Marshall Amplification Field 4000, Owner'S Manual

Page: 34 / 40

Page 34
Field Marshall Tracking Receiver
Each time you take a bearing always scan a full 360
°
radius first to
find the peak response. Remember that every yagi has a certain
response 180
°
from the peak and if you’re not careful you could go
in exactly the wrong direction. To get the most precise bearing,
turn the volume down so you get no signal except on the peak.
If the signal is weak, rotate the yagi around it’s boom to find the
best polarization, vertical or horizontal. If both polarizations are
about equal, the signal could be coming from a reflection off a hill
or mountain or could be diffracting over a hill. If you suspect you
have a reflection, try to figure out where the transmitter would be if
it is in the direction of the bounce (remember that radio waves
reflect off a surface exactly the way light waves do in a mirror; the
incoming angle equals the outgoing angle.) Check for a weaker
signal coming from the true direction to the transmitter masked by
some obstacle.
As you move the yagi across the horizon, is the peak response
sharp or broad? A broad response may indicate a reflection or
diffraction over an obstacle. As you drive away does the signal
drop off rapidly? This may indicate the transmitter is low or is
behind an obstruction rather than high.
Note whether there are any buildings, wires or fences in the
vicinity, or other obvious objects the signal could be reflecting
from. The signal can travel many miles along wires. Also
remember the signal intensity on your Meter for comparison with
the next reading you take.
Triangulation
After you take your first bearing, the natural tendency is to go
straight in that direction toward the transmitter. And if you have a
pretty good idea where the transmitter is that’s not a bad idea.
However, usually you will end up taking a lot of extra
measurements that way because you lack information about the
transmitter’s distance. If the signal is weak it doesn’t always mean
the transmitter is far away.
Another approach is triangulation. Instead of moving directly to-
ward where you think the transmitter is, you move closer and side-
ways. By taking only two bearings you can theoretically pinpoint