<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
</head>
<body text="#000000" bgcolor="#FFFFFF">
So you're actually the first site with all 3 sectors installed, and
this has exposed a problem I hoped would be better mitigated with
the open air environment your site provides. I knew there was
antenna crosstalk, I measured it @ 60dB of isolation in my garage.
I was hoping it was mostly reflection-based, and it was. But the
crosstalk up on the tower is as bad as 72dB isolation, which is
still more than I'd like to see. If we can chop another 40-50dB
down from that, I'd be happy. The fact that there are other
antennas in front of the sectors isn't helping either, as the signal
scatters and bounces back to neighboring sectors.<br>
<br>
I'm gonna fab some retrofit isolation shields to put on the
sectors. Aluminum alloy 1100-H14 seems to be good for microwave
work. First test, we'll try to eliminate the backwards radiation
from the dish making it into the north sector. It's hard to verify
this kinda stuff on the ground cuz the isolation levels required are
so high, and ground reflections are a plenty. It's really gotta be
done without echoes, so the tower provides a good environment for
that.<br>
<br>
<meta http-equiv="content-type" content="text/html;
charset=ISO-8859-1">
<img src="cid:part1.07000308.05070500@bartk.us"><br>
<br>
Here's what sector 1 (north facing) is seeing. The part shaded
"WiFi" is the dish pointing back @ Capitol Hill. That -55dBm signal
should not be entering the north sector. It's also seeing a lot
more signal bleeding from sector 2 (120 deg facing) which peaks
around -40dBm. And finally you can see sector 3 (bearing 240 deg)
leaking in at about -55dBm. The "WiFi" marked leakage is only 5MHz
bandwidth, whereas the other 2 channels are 20MHz wide.<br>
<br>
Hopefully with the shield in place we'll bring that leak down. If
not, we might also have to augment the dish itself, as it's probably
scattering the signal around the main reflector edges. Fabricating
a shield for that is a little more tricky, so I'd like to augment
the sectors first.<br>
<br>
BTW, a pretty nice little spectrum analyzer built into the modems,
huh? :)<br>
<br>
--Bart<br>
<br>
</body>
</html>