[RFI] Internet Modem / Router Cable Length and RFI

[Roger (K8RI)]

I think I'd rephrase what Jim says to "every cable can and may" radiate 
trash.

I've been running 4 and 5 computers on a CAT-5 gigabit network. I've had 
various cable modems, routers and switches. I run 160 through440 with 
the legal limit on all bands except 144.  I run the legal limit on 440 
because I'm too far S and E in Michigan's lower peninsula, or rephrased, 
too close to Canada so the legal limit is pretty low.

So I can't speak for the higher frequencies, but so far I have had no 
detectable egress or ingress from 160 through 440.  The current set up 
is a Motorola cable modem with 100 Mb/s service, the router is a Netgear 
Nighthawk, feeding  a Netgear 8 port gigabit smart switch with a 16 TB 
and 24 TB server, 2 4 GHz 8 core processors with 16 GB of DDR3 RAM. (One 
is soon to be replaced with an 8 core water cooled, CPU running 5 GHz 
and 36 GB of RAM. Those are fed with two 130' runs of CAT-5. Right now  
the second computer in the den is down, but this one is an 8 core CPU 
running a tad over 4 GHz with 16 GB of DDR3 RAM.  My wife's computer, 
smart phone and tablet are on dual wireless from the Nighthawk.  Both 
rigs in the shop and the ones in the den have been hooked into the 
adjacent stations at times

Now that the massive storage is on the two servers, the shop computers 
will be going wireless.

I listed all of that, to show that having bleeding edge computers, a 
high speed network, and the legal limit ham stations does not mean you 
will have RFI.  That I don't certainly does not mean you won't either.  
I have taken no special RFI protection steps. I have no toroids on any 
cables in or out of the computers, network, or ham gear.  I do have 
common mode current chokes at the feed points on most of the antennas .

All coax has the shield grounded at both the top and bottom of the tower 
and at the CPG plate at the house entrance and shop entrance although 
much is in a state of constant change.

Having said all that, each time I make a system change, I have "THAT" 
fear until I've checked every thing out which may take weeks, or even 
months as all equipment, or antennas may not be in operation.

I do have an elaborate grounding system, but not getting that CAT-5 
network disconnected last summer proved to be a very expensive mistake.

Fortunately non of the rigs were hooked into the system. Still, I lost 3 
out of 4 NICs, one motherboars, a  combo wireless and 4 port switch as 
well as an 8 port smart switch and the protective CAT-5 connectors in 
one 2 KW UPS were welded and melted together.   Unfortunately, 
semiconductors can be "hurt" and have their lives drastically shortened 
so I still do not know the total cost  of that nearby strike in money or 
labor.

At least I'm relieved that none of the rigs were hooked up.


73

Roger (K8RI)

On 6/1/2015 1:11 AM, Jim Brown wrote:
> Think of EVERY cable connected to digital electronics as an antenna 
> radiating trash. That includes the router's coax or telco cable, and 
> all the power supply cables.
>
> Guidelines.  Shorter cables are better, ALL with common mode chokes.If 
> cables are longer than about 1/10 wavelength at frequencies you care 
> about, choke both ends. Make maximum use of WiFi for routing.
>
> Replace all switch-mode power supplies with linear ones that you have 
> scrounged or saved or bought at hamfests or second hand stores.  My 
> router and cable modem run from a vintage lead-acid battery that is 
> float charged by a linear wall wart. No noise from the PSU, and it's 
> also a UPS for that gear. My internet doesn't die in a power failure 
> until the cable company's batteries run down (they're good for about 6 
> hours, our outages here in the mountains can get a lot longer).
>
> For conceptual discussion and lots of specifics, see k9yc.com/RFI-Ham.pdf
>
> 73, Jim K9YC
>
> On Sun,5/31/2015 9:53 PM, Tony wrote:
>> All:
>>
>> I'm about to relocate the wireless router in my home which currently 
>> sits next to my internet modem connected with a short run of CAT5. 
>> Relocating the router will lengthen the CAT cable by 40 feet so I'm a 
>> bit concerned about RFI birdies, noise etc.
>>
>> That said, I was wondering if it's better to relocate both cable 
>> modem and the router to the same location to keep the CAT cable 
>> short. I would then have to lengthen the coax cable for the modem 
>> instead of the CAT cable.
>>
>> The question is: which setup has more potential for RFI?
>
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