[NLRS] 24 Ghz Rain Scatter and other Notes

[Mike King - KM0T]

Hi all, wanted to post this earlier, but got tied up with weekend yard work
and watching the skys down here for storms and such.

Friday morning I was down in Dennison Iowa (EN22) for a meeting, which
happens to be in Genes - N0DQS's back yard.  So I brought along the 24 ghz
dish for some messing around in the afternoon.  Gene and I had been trying
some longer paths in the recent months, but have had little success, so we
wanted to "get close" and test the gear again to see if things were messed
up or not.

As some of you saw at Aurora, I have scopes mounted on the dishes as a
pointing aid.  Donn and Bob both suggested that Gene and I get close enough
to see each other thru the scopes in order to make sure there wasent any
"skew" on the dishes from what the scope was showning.

The current pointing method is to use a KVH "Datascope" which is an
electronic monocal scope with a built in digital compass.  The compass
within the Datascope is accrurate to within .5 degrees when calibrated
properly and readout resolution is .1 degree.  In addition, the Datascope
has E or W declination offset, settable in .1 degree increments.  Here is a
link to the item.

http://www.kvh.com/products/product.asp?id=10

So the idea is to get the path in degree headings, use the Datascope to pick
out an object on the horizon (and it sees pretty far for a monocal) with the
calculated heading, then use the scope on the dish to point at that
reference found with the Datascope.

So Gene found a few spots about 2.5 miles apart where we had a LOS path.  He
also used Topo-USA for calculating the beam paths, in lieu of using a grid
program that goes between the centers of subgrids.  He had a GPS marker on
his laptop for his location, and we had a clearly marked road with landmark
turn in the road for verification of my location.  He then used the path
portion of Topo-USA for the heading.

We could see each other thru the scopes on the dishes and attempted to
determine if the peak was right on visual heading.  However, on the lowest
power setting, still full scale S meter readings were present.  So one of us
at a time point the dishes 180 degrees away and we reduced mic gain to
minimum.  This way we were able to see a general peak.  My dish seemed to be
right on path with the scope, Genes was skewed a bit and he did some azmuith
adjustment on the scope setting to bring it back in.  We had anticipated
this on his 24 Ghz dish as one could actually see that the waveguide out in
front of the dish appeard abit off to one side.

As for the Datascope compass heading.  From Genes projection on Topo-USA, I
was within a degree or less, so that was good enough figuring all other
potential variables, thats all one could expect on accuracy.

I headed to the next place, about 15 miles away, a non LOS path and we
repeated testing.  Again with good landmark indicators so Gene could plot
the path on the topo program.  Again signals were full bars, but everything
seemed to peak at the right direction, and the Datascope heading was again
within a degree, but signals too strong to really get a real defined peak.

I then head to another spot, 45 miles away.  Again, defined locations all
showing on Topo-USA for exact placment of my location.  Gene gave me the
heading and and I used the Datascope to get the landmark.  Sure enough I had
choosen a bad location for off in the distance was a knoll with a farm house
and trees.  However the heading was right at a telephone pole on the farm
property and I used that landmark to line up the dish scope.

Gene was right on heading albeit weak, just cw copieable as Im sure the
obstructions were messing us up.  He too said on his side the peak was right
where his landmark with the dish scope pointed there.  At that point we felt
that we were calibrated as close as we were going to get.

We then started to mess around with scatter paths as there were lots of
clouds and potential rain between and all around.  The day had started off
really hot and humid, now it was humid, but slight sprinkles on and off at
both location.

Gene started by elevating his dish while I was transmitting.  He got a
definite peak at 8 degrees above horizon.  I then in turn elevated my dish,
and I also peaked his signal at 8 degrees.  Signals at that point were S7 to
S9 and Auroral in nature.  This was on direct heading but elevated.  We
switched to FM and had good communications for about 10 minutes.  This rain
squall must have been pretty much centered between us, as neither of us
could see it.

We then noticed the signals dropping as whatever was scattering us was going
away, we repeaked and the path started to move a bit east.  Elevations were
changing too.  Over the next 10 minutes or so, the rain squall was
continuing to move.  At the best time, I was peaked at 17 degrees above
horizon and Gene was at 11 degrees.  I could see the squall as it was closer
to me.  The signals there were full bars on the FT-817 and about 30 degrees
off of direct heading.  Dropping the dish to horizon resulted in a total
loss of signal so it was definitly rain scatter.  FM was great, SSB on this
skew path was completely unreadable.  CW was just like 2M Aurora!

This was not a serious storm, it was just rain, and was an excellent
scattering medium.  This was also a good test as our first rain scatter for
70 some miles was a very skewed path and was a serious storm.  We had no
idea that just general rain would be good enough to do it on 24 ghz.
Incidently, when we did the 70 mile path, we were at it for over a half an
hour with nothing heard.  When we finally decided to elevate the dishes, we
found each other in about 5 minutes.  We did not have the digital protrators
for measuring the elevation at the time, but I would estimate that is was
about 5 to 10 degrees.

So, I believe the conclusions would be that general rain is good, probably
for much longer paths.  SSB will work on direct path rain scatter, but not
skewed paths to a point.  And dont be afraid to really elevate the dish!
Low power was used most of the time, switching to high power did little to
improve the signals when we were peaked on one another.  This would be from
2.5 watts on the high side to a 1/2 watt or so on the low side.  For a short
time, I was also listening to Gene when I was getting totally rained on, no
real apparent loss of strenght.

Incidently, the method as some of you may have seen at Aurora, to measure
the elevation in degrees is a digital protractor, Lucas Anglestar - DP45.
Picked up on ebay fairly cheap, ~$50 to $60.

It will take some more practice, but it seems that longer paths are quite
possible.

Just FYI.

73

Mike - KM0T EN13vc