I've built a VHF/UHF Log Periodic for monitoring. It works quite well,
but I've been curious about one thing... Using an analyzer, I get a
good SWR on the majority of the antenna (below 1.5), but there's some
areas that are fairly high. Well above 3.0. Obviously this isn't
crucial for just monitoring, but I use that as a guide as far as
seeing how well the antenna is designed. The thing that I don't
understand is that the SWR readings will change if the cable is moved.
The cable comes off the front end of the boom, travels back to the
middle, then comes off across a horizontal mast (the antenna is
vertical) and down the vertical mast. All that part of the cable
doesn't move... But if I pick up the cable on the ground and change
the location a bit, the SWR will change quite a bit... Areas that
before had a poor SWR now have a good one, and areas that were good
change to poor. Why would this happen? Also, would using a longer
cable possibly make the SWR better? Typically I have tested with a 20'
RG8X cable. I also usually test with the antenna about 8' off the
ground. Would it improve with better elevation off the ground as well?

Thanks for any ideas.

Dave