On 3/4/2020 9:27 AM, Bob Turner wrote:
I've noticed that one of my computers emits RFI, but it's coming from the HDMI
cable. I notice it mostly on 70cm/445.200. To a lesser extent, I've
noticed it on the 6m SSB portion of band. It's very weak on 6m, but it has
made me loose a couple of 6m contacts. If I pull cable from computer, but not
TV/monitor noise is gone. If computer is off, noise is also gone.
I figure I need a high quality shielded (properly) HDMI cable with toroids.
Any known good ones? Any other comments on this?
Because most equipment is built with a Pin One Problem (improper
termination of a cable shield), shielded cable may not solve a problem
like this. For 6M, #31 clamp-on cores are the weapon of choice. Wind two
6-in diameter turns through a single 1-in i.d. clamp-on, or two turns
through two 0.7 i.d. clamp-ons. For chokes, the number of turns is the
number of times the cable passes through the core, so one turn outside
the core. To see the impedance curves for these parts, go to this link,
click on the part number for the core that fits your cable, and scroll
down to the bottom of the data sheet for that part.
https://www.fair-rite.com/product-category/suppression-components/round-cable-snap-its/lower-broadband-frequencies-1-300-mhz-31-material-round-cable-snap-its/
For 2M, use multiple #31 clamp-ons, not wound, just clamped onto the
cable. For 440 MHz, the best Fair-Rite material is #61, also multiple
clamp-ons along the cable. Fair-Rite part nr 0461164181 is 1/2-in i.d.
You can see the impedance curve here.
https://www.fair-rite.com/product/round-cable-snap-its-461164181/
These guidelines work for any kind of cable -- coax feeding TX or RX
antennas, power cables, wired ethernet, cables between equipment. They
also work for other noise sources.
For a long boom 6M Yagi I just had installed, I used five 2-turn chokes
in series. The chokes are clamped to the bottom of the 1 1/2" boom using
nested lengths of 1 1/2-in and 2-in PVC conduit slit in half as spacers
to the boom. Spacers are required so that the boom does not add stray
capacitance to the chokes, which would move their resonance down in
frequency, defeating the choke.
To clarify the spacers, the two lengths of PVC were first split using a
table saw, then cut into suitable lengths (to fit between mounting
hardware for the elements). The 1 1/2-in fits to the bottom of the boom,
the 2-in nests under it, and the cores are then clamped to the boom.
Chokes are spaced from each other along the line so that there's at
least 3-in between the turns of adjacent chokes.
73, Jim K9YC
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