Thanks for the comments. I ordered one of the newer 8K cables and will give
that a shot. The description was bragging about how its "triple shielded", so
maybe it's better. I recall the ones I'm using were low cost, so probably not
the best quality. If the 8K cable does not do the trick, I may try using my
laptop as the 2nd logging computer. It will not have external HDMI cable. I
may also try the Jim Brown solution.
7 3 Bob
N2SCJ
-----Original Message-----
From: RFI [mailto:rfi-bounces+n2scj-lists=outlook.com@contesting.com] On Behalf
Of Jim Brown
Sent: Wednesday, March 4, 2020 2:10 PM
To: rfi@contesting.com
Subject: Re: [RFI] Any known to be good shielded HDMI cables?
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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