It's a bird! It's a crane!
This tale recounts how a cigarette break resolved a perplexing microwave failure.
Earl Schlenk, Engineer -- EDN, December 15, 2011
At our corporate offices, we started getting alarms on
our microwave terminal, which supplied the communications
link for the telephones and data. The
chief executive officer was upset over losing his calls
at various times, so I was assigned to find the cause
of this problem and fix it PDQ! The alarms would
start at approximately 9 a.m. and would occur intermittently all
day long for a few seconds and then stop at about 4 p.m. I went
to the corporate offices, checked the radio equipment, and found
no problems. I even looked for a fault in the alarms circuitry. I
found the radio equipment to be functioning properly. All levels,
power, and voltages were within specifications. I arrived the next
day at 7 a.m. and started to monitor this equipment, hoping to
find the problem. Sure enough, at 8:53, the alarm lights on the
equipment lit up like a Christmas tree. Before I could even begin
to check anything, the alarms went out.
I remained at my post, and, in about
20 minutes, the alarms went off and
again lasted for only a few seconds. It
now became obvious that we were experiencing
a path loss. What was blocking
the signal path in both directions for
just a few seconds? Could it be a flock
of birds? Airplanes flying by?I tried to detect a pattern of this
loss by timing the periods between the
losses. Whenever I thought I had found
a cadence to the failures, however, they
would change. The only pattern I did
find was that the failures ceased between
12 noon and 12:45 p.m. That discovery
was telling me something, but what?
Was it telling me that it was lunchtime?
If so, I wondered how lunchtime figures into a path loss. I ran through all the
options I could think of that could be
causing this trouble: a helicopter flight
above the path, laser sighting between
the dishes, vehicles driving around the
path area, and so on.
Pondering these things, I opened a
window to have a smoke. While looking
at the view from this 10th-floor perch, I
noticed a glimmer reflecting from something
just as the alarms went off. Was
there a connection between the glimmer
and the alarms?I remained at the window, checking
the horizon and again saw the glimmer,
and the alarms again went off. I tried to
identify a landmark where I saw the glimmer
and proceeded down to my vehicle
and drove to the area of the landmark.
As I approached the landmark, I saw
a construction site with a huge crane,
lifting steel up to a multistory building
under construction. I remained at this
location, and, as the crane lifted a steel
girder, I called the office and had them
monitor the alarms while I watched the
crane. Sure enough, the office verified
a failure. I stayed where I was, and the
failures again coincided with the crane
lifts. I realized that, when construction
was complete, we would have no path
at all. This new building—ironically, a
communications company—would permanently
block our path.
Our only choice was to find another path through the city to the microwave hub—an impossible task. So we came up with a temporary solution: using a passive dish system on top of another building, placing the dishes back to back, turning one dish toward the microwave hub, and turning the other one toward our building. We mounted two new dishes on the roof of an adjacent building, aiming one at the distant hub’s location and tying it, through a waveguide, to a dish pointing at our building’s dish. Getting enough signal to pass between these two dishes without any amplification was a major challenge. The alignment of the dishes on the buildings proved difficult at these low signal levels.
Earl Schlenk is a retired engineer for Burlington Northern Railroad. He resides in St Louis, MO.
Talkback
-
A telecom service guy once told me that they had strange breakdowns of the telephone network of one of their customers. These breakdowns usually occured shortly after 7 a.m.
He told me that back then there was a large building site near the building, a new subway line and station was under construction. They finally found that there were enormous glitches on the AC line - resulting from starting lots of powerful electric machines on the building site which all were started shortly after 7 a.m.
Winfried B - 2012-1-1 23:40:31 PST -
Earl... your solution was what some in microwave would call a "Beam-Bender"! For recieve only applications if there is too much loss, an amplifier and filter section would be placed between the two passive dishes. That had been done a lot with Wireless Cable TV installations, especially in urban areas. I had a microwave path signal loss one winter where we beamed several channels from one Headend to the next... overnight, there was a complete loss of signal for our premium channels that were recieved off satellite at the first headend. All of the equipment checked out fine at both ends, but no signal was getting to the second headend at all. A path drive-out the next day revealed that there had been a new municipal water tower built UNDER the path, about midway between the transmit and recieve sites... but the tower itself wasn't the issue... as I said, the tower was under the path, but just barely... the REAL culprit, again, this was during winter... was a pressure relief valve at the very top of the tower was leaking water which was freezing over and ice was building up right into the microwave path just above the tower! The municipality was notified and somehow they were able to get the leak repaired and the ice removed. Our path was restored within a few days.
Larry Mackowiak - 2011-15-12 12:15:14 PST -
My immediate reaction was, why not use a passive reflector, a flat sheet of metal?
I have heard of a flat sheet used on a hill-top to receive in a valley below.
However, on further reflection (not a pun!) I wonder how the efficiency of two passive dishes, one receiving and one emitting, would compare to a flat sheet, of the same size, or possibly much larger?
Rod Dalitz - 2011-15-12 10:35:09 PST -
He should still stop smoking!
j. robert rosenthal, M.D. - 2011-15-12 09:09:43 PST
