"Dave Platt" wrote in message
...


snip

I haven't read any reports of actual experiments which attempted to
quantify the tuning stability of the classic J-pole ...

snip

That, sir is about to change:

I wondered about the effect of mast material, so I took a freshly built
j-pole and mounted it in the clear. I noted the XMIT reflected power (low)
and moved a ten-foot piece of galvanized steel mast around the antenna, I
found I had to get within about an inch to see the reflected power increase.

Next, I found a repeater with nearly continuous chatter and connected the
j-pole through an attenuator to my radio. I set the attenuator so the
received audio was very noisy -- another dB or two and the conversation was
unreadable. That same piece of mast material had essentially the same
effect: had to get really close to louse up the antenna's performance.

That said, if I take a j-pole and walk around with it, periodically
measuring the reflected power, I will see physical locations where it bumps
up. That's probably not detuning; it's probably reflected power off a wall
or fence, which could be what's happening in the OP's attic. (Navy ship
radars' reflected power monitors will "bump up" every time the antenna
rotates past the mast or some other bit of ship's structure.)

Is this great science? No. Is it worth noting? I hope so.