[TowerTalk] 3M™ External PIM Absorber 1000 | 3M United States

[Jim Lux]

On Sat, 18 Nov 2023 21:36:15 -0800, Jim Brown <jim at audiosystemsgroup.com> wrote:

On 11/18/2023 8:57 PM, Jim Lux wrote:
> I was using 31 mix more as an example.  The data sheet for the tape or
> sheet gives curves for R and X. The big issue is, for HF, you don’t want
> the stuff for microwaves (which is what most of them are).

Yes, exactly what I was thinking once you described what worked for you
-- it's a new way of doing what W2DU did with beads 50 years ago.
>
> I’ll see if I can find what we used - we were in the 100kHz -23 MHz
> range.  We got rolls of stuff that we could wrap our cables with, or put
> around holes in the chassis where the wires came through.

That sounds like a shielding failure if the conductor isn't either
bonded or bypassed at the point of exit. I'm sure I'm not the only guy
here old enough to remember feed-through caps! I don't see stuffing the
hole as a solution. I DO see a length of a suitable material forming a
W2DU-style choke.

>> typically, for space flight stuff in cubesats, there’s little or no attention paid to EMI/EMC for lower frequencies - the concern is all about self compatibility with the telecom radios at UHF (less common these days), L-band (GPS), S-band (2-2.2 GHz) and X-band (7-8.5 GHz), where even if there are harmonics from switching power supplies, they’ve faded out by the time you get up to 2 GHz.   And yes, there are connectors with built in low pass filters, but they tend to be bigger and bulkier, and when you’re trying to squeeze everything into a few liters, people tend to use tiny, tiny connectors, like Nano-D.    But more, it’s there’s often no Conducted Emissions or Radiated Emissions Spec, do neither do they design nor do they test for it.





>
> I should point out that this is a “fix” - people should really design
> things that don’t radiate on their wires (other than antennas!) or are
> susceptible to signals, or that have nonlinear characteristics.

Right -- like practicing proper shielding of equipment, proper bonding
everywhere.

>> In this case bonding usually isn’t a problem, spacecraft systems are well bonded because of concerns about charging from charged particles, although inevitably there are loops: coax shields providing an alternate path(s).  Here it’s plain old filtering on the power supply inputs inside the box to prevent switching currents from propagating back out. 

>> I should point out that we have spacecraft with a payload that is *very* sensitive - we’re galactic background noise limited, so in a MIL-STD-461 sense, we’re sensitive down to about -40 dBuV/m, which is a good 60 dB below the usual 24 dBuV/m Radiated Emissions limit.   But in any case, we are far from the only spacecraft that runs into this kind of problem. 



73, Jim K9YC

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