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

Charles g3wdg at moon-net.eu
Wed Sep 16 14:26:34 CEST 2020


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
>
>
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> Den ons 16 sep. 2020 kl 09:41 skrev Ingolf, SM6FHZ <ingolf.fhz at gmail.com>:
>
>> 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
>>
>>
>>
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>> 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
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
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>>>
>>>
>> --
>> Ingolf, SM6FHZ
>> http://www.2ingandlin.se/SM6FHZ.htm
>>
>


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