[Moon-Net] Prime-focus vs. Offset-dish

Edward R Cole kl7uw at acsalaska.net
Tue Nov 22 22:00:48 CET 2016


Manfred,

With great respect of your accomplishments (and particularly helping 
me with mods of 9cm Toshiba amp):

I guess it goes without much understanding that choosing the proper 
feedhorn is important whether for center feed or offset 
feed.   Certainly hams probably have much more experience setting up 
a center feed, and probably more info on doing that (without a lot of 
theoretical understanding).

My first encounter with offset fed dishes was modifying a 85cm dish 
for use receiving 2.4 GHz telemetry signals from AO-40 when at 65,000 
km range.  I had experience making helical beam antennas so 
calculated that I needed 6 turn helix to match illumination angle for 
the dish.  I mounted the LNA at the feed and it worked superbly even 
when AO-40 was aimed at 49-degrees "squint angle".  A lot of credit 
is to the 0.6 dBNF 40-dBG LNA from db6nt (circa 1999).

I used the feed support arm supplied by the dish mfr.  I did have to 
make a holder for the dish that compensated for proper pointing 
(plane of the dish face is 26.5 degree offset from direction of 
radiation.  Not all offset dishes are designed with the same offset 
angle, so that is one caveat and a technical issue to be handled.  If 
you have to determine focal point location then it becomes more 
complex than setting up a center fed dish.

Next I constructed a 10-GHz dish using 46cm Direct Satellite dish 
which also had the original feed support arm.  Pretty much 
duplication of the larger dish.  I have the dual-band feed made by 
W1GHZ for this dish plus the conical corrugated horn .

For eme, I assembled a surplus 4.9m dish that has f/d=0.5 so my 
original square septum beam was too wide which caused excessive 
illumination spillover such that Ysun/cs was 8-dB.  Replacement using 
W2IMU horn doubled the Y-factor to 16-dB which is about 1.5 dB below 
that shown by VK3UM EME Calc.  I tried adjusting for focal distance + 
and - the geometric focal point with no improvement.

At that point I decided maybe good enough to try eme.  It is - 
receives eme very nice.  Could it be better?  Probably, but each 0.1 
dB is going to be much harder to realize.

Feed shadow of 10-GHz is probably not too bad.  23cm feed on 2.4m 
dish is more significant.  On even smaller dishes its worse.  Thus my 
suggestion to Serge on choosing a small dish for use on 23cm.  It 
might not be enough of a problem if he uses a 2.4m dish or 
larger.   The offset fed dish will be harder to find the appropriate 
feed.  Not sure a W2IMU is narrow enough.

I have two 10-GHz rated dishes: Andrew 2.4m center feed, and 1.8m 
offset feed.  If I ever decide to pursue 3cm-eme then I will have to chose.

73, Ed - KL7UW

At 10:57 AM 11/22/2016, dl7yc at snafu.de wrote:
>Just another comment regarding the discussion offset dish vs. prime focus
>dish:
>
>
>Both "sides" claim better results, but reality shows the truth, hi.
>
>If you calculate the "shadow" of the feed-system (in my perspective
>including the WG-switch, preamp, transverter AND TWT with power supply for
>10 GHz OR 24GHz) its 1-2% of the whole surface only !! (based on my 2.4 m
>precision prime focus dish)
>
>The results for such a setup showing moon- and sunnoise results very close
>as predicted in VK3UM`s programs. Compared with other well equiped stations
>nothing is worse, therefore the effiency should be as high as caculated.
>And...... the feed design and focal point adjustment / calculation is VERY
>easy.
>
>Looking to my friends having offset dishes the same size, its all a bit
>motre complicated. I know all the recommendations from Hannes well and they
>where all very helpful. Even the "trick" with the waterhose, I think Dick,
>PA2DW told this,  is very nice and helps a lot....
>
>But the offset feed design and adjustment to its phase center is much, much
>more complicated than it looks. I personally know some very wellknown
>Microwave EME friends, struggeling many times to "optimize" their dish
>performance.
>
>If one loose 5% effiency due to a "bad" phase center, not perfect adapted
>feed in respect to the dish`s f/D (i.e. wrong feed system selection) and
>therefore spillover results in AZ or EL orientation due to "breaking"
>the -10dB rule in one direction at least, this all together is MORE worse,
>than a 2% "feedbox"-shadow of a prime focus RX/TX setup with no extra line
>losses !! Its true ... , especially for the more rectangular beam of the
>Super-VE4MA-Feed (Kumar feed / see W1GHZ handbook).
>
>Therefore, it could be allready an open question!!  If one have a PERFECT
>offeset dish setup, this might be 2% better compared to the same prime focus
>dish setup ?? But if NOT, hmm....
>
>My reccommendation:
>
>Try to put the equipment you need for the given band straight inline with
>the prime focus feedhorn and beat your neighbour with more moonnoise
>performance, hi. Forget struggeling and time consuming processes to optimize
>overall performance of microwave EME dishes.
>But, .... you really will loose the possibility to SIT on the ground for
>adjustment of your offset feed, because NO adjustment is neccessary
>anymore!! And the focalpoint of an 2.4m prime focus dish is 1.2 m far from
>the surface only (plus some extra cm) Feed change is not a dangerous thing !
>
>I NEVER re-adjusted  the feedsystems in its focal point after a perfect
>calculation and mechanical measurement by a laser distance meter from a
>hardware shop.
>
>BUT at the end: its much more important to be QRV than to grab the last
>tenth of a dB !!!!!!!!!!!!!!!
>
>Vy 73 Manfred, DL7YC
>
>
>2.4m precision prime focus dish (Areal OY Finnland), f/D 0.4 used for 10 GHz
>and 24 GHz
>VE4MA "Super feeds" for both QRG`s
>0.6dB NF (10GHz) 1.3dB NF (24 GHz)
>DB6NT transverter with 10 MHz time base connection (GPS)
>modified RW1127 TWTA`s with RWN 320 PSU`s
>Output: > 90 watts (10GHz) 40 watts (24 GHz)
>Autotracking NITEC rotator
>

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag Business e-mail:
     dubususa at gmail.com 



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