A Ubiquity Nanostation Loco M5 5 GHz radio like the one I use to bring internet to the garage. Its PoE power supply is on the right.  The unit is capable of 300 Mbps and its range is up to four miles.  This one was available used for just $29.00.

The Ubiquiti radios get their power from the AC socket in the house to the box outside using ‘Power over Ethernet’ or PoE where the Ethernet cable itself carries the necessary DC voltage.  Ethernet cable typically uses only two of the four pairs of wires, the orange/white-orange and the green/white-green to carry data, leaving the other two pair otherwise unused.  In a PoE system, the blue/white-blue pair and brown/white-brown pair are used carry plus and minus 24 VDC to the outside unit.  DC is fed into each of the paralleled pairs that go to the Ubiquiti unit, but not to the rest of the LAN network wiring.  A ‘feature’ is that you can reset the unit to factory defaults by sticking the end of a paperclip into either of two small holes, one on the outside radio unit and the other on the indoor PoE power supply.  To cause this ‘hard’ reset, when the paperclip is inserted, the DC is additionally carried on the data pair (orange and green pair) of the Ethernet cable which is used at the radio end for the reset. The data pair get DC injected by using small, surface-mount, center-tapped transformers in the power supply that pass data but isolate the DC from rest of the network. A Zener diode in the radio prevents the reset voltage from exceeding a certain limit.

A PoE injector circuit for ordinary power on the non-data or ‘spare’ pairs.

A PoE circuit where DC and data flow on the same pairs.

I read the Ubiquiti Nanostation M5 instructions which say to hold the reset button down for 10 seconds to return the unit to factory defaults. Suddenly the light flashed in my head! The difference between the CW and FT8 RFI is that CW runs in very short dits and dahs while FT8 is key-down for 14 seconds. The tower, which is shunt-fed on 160 meters, is less than 1/4 wavelength from the Ubiquiti radio and its 175’ of shielded outside Ethernet cable. Even at 400 Watts, enough RF was being induced into the cable and rectified by the Zener to cause a factory reset. This was not a common-mode problem, but rather a differential-mode problem, which cannot be cured with ferrite chokes on the outside of the cable.

Now, How Do We Solve This?

There have been very few Ethernet RFI filters available for HF radio interference until recently when DX Engineering came out with their ISO-PLUS, a small box that has both common-mode and differential mode filtering. While the specifications are excellent, the unit does not pass PoE voltage. This is good news and bad news. The good news is that the reset voltage on the data pairs get blocked. The bad news is that the Ubiquiti radio needs PoE voltage on the non-data pairs to run. The solution: extract the PoE voltage from the non-data pairs before the ISO-PLUS and re-inject it after, wrapping the wire between the two injectors around a ferrite loop to kill any residual RF. This is relatively simple using commonly available PoE injector boxes which I purchased for $7.49 each on Amazon. All parts were mounted in a small watertight plastic box and secured to the 10’ mast where the Ubiquiti radio is mounted.

The finished filter box with two PoE injectors, ferrite choke for PoE voltage and DX Engineering ISO-PLUS Ethernet filter.  The 175’ shielded CAT5 cable from the shack is fed into the injector on the left.

Does It Work?

And how! Since installing the filter I have made over a hundred transmissions at 1,500 Watts on 160 FT8 without a single problem and in the first week after installation, I added over a dozen new countries to my confirmed list on the band and even started a couple of JA pileups! This puts me as of this writing within three QSL’s of 9-band DXCC. Although 6 meters has not been open, I have done full-power tests with no problems.

This was truly an obscure RFI problem with virtually nothing written about it on the internet, so I have posted this on my site and hopefully others having similar issues in the future will find it. In solving this problem, I learned a lot about PoE and general network infrastructure. Look for me on 160-meter FT8!

The completed filter installed. The 70’ tower and DB42 are in the background.
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