[NLRS] 10 ghz scatter!!???

Fri, 02 Aug 2002 12:17:58 -0500

Good Afternoon All,

I only quickly read the correspondence but based on microwave experience
with dishes on terrestrial microwave systems I don't think there is any
magic going on here other than you obviously had  good conditions between
you.  These good conditions gave you big signalsas if you were in a local
line of sight QSO.

Once you have lots of  signal level you don't need the gain of the dish at
one end to have a QSO.  Clearly when you go off beam with a dish the "gain"
drops dramatically after a few degrees at 10 GHz  but it does not drop
infinitely.  Consider that a 1 ft dish has "only" 30 dB of gain on
axis....even the basic feed antenna has 6 or 7 dB gain by itself.

Where am I going with this.... when Mike pointed his dish straight up, the
feedhorn itself being a low gain antenna probably still had  reasonable gain
compared to a dipole (say 10 dB loss), but that is only 40 dB down from when
the dish is pointed at the other station.  If you have 59+20 signals to play
with, assuming 4dB per S unit?, thats 56 dB of signal above the noise, and
16 dB S/N left when pointed straight up !

There will be a pronounced gain peak on the back of the dish...I don't
recall what front to back to expect.

Commercially the fact that dish feedhorns can still see signal sneaking in
from the side, is a problem that is dealt with by putting a  shroud (short
cylinder) lined with RF absorber to shield the side view. These antennas
usually have a white flat radome over the front so keep the snow and birds
out and improve the wind load.

Have fun !

Best 73

Barry VE4MA
----- Original Message -----
From: "Jon Platt" <[email protected]>
To: "Mike King - KM0T" <[email protected]>; <[email protected]>
Sent: Saturday, August 03, 2002 9:52 AM
Subject: Re: [NLRS] 10 ghz scatter!!???

> Pretty interesting stuff, especially given that Gene was 73 miles distant.
> If you look at the current upper atmosphere plots from the NWS (called
> Skew-T plots) from http://asp1.sbs.ohio-state.edu/skewt.html you can see
> that there is a LOT of upper atmosphere "strucuture" with respect to
> boundary layers comprised of sudden changes in temperature and sudden
> changes in humidity with elevation (on the skew-t plots, the right hand
line
> is the temperature and the left hand line is the dew point).   Skew-t
plots
> are taken twice a day by the NWS stations and plot temperatue & humidity
vs
> altitude.  Does this "structure" provide for boundary layers for
scattering
> on 5.7 and 10 GHz ?   Got me, .... perhaps someone else would know this.
> The evidence would tend to say so.   Try the same experiment in January
when
> the skew-t plots show very little upper atmospheric boundary structure and
> see if you get the same results .... I would bet not.
>
> The good news Mike is that you don't need that 0.2 degree accuracy on the
> rotators any more.
>
> 73, Jon
> W0ZQ
>
>
> ----- Original Message -----
> From: "Mike King - KM0T" <[email protected]>
> To: <[email protected]>
> Sent: Saturday, August 03, 2002 12:33 AM
> Subject: [NLRS] 10 ghz scatter!!???
>
>
> > Hi all,
> >
> > Gene, N0DQS and I were testing all the bands tonight in preperation for
> the
> > UHF contest.  We spent a considerble amount of time ragchewing on 10 and
> 5.7
> > ghz just to get a feel again for how it all was operating.
> >
> > Gene was in EN22GE, about 73 miles away.  Signals were 5x9+10 to 30 over
> on
> > 10 ghz and slightly lower on 5.7.  This was holding up very well over
the
> > course of an hour or so as we ran the other bands, so we kept going back
> to
> > 10 and 5.7 to see how conditions were changing.
> >
> > Anyway, I started to screw around with my elevation control and while we
> > were on heading with each other, I started to point the dish at the
> ground.
> > We still copied each other well when my dish was pointed 45 degrees
below
> > horizon.  So after that, I decided to point the dish up.  I have the
rotor
> > setup so I can point straight up in the air so I can park the dishes
> durning
> > windy conditions.
> >
> > Well, we copied each other all the way up, even when my dish was pointed
> > straight up.  Scratching our heads led me to say, "well, if the dish is
> > pointed straight up, then I should be able to rotate any direction in
> > Azmuith"
> >
> > I proceeded to rotate around the clock and copy was still managable all
> the
> > way, thus proving that the dish was straight up.  Not believing this
> myself,
> > I went outside to check the tower.  Yes indeed, the dish was pointed
> > straight up.
> >
> > We tried 5.7 ghz and experienced the same thing!  Even with 250
milliwatts
> > on that band.
> >
> > The next step was to rotate the antennas back down to the horizon and
> start
> > swinging my dishes around.  Well that led us to hearing each other
almost
> > all the way around the clock.  The strongest was when I was pointed
> directly
> > away from him (other than direct dish heading)  We also heard each other
> on
> > 5.7 ghz with the 180 degree dish heading away from him.
> >
> > The best I can figure is that there must have been an inversion up high
> and
> > that was scattering the signal.  It was 66 degrees here tonight when it
> > happened (about 0400utc) from an 85 degree high.  Humidity was not that
> high
> > here today, pretty comfortable day, one of the nicest of the summer.
> >
> > Anyway, when I got pointed back on him directly and on horizon, the sigs
> > jumped up to 30 over, so I know I was on the right IF and 10 ghz band!
So
> > Gene then did the same thing on his side and sure enought, about the
same
> > was repeatable on his end, the band appeared to be fading abit by that
> time.
> > Direct heading sigs where then 5x9 on both bands.  Which may account for
> not
> > as good sigs when he was pointed straight up.  (he has az and el control
> on
> > his dishes as well.)
> >
> > One note, while I was pointed straight up with Gene coping me no
problem,
> he
> > rotated off the beam heading and lost me right away, so it looks that at
> > least one of us has to point at the other.
> >
> > So, from now on, I will just point my dish straight up and not worry
about
> > the dish heading.  I will let the rest of you worry about it :)
> >
> > Seriously, we will try it again tommorow, my guess is that it will not
be
> > repeatable when it warms up.
> >
> > So, any thoughts?  We sure had a good time giving the equipment a
workout.
> > As for drift, pretty much non-exisitant.  I think with the cooler wx
> > ambient, that may be a factor.  The drifting we experienced weeks ago
when
> > we all worked, it was hotter than you know what!
> >
> > 73
> > Mike - KM0T en13vc
> >
> >
> >
> > _______________________________________________
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> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/nlrs
>
>
>
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