On Thu, 3 Mar 2005 16:12:03 -0500, "G. Doughty"
wrote:

I have an issue on 40 meters from my tentec scout 555. After the rig is
tuned (mfj 901b) and I tx cw, I get an AC hum in my headphones on every key
down. This doesn't happen when my audio comes from the rig. I am using an
alpha delta loaded antenna in the attic and it is almost certainly putting
the tuner to a test. Is there something I can do to alleviate this problem?

Since you are using an attic antenna, I guess that you must have quite
a large RF field strength inside the house. When there is a headphone
cable connected to the transceiver, quite large RF voltages can be
induced into the wire, feeding it into the headphone jack and possibly
causing some havoc inside the rig.

During a QSO, did you ask about any hum in your transmission when the
headphone was used or when the speaker was used respectively ?

What exactly happens is hard to predict on this description, but here
are some possibilities.

If your rig and antenna are badly grounded, the RF current induced to
the headphone wire will flow through the rig signal ground to the
power supply rectifiers and through the mains transformer stray
capacitances to the mains neutral and then to the ground. Since the
rectifiers are not conducting constantly, but will cut off 100 or 120
times each second, causing some hum. This has been a problem with some
mains operated receivers, requiring some capacitors across the
rectifier diodes or a proper grounding. However, I have never heard
about this with transmitters.

Depending on the wiring code used in your area and if separate
grounding electrodes are used for your antenna and electric system,
some neutral current may flow through your rig to the antenna ground
electrode and from there to the mains ground electrode. This might
cause some strange effects in some special situations especially with
resonant wire sections in the house wiring.

The first thing to try is to wind the headphone cable around a ferrite
rod very close to the headphone plug and check if this helped. Also
putting some ferrites on the power supply mains and/or DC may help,
since apparently there is a quite high RF field strength around, due
to the attic antenna, which can cause all sorts of problems that might
be hard to diagnose.

Paul OH3LWR