I've been thinking about putting 20m and 10m dipoles up in my attic (HOA), but I also intend to soon put some Ethernet cable to a WAP (with PoE) through the attic.

I was intending to feed this with CAT5e cable because I have lots of it and it's cheap, but I recently started thinking about the problems that the two might cause for each other in a proximity of 4-6 feet.

Should I worry about the data on the CAT5 being affected by CW/Data/SSB at 100 W on a resonant dipole whose elements are only a few feet away? Will activity on the CAT5 be noticeable on my dipoles? Does using shielded cable make a significant difference here?


Having Ethernet and your antenna co-located is not an ideal situation. But then most amateur antenna situations involve compromises. The general idea of the following recommendations is to take as many precautions as practical to minimize the interference possibilities.

I recommend that your Ethernet cable to your WAP be a CAT6 shielded cable (STP). While obviously not needed for your Ethernet environment, the improved twists of the CAT6 cable along with the foil shield will improve the immunity of the Ethernet cable to ingress from your antenna while transmitting as well as increase the attenuation of leakage from the Ethernet cable to your antenna while receiving.

Your Ethernet cable should also have a few toroid cores of type 43 material spaced along its length to further improve the performance of the shield. Wrap 4 to 6 turns of the cable around each toroid. This attenuates common mode currents that may be flowing along the exterior of the Ethernet shield.

Your transmission line to the antenna should be a double shielded type coax (braid plus foil). This helps to minimize ingress / egress from your transmission line.

Use a high quality, ferrite 1:1 balun at the feedpoint of the antenna. This helps to reduce common mode currents on the cable that can cause interference to your Ethernet from your transmitter or cause interference from the Ethernet to your receiver.

Ensure that your coax length is not close to an odd multiple of 1/4 wavelength (electrical length) for the bands on which you wish to operate. An ideal length is an odd multiple of 1/2 electrical wavelength as this presents the highest impedance for common mode currents. Note that this electrical wavelength relates to the outside of the shield and is influenced by the type of jacket that is on the exterior of the coax cable.

Pay attention to the layout of the cables and antenna. To the greatest extent possible, have cables cross in a perpendicular fashion instead of running or meeting in a parallel fashion. Similarly have the antenna in a perpendicular orientation to the Ethernet cable. This helps to minimize the coupling between the two systems.

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    $\begingroup$ Above and beyond the call of answer duty answering questions I hadn't even though to ask - thanks! $\endgroup$ – William Jan 9 '18 at 3:57
  • $\begingroup$ I took your and Edwin's advice and am now in possession of some Cat-6 SFTP that I'll use for this. $\endgroup$ – William Jan 23 '18 at 22:41

Ethernet cable has a minimum bending radius (check the specs of your cable). When feeding it through toroids, don't wind it tighter than that. Exceeding the bending radius causes the twists in the twisted pair to come undone somewhat, and will make the cable leak.

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  • $\begingroup$ Good point. I'll be sure to get some decent snap-on beads. Given the usual "four times the outer diameter" min. bend radius, a toroid will mostly be wasted. $\endgroup$ – William Jan 12 '18 at 14:18
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    $\begingroup$ I wound 14t of CAT5 through a Fair-Rite #31 2.4" core, and pretty tightly because I needed to reach my computer. No harm done (this time :-). Anyway, it solved my problem of 75m RF getting into the PC. $\endgroup$ – Mike Waters Jan 12 '18 at 15:22
  • $\begingroup$ @MikeWaters - Are you running pretty high power, or is that kind of thing fairly common? I've never had issues with RF and my networking as yet. $\endgroup$ – William Jan 12 '18 at 16:00
  • $\begingroup$ @William-Rem Yes, high power sometimes (but not always). I have unshielded LAN cables running all through my house: basement, first floor, and second floor. The dipole isn't far enough away from the house. Maybe a LAN cable is resonant on a ham band, I don't know. $\endgroup$ – Mike Waters Jan 12 '18 at 16:08

I have multiple antenna's, feedlines and Ethernet cables in close proximity.

To add to the great answer already given:

I have used STP in stead of UTP for all my Ethernet connections. This reduced cross interference greatly.

I did not have an affect on RX by activity of the Ethernet connections, but I certainly had "unexpected network behaviour" when TX-ing.

You can get STP in Cat5e and Cat6. I would recommend Cat6-STP, but it is more expensive than Cat5e and/or UTP variants.

I understand that you have Cat5e already at hand, so maybe this is not an option for you.

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  • $\begingroup$ Thanks! I have some CAT-6 SFTP now and will fish it up to the attic soon. $\endgroup$ – William Jan 23 '18 at 22:40
  • $\begingroup$ Quick clarification question. When you said "SFTP", do you mean "SF/UTP" (Both an overall braid screen (S) and foil shield (F) with unscreened twisted pairs (UTP)) --- or "S/FTP" (An overall braid screen (S) with foil screened twisted pairs (FTP). The “shield” underneath the jacket is a braid, and each individual pair is surrounded by its own foil barrier.) It seems that both types are available. How did this work for you? (RFI and difficulty)? I have the same dilemma as the OP. $\endgroup$ – whiskeychief Oct 15 '18 at 11:13
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    $\begingroup$ @whiskeychief I used SF/UTP although I called it STP in my answer. $\endgroup$ – Edwin van Mierlo Oct 16 '18 at 9:04
  • $\begingroup$ SF/UTP (category 6a?) from a reputable company seems to be quite difficult to find — F/UTP is readily available. Is there a CMR rated cable available that I haven’t seen? $\endgroup$ – whiskeychief Oct 17 '18 at 13:18

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