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

TexasRF at aol.com TexasRF at aol.com
Tue Nov 22 23:16:41 CET 2016

One added comment re W2IMU feed beamwidth: The original article showed  
design data to accommodate various output section diameters to achieve the  
desired beamwidth. The transition section angle has to be selected to make it  
all fit together properly.
The commonly accepted design fits a .5 f/d and as has been mentioned, many  
o/s fed dishes are more like .6 to .7 f/d.
We should consider the W2IMU feed more as a concept than a rigid design  
when used with modern o/s reflectors.
Gerald K5GW
In a message dated 11/22/2016 3:01:00 P.M. Central Standard Time,  
kl7uw at acsalaska.net writes:


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  

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
>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 
>nothing is worse, therefore the  effiency should be as high as caculated.
>And...... the feed design and  focal point adjustment / calculation is  
>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 
>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, 
>more complicated than it  looks. I personally know some very wellknown
>Microwave EME friends,  struggeling many times to "optimize" their  dish
>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 
>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 
>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 
>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
"Kits made by  KL7UW"
Dubus Mag Business e-mail:
dubususa at gmail.com  

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