HF TRANSCEIVER GROUND (EARTH) WIRE USING CO-AXIAL CABLE.
Living in a flat has it’s own problem where radio frequency (RF) grounding for a HF transceiver is concerned. Improper grounding/earthing system can cause several unwanted RF interferences (RFI) to television sets, broadcast receivers, hi-fi sets and telephone sets.
I have these RFI’s when using my transceiver on 40m, 20m, and 15m. I have not tried using 10m because my home made magnetic loop antenna was unable to operate on that frequency. By the way, my ground wire is connected to the 4-inch diameter cast iron sewage piping system about 35 feet away.
With the assistance from my XYL, who is quite familiar with the operation of a HF transceiver, I started my troubleshooting for the source of RFI’s. Using the multi-function ‘Dinky’ SWR meter model SS-51 by Oskerblock, I switched to field strength meter (FSM) function. While my XYL was keying-up the transceiver at preset intervals with 30 watts on a clear 40m frequency, I adjusted the sensitivity of the FSM for minimum deflection. This is because I do not want to be confused with the RF picked up from my home made magnetic loop antenna that is mounted at the balcony of my flat.
After all the necessary adjustments made, I started moving the FSM close to all wires laid out from the shack including the ground/earth wire. To my surprise, the needle of the FSM was deflecting strongly when I brought the FSM close to about 3 to 4 inches from the insulated ground/earth wire that was connected to the sewage pipe. As I moved the FSM along the wire, there was strong deflections. No meter deflections was recorded near the sewage pipe. This means that my ground/earth wire is radiating some how!. It was the same on 20m and 15m. Labouring with my thoughts, how am I going to solve this problem?. Using a low pass filter does not solve this either. I need a method to shield the ground/earth conductor wire that is something like my antenna feeder cable.
With a sufficient length of RG-58 co-ax cable in hand, I connect the center conductor with one end to the transceiver’s ground and the other end to the sewage pipe. This leaves the co-ax brading ‘floating’. I thought that the braiding can act as a shield. After keying-up the transceiver, the RFI problem persist!. After reading through some articles about RC and LC circuits, I soldered at both ends a 470pF 1KV ceramic disc capacitor (C1, C2) across the center conductor and braiding of the co-axial cable. Please refer to the diagram below.
When the transceiver was keyed up by my XYL, I noticed that the FSM meter deflection was only about one third compared to earlier deflections. This shows that the co-axial cable braiding is partially shielding the center conductor!. Traces of RFI’s to my television set, broadcast receiver, hi-fi set and telephone set was now non-existent. Then I decided to replace C1 and C2 with value of 820pF 1KV. Builders can try different capacitor values. Further tests show that the FSM meter deflection was minute or no deflection at all even at maximum sensitivity.
The co-axial cable braiding now acts as an extremely effectve shield and any RF that reaches and absorbed by the braiding will be transfered or bled back to the center conductor via the capacitors. This is because the path of lowest resistance is through the center conductor.
I have been using this co-axial cable ground wire system since the middle of 1989 and no RFI’s has been encountered in my QTH or received complaints of RFI’s from my neighbours.
If you have RFI’s possibly due to your grounding system, then experiment with this system. It might help you. So please give it a try. Thank you.
73 and all the best,
Penang Island ( IOTA AS015 )