[Moon] [Moon-Net] Finding the focus point in a dish

Peter Blair g3ltf at btinternet.com
Wed Sep 16 23:18:08 CEST 2020


Just to support what Ingolf is saying about the need to achieve a good cross 
polar performance over the whole aperture,  I can say " Been there done 
that, got the T shirt"
A lot of years ago using a 23cm W2IMU dual mode feed with a post (or screw) 
type polariser I decided to try a different way of setting up the screws. 
Leaving the detail of the (crap!) method to one side, when I finished and 
put the feed back into the dish the sun noise was unchanged but my echoes 
were noticeably weaker. I went back to the original method and settings and 
the echoes returned to normal. Nowadays with sophisticated  modelling of the 
interaction of septum, waveguide feed and choke good X polar performance can 
be pretty much assured, just build to the modelled dimensions, much easier 
than the post polariser.

Just for interest, as someone pointed out the dish reflection is only a real 
issue when the reflector is, centre fed, small in terms of wavelengths  and 
the polarisation is linear, I matched my dual dipole feed in my 6m (8.6L) 
dish, again many years ago, and the work is described here. 
http://www.nitehawk.com/432_MHz_EME/432_eme_feed_v4.pdf

Hope to see you in the ARI at the weekend.
73 Peter G3LTF

-----Original Message----- 
From: Ingolf, SM6FHZ via Moon
Sent: Wednesday, September 16, 2020 5:56 PM
To: Charles
Cc: DW Harms (Dick ; Moon-Bounces
Subject: Re: [Moon] [Moon-Net] Finding the focus point in a dish

Highly Honoured Dick.
With the risk to stand out as the stubborn old man I am, I need to dispute
your statement of moving the choke ;-)
As Charlie points out, it is a big difference in adjusting the choke using
noise and using a co-polarised signal.
Solar noise, and Moon noise as well, contains ALL polarisations. So, when
using noise to adjust the choke you do just that, you adjust for
optimal reception of NOISE i.e. Total Power. The risk that you do just that
is very high.
But that is not what you want to optimise for. You want to optimize for
reception of a Co-polarised signal, in this case the returns from the Moon.
When adjusting the choke you will impair the Cross Polar Discrimination
(CPD) performance and by that get noise contribution from the Cross Polar
power as well. This is explained in the referenced pages in the
presentation linked to in my first mail. It was also discussed and
explained during my presentation at the Swedish EME-meeting in Örebro.
The graphs there clearly show that you will destroy a good feed design by
moving the choke from the prescribed position, at least for my feed
designs. The feed position is optimized for the overall best performance
taking several parameters into account. I can not comment on other feed
designs, as I do not know how they are optimized, if optimized at all for
CPD-performance.
The bottom line is that you most likely foul yourself by adjusting the
choke position by using noise.
I am sorry if I have been superfluous in my answer.
73 / Ingolf, SM6FHZ


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Den ons 16 sep. 2020 kl 17:43 skrev Charles via Moon <moon at moonbounce.info>:

