Folks,
As most of you know, the topic of RFI issues with switching (or "switch mode")
power supplies pops up here quite frequently. I was delighted to see that the
February issue of QST addresses the subject with lab tests on 4 more such
power sources. (See pages 56 thru 59 for all of the details.) Make certain
that if you are interested in noise issues with switching power supplies, you
read the entire article carefully. It is also worthwhile to track back and see
the prior power supply test articles the League has published, and which are
referenced in the footnotes.
Keep in mind that all switch mode power supplies have both radiated and
conducted emissions. The name of the game is to buy those having the lowest
levels of emissions in YOUR operating frequency bands of interest. Thus, study
the plots and tables well if you are primarily an HF operator, as most of the
emissions occur in the HF bands. (An exception was the TenTec supply, which
was very noisy in the 160m band, considered to be MF.) Those who operate with
HF antennas inside their houses (especially in the attic) or with antennas
close to the house and the house wiring is not shielded (no conduit or metal
armor cladding), might very well end up with RFI issues from a power supply
when trying to receive weaker signals in some of the HF bands. While there are
ways to reduce conducted emissions from the power supplies, the best
alternatives might be to either stay with linear (heavy) power supplies or to
move the antennas further away from the house wiring.
Related comment: This reflector also gets frequent posts regarding industrial
switching power supplies for use in amateur radio applications, especially for
28 VDC output or very high current at 13.6 VDC. If a power supply was intended
for the industrial or commercial market, the most it may have to meet in the
way of emissions requirements is set forth in FCC Part 15, Class A limits.
These limits are far more lax than the Class B limits for computing devices
used in a home or office environment. In short, such switching power supplies
are almost guaranteed to be "screamers", expecially in the HF region. Stay
with linear technology supplies unless you are prepared to test and design
filters or RFI suppressors for these special power units. The cost of the
"fixes" could easily add up to the purchase cost of the power supply alone, or
even exceed it.
My thanks to ARRL for another excellent article in helping hams make better
buying decisions.
73, Dale
WA9ENA
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