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Author Topic: Cat5 cable resonant lengths?  (Read 2564 times)

Posts: 827

« on: April 06, 2005, 12:25:09 PM »

I'm about to replace a couple of cat5 computer cables with Cat5e STP (shielded) cables in an attempt to cut RFI/TVI from them, mainly in the 2m band.

I was wondering if the length of the cables (shielded OR unshielded) could itself be significant, i.e. if they fell into a multiple of the 2m wavelength would that make them more prone to radiate noise?

My new cable choices are 7' or 14' premade, making the cables pretty close to 3x and 6x the 2m wavelength unless I've gotten my numbers mangled as usual. (No specs for the cables.) Probably will need the 14' length, no other stock sizes from this vendor.

Should I have one custom cut to be sure it isn't resonant? Or just try what I can get off the shelf?

Posts: 17411

« Reply #1 on: April 06, 2005, 12:55:53 PM »

Actually, 7' would be just over 1 wavelength on 2m, except
that the dielectric constant of the insulating material
needs to be considered, and that is a bit of an unknown.

I'd be concerned about resonant cable lengths on the lower
HF bands (a pair of 30' speaker cables makes a great 40m
dipole) but not as much on VHF.  If you have troubles,
put a couple ferrite split beads over the cable 18" apart .
That should kill any resonances in the cable.

Posts: 21758

« Reply #2 on: April 07, 2005, 08:46:20 AM »

A crimper from Tyco goes a long way in saving on preassembled CAT5-CAT6 cables, shielded or not.  I make all my own for about 20% of what they sell for preassembled, and the tool was only about $30.

But I agree with Dale.  Not sure if the RFI you're addressing is outgoing (from your router to your 2m rig) or incoming (rig to router), but wrapping the cable through a ferrite core a few turns usually does the trick, even with unshielded TP cables.


Posts: 827

« Reply #3 on: April 07, 2005, 02:35:04 PM »

I agree with you about the crimper and have several (including the good ratcheting crimpers) in my bag. But somehow not an RJ-45, the time and effort to make and test cables isn't worth it for installation jobs. For myself I've borrowed one in the past, but I thought I had enough cables to last the rest of my lifetime. Never planned on using STP though.<G> These two cables will cost me all of ten bucks including delivery, so I'm going to pass on another crimping tool for now.

The RFI/TV is coming from the router. Very little from the router, enough to be distracting from the cables though.


Posts: 555

« Reply #4 on: April 14, 2005, 06:41:43 PM »

CAT-5UTP is twisted pretty tightly.  This is for common mode rejection of interference coming into the cable.  This usually also keeps signals from radiating from the cable as well.

I guess I would start by trying a CAT-6UTP cable. it's twisted even tighter than CAT-5.

If the cable is radiating, it is possible that the shield on CAT-5STP will radiate just as badly.

Good Luck ES 73


Posts: 52

« Reply #5 on: May 05, 2005, 10:21:14 AM »

CAT6 cable has exactly the same twist and characteristics as CAT5E.  The difference lies in the fact that all four pairs are twisted the same and terminated the same.  This is so that GIG-E can use all four pairs to achieve 1000Mbps.  CAT6 cables cannot be made in the field.  that require special fabrication techniques to keep the proper twist and pair isolation down to the connectors.


Posts: 229

« Reply #6 on: May 08, 2005, 10:21:40 PM »

Much of the interference a network causes is due to common mode current from computers, servers and other network equipment. Some is from data current common mode conversion in imperfectly balanced cables and interfaces, and from coupling to other nearby conductors.

Shielded cable can help -- but only if the shield is properly terminated at each end. Shields need to be terminated all around the connectors, and right to the chassis, too, *not* to shells "grounded" only to the boards on which they're soldered. That's all too likely to increase emissions instead of decreasing them. Additionally, common mode current on power or control wiring will also flow on the surface of STP, so those wires must also be suppressed. I've seen a persistent common mode problem go away when a "wall-wart" AC power adaptor ground pin was disconnected. (Ahem! This is NOT a viable fix.)

Some RJ45 equipment interfaces have EMI filtering built in, and this can help.

Unfortunately, grounding STP shields to the chassis of AC-powered equipment may result in problems.  

"There is always a risk of bridging two branch circuits with the ground on one of these cables. Any significant difference in the impedance of the two AC grounds can induce current flow in network cable ground that, at the least, can potentially destroy the cards at either end. "

". . . cable shield can start carry grounding current which affect the cable performance. If a ground potential differences of less than 1 V r.m.s. cannot be met, this can cause problems for STP installation. In those special cases, the wiring system needs to use non-continuous grounding where . . .

There may also be issues with fire and electrical codes. Equipment safety ground is provided by AC power "green wire" connections to Neutral and building ground at the AC power entrance panel. Equipment grounded elsewhere may upset this requirement.

You might want to consider ferrites at VHF frequencies, since the commonly available "RFI" ferrites are quite effective at 100 MHz and upwards.  There is often some benefit to sliding these along cables until interference is minimized; ferrites work best where common-mode current is highest - and worst (or not at all) a quarter wavelength away from these points, where common-mode current is least.

Lastly, resist the temptation to throw ferrite at everything. If one ferrite helps, two will help more, and four even more, but four is just about the point of diminishing returns and you might be better off using the last two ferrites on another cable instead.

Good luck!


Posts: 3288

« Reply #7 on: May 15, 2005, 08:24:58 PM »

You don't exactly explain your interference problem but here is my thoughts.  

1. Your effort should be put into finding the EMI source and fixing or replacing it.  My guess is that it is a network component problem e.g. router, switch, NIC, attached device, rather than a radio emission problem.

2. Reasons why it is probably not a radio emmission problem. A computer network is packet based, not waveform based. If packets are missed they are retransmitted.  It is unlikely that your radio is going to be tramitting on exactly the same frequency that the network is operating.  It is unlikely that your transmissions would be long enough to completely degrade the network ops.  

3. Standard Cat5 cable is Highly resistant to any RFI even in high density ops. I don't think cable length will have any effect.  Additional shielding is a nice to have feature.  

I worked developing LAN based systems for tactical military applications for the last 4 years.  We found that we had NO RFI problems operating 16+ radios from HF thru SHF with antenns within 100ft.  We had up to 20 computers and peripherals plus lots of LAN controlled equipment.  We found no difference between shielded and unshielded Cat5.  This experience is based on dozens and dozens of fielded systems.

Good luck.    
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