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

Edward R Cole kl7uw at acsalaska.net
Wed Nov 23 05:59:40 CET 2016


I suspected that but kept my reply conservative without explicit 
info.  I probably will examine the design if ever needing a offset 
feed that is CP (e.g. 23cm feed for my 1.8m dish).

First use of that dish probably will be linear-pol on 3cm and be 
Rx-only.  The challenge is a 35w+ sspa on a retirees budget.

I do have a DEMI 10368/144 transverter which will get a A32 synth LO 
next year and maybe I will finish the 2.5w DEMI PA.  This is for 
terrestrial use, of course.  I have a 18-inch (46cm) offset dish for 
that plus a 22-dBi rectangular horn.

73, Ed - KL7UW

At 01:16 PM 11/22/2016, TexasRF at aol.com wrote:
>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 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
>      "Kits made by KL7UW"
>Dubus Mag Business e-mail:
>      dubususa at gmail.com
>Moon-Net posting and subscription instructions are at 

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