on a clear
frequency close to the HAM band.
So I saw, that close to or house the noise level was highest, and walking away
the noise disappeared. Then I cut the current of our house - and the noise
remained.
Now I went carefully around our house and noted, that next to the neighbours
house the noise was strongest.
I pushed the bell of the neighbour and asked if I may cut the current of its
house to find out, if the source of the noise is from there. And really, after
"power off" we had silence on the radio. Then I asked permission to check
inside the house, and by entering the living room it was easy to detect, that
the cause of the noise was a DVD player (Brand Daewoo). This equipment was
purchased little time before.
Next day the neighbour went to the warehouse where he buyed it. They told him
that the guarantee just expired. So I offered him to denoise this apparatus.
The cause of the noise was a switched power supply which had no adequate
filtering.
Interesting was, that the power ON switch of the equipment was behind the
switched power supply, therefore the noise was generated as long as the
connector was plugged to power ...
Remedy of the noise was a filter circuit which was installed inside the DVD
player, just at the power line input.
Required material for the filter:
2 ferrite rings FT114-43 (or FT140-43) material 43 is much better than material
77
2 capacitors of 10000 pF (0.01 uF), 1400 V DC, ceramic disk (low inductance)
isolated litz wire
Ring 1:
2 isolated litz wires wound individually on the ring.
First wire: 8 turns beginning from the top front of the ring through the hole
on the right half side of the ring.
Then with the second wire: 8 turns beginning from the top front of the ring
through the hole on the left half side of the ring.
Seen from the front: both wires are beginning from the top and are ending
behind on the lower side.
All 4 ends have to be prepared to be soldered by stripping off the isolation at
the ends.
Ring 2:
2 isolated litz wires are wound in parallel on the ring. 8 to 10 turns
distributed over the full ring. Also these 4 ends have to be prepared for
soldering.
Now solder the output of ring 1 to the input of ring 2.
Now, the power line inside the unit has to be cut just behind the enter point
into the unit and both ends have to be prepared for soldering.
The arriving power line is to be soldered to the input of ring 1.
Then solder the output of ring 2 to the power line which continues inside the
unit.
As next, locate a ground connection close to the enter point of the power
line.
Eventually, the metallic housing has to be scratched to allow soldering of a
thick ground wire directly to the metallic housing (using a strong soldering
tool).
Now, solder the first capacitor between ground connection to the first
soldering point between ring 1 and ring 2.
Then, solder the second capacitor between ground connection to the second
soldering point between ring 1 and ring 2.
As next, all soldering points have to be isolated carefully with PVC tape, and
fix the two rings from moving around.
Close the housing. - Finished.
Power on and test:
Before installation of the filter, strong noise was heared from medium wave to
all SW bands.
After filter installation no noise was detectable.
Test on HF band with radio equipment:
The previous noise of S9 disappeared, the band was clear with a ground level
noise of S1.
---
At the second test location, 1000 km south of Santiago, the same problem:
The full HAM band with noise of S9 and more.
Again searching the source: 3 equipments were found which were causing
interference
in the same house where the antenna was installed:
1. = Computer with switched PS in miditower.
2. = Fax Unit Panasonic KX-F580
3. = AIRSPAN, Subscriber Terminal Indoor (= wireless phone and Internet
connection)
In all cases, the noise came from inadequately filtered switched power
supplies.
Depending on the type of switched power supply, the created noise differs. Some
produce a broadband "white noise", others a broad, unstable noise which is
heared abt. every 40 to 80 KHz on the bands.
At all 3 units the same filter circuit was used to denoise. With the only
difference, that in the third unit no space was available to install the
ferrite rings. There I had to wind air coils, which could be placed between the
switched power supply and the circuit board.
After installing all filters the ground level noise on a free frequency dropped
also to S1.
Note: You have to be aware, that if the unit where you installed the filters,
may cause another problem: If it fails later on, they may try to make you
responsible for any other defect.
Therefore you should be aware of that.
In some cases it is recommendable to install the necessary filters externally,
even so the effect of the filter would be best, if the filter is inside the
unit.
Vendors of Ferrite Toroids:
------------------------------------
- http://www.oselectronics.com
- http://www.palomar-engineers.com
- http://www.cwsbytemark.com
73, Felix, HB9ABX ( felix-abx at gmx.ch )
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