> Hi Dick
>
> You would not want to replace the centre of the dish surface with
> teflon, as then there would be a transparent hole in the centre which
> would allow the feed to see hot ground behind the dish and thus increase
> your receiver noise. Putting teflon over the metal would do little as it
> is pretty much RF transparent.
>
> What Ingolf was saying, and my reference backs up, is that a properly
> functioning CP feed should not see the effect of metal in front of it,
> in terms of the VSWR you see at its ports.  The dish reflection should
> appear simply as a loss of isolation between CP ports and not as a
> reflection from the port whose VSWR you are looking at.  If you do see
> large changes then its an indication that the feed itself is not working
> properly, and would likely have less than ideal circularity (axial
> ratio). As Ingolf has also noted, adjusting the choke
> dimensions/position can have significant effects on VSWR, isolation and
> circularity.  Some of these changing may not be evident on sun noise for
> example, but could reduce the S/N of signals you receive, and level of
> your transmitted signal.
>
> The shorting plate test I describe in the reference is quite a simple
> way of determining whether your feed is working properly.
>
> 73
>
> Charlie
>
>
> On 16/09/2020 15:02, DW Harms (Dick wrote:
> > Hi Charlie (and Ingolf),
> >
> > First of all thanks to Ingolf for his explanation. It is of course
> > true that the feed should match the dish f/d etc. but I am sure that
> > by tweaking the choke, overspill can be reduced.
> > What I did was obviously rather designing/optimizing it in an
> > emperical/practical way. I am a mechanical engineer and by no way an
> > antenna designer ;)
> >
> > And what you wrote about the absorbing material is interesting. Of
> > course lossy material causes noise, so maybe replacing the metal
> > middle of the dish surface by teflon is a better solution.
> >
> > For Charlie:
> > I think you might have misunderstood me. I do not suggest to hold the
> > metal plate in front of the dish, but rather behind it, thus extending
> > the dish surface locally. In this way you can easily check for 
> > overspill.
> >
> > 73, PA2DW
> >
> >
> >
> >
> >
> > On Wed, Sep 16, 2020 at 2:43 PM +0200, "Charles via Moon"
> > <moon at moonbounce.info <mailto:moon at moonbounce.info>> wrote:
> >
> >     Hi Ingolf, DIck and all
> >
> >     As Ingolf notes, a properly functioning CP feed should show very
> little
> >     change in VSWR on its TX (or RX) port when a metal plate (or dish) 
> > is
> >     placed in front of it and moved.
> >
> >     This is borne out by some tests I did on Ingolf's 10GHz excellent
> 10GHz
> >     designs, where I had better than 20dB return loss with the feed 
> > mouth
> >     open and with a metal plate shorting across the feed mouth.
> >
> >     More info contained in this paper:
> >
> https://drive.google.com/file/d/0B116IwQIUFNTa0t5bl9PX2Y3djA/view?usp=sharing
> >
> >     73
> >
> >     Charlie
> >
> >     On 16/09/2020 13:43, Ingolf, SM6FHZ via Moon wrote:
> >     > Dear Dick and All.
> >     > I would like to elaborate some more on the reflection discussion,
> feed -
> >     > dish combination.
> >     > With a CP feed, IF you see reflections from the dish in the
> Tx-port when
> >     > transmitting in that port, the isolation has most likely been
> reduced due
> >     > to lack of polarisation purity. You are not truly circular, but
> more or
> >     > less elliptical in your polarisation. Probably a lot more than you
> >     > should be. I.E. Cross Polar Discimination / Axial Ratio is not
> good.
> >     > This may come from the dish surface (less likely), the feed itself
> (yes
> >     > most likely), the dish surface is not in the far field of the
> feed, you
> >     > have been fiddling with the choke position (also possible). See 
> > the
> >     > reference link in the first mail.
> >     > Check this before doing a lot of other complex adjustments that 
> > may
> >     > cause other problems.
> >     > 73 / Ingolf, SM6FHZ
> >     >
> >     >
> >     > > Virus-free. > www.avg.com > >
> >     <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> > > Den ons 16 sep. 2020
> >     kl 09:41 skrev Ingolf, SM6FHZ : > >> Dear Dick and All. >> I must
> >     object to two of your statements, Dick. >> 1) Do NOT put any
> >     absorbing material in the vertex of the dish (or in any >> place
> >     of the dish surface) as you will introduce noise right into your
> >     >> receiver. The absorbing material has the same physical
> >     temperature as the >> ambient and will have a noise temperature
> >     corresponding to that. When using >> circular polarisation the
> >     reflection from the dish (when Tx-ing) will NOT >> be seen in the
> >     Tx port if the isolation is as it should be. It will show up >> in
> >     the Rx-port, deteriorating the Tx to Rx isolation. If you see any
> >     >> significant change in Return Loss on a CP feed when putting it
> >     into the >> dish you probably have a isolation problem. With
> >     linear polarisation it is >> another story. >> 2) Moving the
> >     choke. I do not know about other designs, but I think they >>
> >     behave very much the same as my designs. On my feed designs the
> >     choke >> position is optimized for optimum total performance. Do
> >     NOT change this >> position. If you move the choke the radiation
> >     pattern changes and you will >> get effects you have no control
> >     over. If you have spillover from your feed >> in your antenna
> >     system you have chosen the wrong feed for the f/D of your >> dish.
> >     This choice would be the first thing you should do when planning
