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Author Topic: Using a lead acid battery as shielding/rfi sink for a switch mode supply  (Read 3743 times)
LA9XSA
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Posts: 376




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« on: December 03, 2012, 06:55:23 AM »

I have a variable voltage switch mode power supply for a netbook, that I use to charge a 7 Ah AGM battery through a self-built trickle charger, and also run my QRP radio at the same time. The PSU is rated at about 4.5 A at 14 volts, and 5 A on 12 volts, which is more than enough for this. This particular power supply generates quite a bit of hash on 17 meters and below, so when using an indoor antenna it tends to wipe out the bands 80 through 17.

The obvious solution is to use the outdoor antenna, or to just run from the battery, but it seems if I take this power supply and rubber band it to the battery itself, keeping the battery between the antenna and the PSU, all the mass of the battery - or perhaps coupling of the magnetic field into the battery - brings down the noise several S-units and makes 17 and 20 meters usable while charging.

I already knew that the battery helps filter any cable conducted DC voltage ripple, but that it would also help shield air-conducted RFI so effectively was a bit of a surprise to me. The PSU already had ferrite beads on its leads, and adding more doesn't seem to make a difference, so it appears that the hash is being transmitted by the PSU box itself and not by the wires.

Anyone else use their battery (banks) as RFI shielding?
« Last Edit: December 03, 2012, 07:01:09 AM by LA9XSA » Logged
KQ6Q
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Posts: 988




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« Reply #1 on: December 08, 2012, 03:51:37 PM »

while the setup with the small switcher is working for you, it might have been even simpler to use an analog power supply - 3A or 5A 12V units are generally under $30. Battery IS the quietest tho, always!

KQ6Q
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