[BC] Microwave path de-coupling...

[Burt I. Weiner]

In the microwave world this phenomena is known as de-coupling; where 
the beam is steered away from the receive antenna by 
super-defraction.  This is quite common along desert microwave 
paths.  Imagine a streamer such as a kite tail tied to the center of 
a fan.  Essentially the same thing.  A more visible and common 
example is the twinkling of stars or distant city lights.  The common 
method for minimizing the effects is to use diversity antennas and 
sometimes receivers.  This is one of the reasons I do not like to use 
large dishes on the transmit end of long paths.  On long paths I use 
the smallest dish I can get away with on the transmit end and as 
large a dish as I can on the receive end.  With the narrow beamwidth 
of a large dish the signal is more likely to suffer from 
de-coupling.  A fat lobe can certainly help to minimize de-coupling.

One other thing that I've seen cause problems is a path that is 
inadvertently aligned on minor side lobes.  When aligning dishes, 
always take wide sweeps to make sure you are not on minor or side 
lobes.  Misalignment this way will certainly kill your fade margin.

When I had my two-meter repeater on Mt. Wilson I was using a RCA 
TVM-1 TV microwave link that I moved down into the 5 GHz Ham 
Band.  The path was 38 miles across the city.  I kept having long, 
deep fades due to de-coupling.  The receive dish was a four foot and 
the transmit end was a six foot dish.  I reversed the two dishes and 
that greatly minimized the deep fades.

Burt

At 07:22 PM 7/1/2006, you wrote:
>From: Robert Reymont <robert at reymont.com>
>Subject: Re: RE: [BC] WLS dead cxr prob. this a.m. at ~6:50
>To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
>Message-ID: <6.2.3.4.2.20060701102937.034ac7f0 at pop.registeredsite.com>
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>
>I have had a number in instances where the best solution for a
>inversion layer is multiple receive antennas at different heights on
>the tower.  One instance in Sacramento where an antenna mounted on
>the roof and one mounted 150' on the tower.  You could watch the
>receive signals on the two receivers... one went up while the other
>went down.  The receivers would switch when the signal was below the
>squelch levels.  Kept the station on the air about 99% of the time.
>
>   I also found the same problem in Orlando with a twenty mile
>hope.  Antennas mounted at 150' and 450' solved the problem.
>
>
>At 09:16 AM 7/1/2006 -0700, you wrote:
> >In this case, where there was an inversion layer causing the STL "beam" to
> >miss the receive antenna, I wonder if another antenna at the receive site
> >would have suffered the same signal loss at the same time. MANY years ago,
> >the TV station in San Luis Obispo got almost all of its programming from
> >its sister station in Salinas with a bunch of long microwave hops. They
> >had two receivers and two receive dishes. A comparator watched the AGC
> >voltage and switched to the receiver that had a better signal. This space
> >diversity system worked pretty well for them. There WERE times when both
> >receivers faded, but more often it was just one of them.
> >
> >Harold
> >
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>Robert Reymont
>Double R Consulting, Inc.
>P.O. Box 42277
>Mesa, AZ 85274-2277
>Phone (480) 820-2439, FAX (480) 820-2514

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at earthlink.net
K6OQK