> >     for >> an EME antenna system if you want to be truly successful.
> >     It is well >> documented what would be needed for different f/D's.
> >     >> >> More on these subjects can be found in: >>
> >
> http://2ingandlin.se/A%20novel%205%20step%20septum%20feed%20suite_PF1.pdf
> >     >> in pages 14 to 18. >> >> 73 / Ingolf, SM6FHZ >> >> >> >>
> >     Virus-free. >> www.avg.com >> >>
> >     <#m_-9056245441693713447_DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> >>
> >     >> Den ons 16 sep. 2020 kl 08:50 skrev DW Harms (Dick) via
> >     Moon-net < >> moon-net at mailman.pe1itr.com>: >> >>> Hi Paul and the
> >     rest of us, >>> >>> I use a simple and practical way to tweak my
> >     dish which might be useful >>> to you, and others of course. >>>
> >     >>> For a prime focus dish, the best way to adjust the feed
> >     pointing to the >>> midpoint of the dish, is to mount a small
> >     laser pen in the center of the >>> feed. Obviously the feed
> >     alignment should make the laser beam point exactly >>> in the
> >     middle of the dishsurface. I am using a 3D printed mound, with the
> >     >>> laser pen in the middle, which I can shove onto the front of
> >     the feed. >>> >>> The next step is top make sure you illuminate
> >     the dish correctly, not >>> over- and not underilluminating it.
> >     The simple method I use, is to point >>> the dish to the sun and
> >     measure sunnoise. Next you take a metal plate of >>> 40x40 cm and
> >     move that slowly along the edge of the dish. If you observe >>>
> >     slight variations in the noise, your feed is looking over the
> >     edge, in >>> plain words you are overilluminating it! This is bad,
> >     as it means part of >>> your feed is looking at the warm earth. If
> >     you don’t see any changes, you >>> might underilluminate and then
> >     you have to start moving your choke ring >>> (never move the
> >     feed!). If you don’t have a choke ring, get one! >>> It is a bit
> >     of work, but next move the choke ring backwards, just until >>>
> >     you start observing effect of your 40x40 plate. Then move the
> >     choke ring >>> slightly forwards again and fix it! By the way, pay
> >     very good attention to >>> the contacts between choke ring and
> >     feed. Corrosion or dirt can deteriorate >>> the contact and this
> >     is of great influence to the performance! >>> Never move the feed
> >     itself away from the focus point though, it will >>> distort your
> >     beam pattern! The focal point can easily be calculated and >>>
> >     measured/adjusted. >>> >>> For an offset the alignment is even
> >     more simple. Point the dish to the >>> sun and wet the surface (a
> >     solid surface is the easiest of course...) and >>> observe the sun
> >     projected on the feed and you can easily optimize its >>>
> >     position. >>> >>> In a prime focus dish, reflection on the
> >     midpoint can disturb your VSWR >>> measurement. It may even damage
> >     your LNA, so always use an additional >>> sequenced relay at the
> >     LNA-entrance. I know, it deteriorates the >>> performance, but in
> >     the end you safe the LNA. >>> I have seen plates of absorbing
> >     material in the middle of the PF dish, to >>> overcome this
> >     reflection and I advise to mount it (same as the size of the >>>
> >     aperture) during measurements but maybe even forever. I don’t have
> >     it by >>> the way, but that’s because I am lazy 😊 >>> >>> Hope
> >     this is useful, wish you success Paul! >>> >>> 73, Dick PA2DW >>>
> >     >>> P.s. I will be at PI9CAM again coming Sunday in ARI-contest.
> >     Finally, we >>> can operate it again! Hope to see you and all 😉
> >     >>> >>> >>> >>> >>> -----Original Message----- >>> From: Moon-net
> >     [mailto:moon-net-bounces at mailman.pe1itr.com]On Behalf Of >>> Paul
> >     Andrews via Moon-net >>> Sent: 11 September 2020 01:27 >>> To:
> >     Moon-Net >>> Subject: [Moon-Net] Finding the focus point in a dish
> >     >>> >>> >>> Hello Moon Folk, >>> >>> >>> I am amazed at how the
> >     measured VSWR of a feed changes when you put it >>> into a dish.
> >     The feed in open space is trying to couple all the >>> transmitted
> >     energy into spare. When the feed is placed in a dish, there >>>
> >     are reflected signals from the test instrument involved that phase
> >     add and >>> subtract in mysterious ways.. >>> >>> >>> I'm now
> >     working with a new dish and a new feed. The measured vswr can >>>
> >     change significantly with small changes in feed position in the
> >     dish. >>> >>> >>> Life would be simple if the dish focus was found
> >     when vswr was minimum. >>> Or, should the feed be placed at the
> >     geometric focus point regardless of >>> vswr? >>> Is there a
> >     predictable vswr event as the feed moves inside the focus and >>>
> >     then outside the focus. >>> >>> >>> I will begin to experiment
> >     with focus vs vswr but I hope others have >>> already blazed this
> >     trail and can provide some hints to what I will find. >>> >>> >>>
> >     73 - Paul - W2HRO >>> >>> >>> >>> >>> >>> >>>
> >     _______________________________________________ >>> Moon-Net
> >     posting and subscription instructions are at >>>
> >     http://www.nlsa.com/nets/moon-net-help.html >>> >>> >> -- >>
> >     Ingolf, SM6FHZ >> http://www.2ingandlin.se/SM6FHZ.htm >> >
> >     _______________________________________________ Moon mailing list
> >     Moon at moonbounce.info /mailman/listinfo/moon
> >     Join eQSL.cc https://eqsl.cc/qslcard/Index.cfm
> >
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-- 
Ingolf, SM6FHZ
http://www.2ingandlin.se/SM6FHZ.htm

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