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

To: jim@audiosystemsgroup.com, towertalk@contesting.com
Subject: Re: [TowerTalk] 3M™ External PIM Absorber 1000 | 3M United States
From: "Jim Lux" <jim@luxfamily.com>
Date: Sun, 19 Nov 2023 12:29:47 -0500
List-post: <mailto:towertalk@contesting.com>
        


 


On Sat, 18 Nov 2023 21:36:15 -0800, Jim Brown <jim@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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