High electrical noise only when coax shield is connected

Question

I'm trying to understand a noise issue on my Yaesu FT-891. I have very high 60 hz electric buzz on most bands with 40 and 15M being the worse at about S9+. My antenna is a MyAntennas 66ft EFHW fed with 25 feet of RG-8X coax. I've noticed that if I unscrew the coax shield to the antenna input of the 891 and just leave the center feed connected the buzz goes away completely. Yet, if I screw the shield back on the noise is there.

It's a bit perplexing - the assumption being that the coax shield is capturing the noise. But, why wouldn't the noise still be there if the shield isn't attached. It implies the endfed wire is not picking up the noise and the shield is. I would think the some of center wire of the coax and the wire antenna would still have the noise if the shield isn't connected. Note that chokes, ferrites, grounds make no difference.

What are thoughts on this? What am I missing here? Why would the buzz go away completely when the PL-259 shield is unscrewed ?

PS - I've shut down power from the fuse box in my shack room and around it as well and no difference (running the rig on battery).

Answer 1

Shutting down power for surrounding equipment is a good diagnostic for eliminating local RFI problems, but when you have a 60hz hum, the problem typically isn't RFI but intrusion from the AC power. Typically, 60hz hum is caused by a ground loop.

A ground loop occurs when there are multiple paths to ground that are different enough to have a voltage potential between them.

There are a number of ways to eliminate a ground loop, but the two most common are:

• break the path to one leg of the loop
• connect all the grounds together at a single point with the lowest possible resistance

When the hum goes away when you disconnect the shield, that pretty much confirms you have a ground loop. However, this probably doesn't make for a good antenna.

Most amateur radio equipment has a ground point on the chassis specifically to ground the equipment. Your solution here is to use that on all your equipment (power supply, tuner, radio, etc.), connect all those grounds together at a single point, and then connect from that point to a good ground. Typically the antenna shield is already connected to the radio chassis.

Typically large flat braid is used for grounding, but any thick wire is better than nothing.

At best, a ground loop will introduce a hum as you have experienced. At worst, a ground loop can introduce currents in unexpected places, which could lead to safety issues or damaged equipment. Improving your ground will also help harden your equipment against lightning strikes, although while it is necessary, it is not a complete solution for that.