[NLRS] Parabolic Question

[Baker, Donn B]

'Morning Scott.

Some comments (hopefully usefull) below....

>Subject: [NLRS] Parabolic Question
>
>I was wondering how the people that are using DSS dishes aim them,
>particularly the elevation
>setting.  Do you somehow use the marks on the original 
>mounting plate?  It
>looks like it would be
>impossible to tell when you have it at zero degrees just by 
>looking at the
>dish.  At least on a parabolic fed
>from the center, you can tell when the dish is level 
>(horizontally).  

*****
The DSS dishes have a built-in offset in elevation.  While it varies from
type to type, it can be calculated.  Or observed.  The "trick" is to look at
the antenna from the front, and have someone tilt it forward.  When the oval
dish looks like a circle, its "on target."  This is only approximate, of
course, but will get you in the ball-park.

Some dishes do have a "reference plane," if you can recognize it.  With that
plane vertical (or horizontal, depending on the dish) the scale for
elevation is accurate, and can be used to set boresite to the horizon.  The
Sony DSS dishes that a lot of us are using require the amplifier mounting
arm to be ~5* below horizontal to bring boresite to the horizon.  The RCA
dishes need the flat surface of the mounting arm to be ~22* below
horizontal.


>I have
>an extra DSS dish that I could use
>for amateur work.  What bands will these dishes work well on?

*****
A dish is not frequency-sensitive... but the size and feed method makes them
more practical for some bands than for others.  The 18" DSS dishes have been
(and are) used primarily for 5.7, 10, and 24GHz. They work best at 10GHz
(big surprise... as they were designed to work at 12 Ghz!)  At 5.7GHz, they
should have about 25dB gain; at 10GHz, 31-32 dB; and at 24 GHz, 37-38dB.
Note that the higher the gain, the narrower the beam-width.  At 10GHz,
they're about 4.5*.  To get a feel for that, clench your fist, then open the
first three fingers (the Boy Scout salute).  Extend your arm full length:
the part of the horizon covered by your three fingers is close to 5*... the
antenna beamwidth is a little smaller than that.  On 5.7GHz, the beamwidth
is ~8.5*; and on 24GHz, its about 2*.  The narrower the beamwidth, the
harder it is to aim the antenna !!  And remember, you have to aim in BOTH
azimuth AND elevation !!!   (Actually, all it takes is some practice.)
Also, the higher the frequency, the more "perfect" the surface and figure of
the dish must be.  24GHz is pushing the accuracy of the DSS dishes, and you
won't get the gain you "should" if its been dropped, hit, beat up in the
car, etc.

>
>Next question:  I have one of those little parabolics from SHF 
>Microwave.
>It is 12 inches in diameter,
>with an f/d of 0.6, or 7.2".  The sheet that comes with the dish claims
>gains of 15 dB at 2304, 17 dB
>at 3.4 GHz, 23 dB at 5.7 GHz, 27 dB at 10.4 GHz and 33 dB at 
>24 GHz.  Do
>these figures seem
>believable for such a small dish?  I only planned to use it at 
>5.7 and 10
>GHz.

*****
3.7GHz ~ 17dB (40% eff.)  
5.7GHz ~ 21dB (40% eff.)
10GHz ~ 26dB (40% eff.)
24GHz ~ 34dB (40% eff.)
Figures from the Scientific Atlanta Antenna Calculator.  40% numbers used
becasue they match the ones from SHF.  50% is the "usual" assumption.
(They're very close... maybe 1 or 2 dB higher for 50%)

>  How do these figures compare
>to the DSS dish?  The small SHF Micro dish has the advantage 
>of easier feed
>as I plan to use PVC to
>support a (separate) dipole for each band.  All I would need 
>to do is swap
>feeds to change bands.

***** This will "work," but you will NOT get the claimed gain.  Especially
on 5.7, the feed horn will block a large percentage of the dish; this will
also happen on 10GHz.  If you actually mean to feed the dish with a
"dipole," it will be even worse.  The dish has to large an f/D ratio to be
well fed with a dipole.  Dipole feeds require deep dishes to work well...
less than, say 0.3.  With a splash-plate feed, you need to have waveguide
from the back of the dish: 3/4" copper water pipe works well for circular
waveguide at 10GHz, but 5.7GHz is another story.  Nothing's readly
available.  Using a horn on either band will produce SIGNIFICANT blockage on
a 12" dish. Plus, you have to feed the horn with coax, etc.

Even though you already have the 12" dish, I would suggest you look for one
of the 18" DSS dishes.  The offset feed arrangement is NOT a problem, and
lends itself to "swaping" feeds as well as what you are proposing.  There
are good designs for dual-band feeds (for 5.7/10 & 10/24GHz) that even
eliminate the need to swap.  ALL of the hardware is out of the beam (NO feed
blockage.)  Used DSS dishes are readly available... free to maybe $10.  Try
to get the LNB with the dish... it tells you EXACTLY where the feed point of
the dish is, and saves a lot of screwing around.  (Its not necesary to have
the LNB, its just easier.)  The LNB does NOT have to work... a
lightning-strike dish is just as good a new one.

A really good source of information in general, and especially for parabolic
dishes is W1GHZ's "Online Microwave Antenna Book."
<http://www.qsl.net/n1bwt/contents.htm>  I STRONGLY urge you get spend some
time looking this over.  Chapters 4 & 5 cover much of what you need to know.
Chapter 6 is parabolic antennas in general... good background.

Good luck... hope to work you this spring on 10GHz!!

73 Donn
WA2VOI/0

>
>Any comments will be greatly appreciated!
>
>Scott
>http://corbenflyer.tripod.com/
>