Mains-borne noise on a low-band receive antenna
A receiving station on 80 m uses a long wire fed with coax through a 9:1 balun. The operator hears a strong buzz that rises and falls with the mains cycle, strongest when the antenna is connected and absent when a dummy load replaces it. The noise is present on all bands but worst on 80 m. Nearby there is a solar inverter, a switched-mode power supply for a laptop, and a street lamp on a photocell. Disconnecting the laptop supply reduces but does not eliminate the noise. The station earth is a single 1.5 m rod.
1. How would you localise the remaining noise source systematically?
Read the worked answer
Work from the antenna backwards and from the building outwards. First confirm the noise is radiated and not conducted: run the receiver on battery, disconnect the mains, and see whether the buzz persists. Then replace the antenna with a short wire indoors and outdoors to see whether the source is near the shack.
Use a portable receiver with a directional loop or ferrite rod to walk the site, watching the 80 m noise level while switching suspected loads off at the breaker. The solar inverter is a prime suspect because its switching frequency and harmonics are rich and it runs during daylight; the street lamp photocell switches at dusk. Switch each circuit individually and note the change.
If the source is a neighbour's installation, record the times and levels and approach the matter through the regulator rather than tampering. Keep a log correlating noise with the state of each suspected device. The systematic point is to change one variable at a time and to distinguish conducted noise entering via the mains from radiated noise picked up by the antenna.
2. What role does the single earth rod play, and what would you change?
Read the worked answer
A single 1.5 m rod is a poor RF earth at 3.5 MHz: its impedance is high, so it does not provide a low-impedance return and may even make common-mode currents worse by giving the coax shield an asymmetric path. The buzz that changes when the antenna is connected suggests common-mode current on the feed line is part of the coupling path.
Improvements: fit a current choke (common-mode choke) at the antenna feed point and at the shack end of the coax; bond the station earth to the building protective earth at a single point; and improve the earth with additional rods or a radial system if a good RF ground is needed. Do not create multiple separate earths that are not bonded, as that creates dangerous potential differences.
Also check the balun: a 9:1 balun on a long wire is a compromise, and the feed line may be radiating or picking up. A proper unun with a choke, or a balanced antenna, may reduce the common-mode noise substantially.
3. The laptop supply is partly responsible. What filter would you use and how would you verify it?
Read the worked answer
Fit a mains filter designed for EMI suppression on the laptop supply, or better, replace the switched-mode supply with a linear one or run the laptop from its battery while operating. A clip-on ferrite on the DC lead, with several turns if the core allows, suppresses the common-mode current on the low-voltage cable; a ferrite on the mains lead suppresses the differential and common-mode noise there.
Choose a ferrite mix appropriate to the frequency: a high-permeability mix for the lower HF bands, a lower-permeability mix for VHF. More turns increase impedance but raise the self-capacitance, so a few turns is usually the practical optimum.
Verify by measuring the noise level on 80 m with the receiver, with the supply connected directly, with the ferrite fitted, and with the laptop on battery. Repeat on several frequencies to see the broadband effect. Record the before and after levels in dB. If the buzz persists, the remaining source is elsewhere and the same measurement procedure applies.