Thank you, Roy and others.

My background noise on 80 and 160 seems high, but it does not sound like
man-made noise (as best I can judge by ear). The antenna in question is a
half sloper. I will try some clamp-on cores the next time I crank it down.
(I just cranked it up yesterday, so my timing is bad.) The coax runs down
outside of the tower (with the standard UST standoff cable guides). Using
the half sloper, the tower itself (and the outside of the coax, and all the
other cables coming down from the tower) are part of the antenna, of course.
Perhaps the ferrite cores will reduce some of the noise pickup.)

I realize that not everyone likes half slopers. I do not have any other tall
supports other than my 55-foot tower. I have a terminated inverted V near
the top (and a Steppir on top). The half sloper is attached at about 40'
(top of second section of the crankup). I have a good number of radials.
The SWR wanders around across 80 meters, but is below 3:1 everywhere. It
seems to transmit well, but seems too noisy on reception.

The "terminated dipole" has a resistor to ground at each end. (It is from
TenTec. Moderately low SWR from 160 to 20 meters; not great; probably not
worth the cost.)

Bill
W2WO