On 5/24/2011 4:43 PM, Dave Platt wrote:
In ,
John wrote:

All of this leads me to think that something else is wrong. I can hit a
repeater 20 miles from me with 5 Watts and its antenna is on a 300 ft
tower. My antenna (half wave end-fed dipole) is on a 20 ft mast.

I wonder whether he might have his antenna sitting in a particularly
deep multipath cancellation null.

I help run a repeater located about 4 miles from my house, which is up
at about 80' AGL on a hospital roof. There's an office building
blocking direct line-of-sight path from my house to the repeater.

For a few weeks, several years ago, I found myself unable to
successfully open up the repeater using my base station, even while
transmitting 50 watts of power from a good-quality commercial 2-meter
antenna located a few feet above my roofline. Had no trouble with any
other repeater in the area, or with simplex operations. The
feedline/antenna system measured out just fine... no reflected power
worth speaking about.

I moved the antenna mast about 2' to one side, and was then able to
open up the repeater full-quieting with 100 milliwatts.

All I can figure is that there must have been a *very* deep multipath
cancellation effect playing out... maybe interference between a
edge-diffraction path involving the office building, and reflections
from nearby trees. Moving the antenna a fraction of a wavelength
changed the path lengths enough to eliminate the cancellation (or
reduce it by a large factor).


That is always a possibility, Dave. I have seen similar situations and I
am not surprised at your findings. It is one reason I suggested moving
his antenna, even slightly.

His latest post may also indicate that there is a repeater problem. So,
I hope he posts his findings after his meeting with his club.

Cheers,
John