[Moon] Focal point question.

Peter Blair g3ltf at btinternet.com
Sat May 13 18:37:30 CEST 2017 

I think you are right Charlie, we do optimise for best G/T, not gain. Its 
another reason why we are nearly always below the VK3UM numbers. ( and there 
is no edge diffraction in the spillover numbers as they are derived from the 
W1GHZ EME2014 analysis pp 69-76  which does not include that effect.)
Steve, You said in answer to Charlie's Q on measurement  " Sun noise and a 
General Radio 1236 noise meter. Obviously moving the feed in and out to find 
the peak. After each change make sure Make sure the dish is still smack on 
the sun."
Did you re-set the cold sky level each time? If you dont do that the slight 
change in phase and amplitude of reflection from the dish can introduce a 
LNA noise change. Its small if the feed isolation is high but its best to be 
on the safe side with a cold sky check each time you move the feed.
Re. Gain and sidelobes vs feed displacement see Paul's paper here 
http://www.w1ghz.org/antbook/conf/Parabolic_Dish_Focus_Zoom_and_Tilt.pdf 
especially fig 4.

The data on my feed positions, maximised on sun noise, compared to the 
design focal length for 23 13 ,9 and 6cm  is (in mm)    -34,  +12, +9 and 
+22 ....or in wavelengths -0.15, +0.09, +0.1, +0.42
The feed type is exactly the same for each band, the phase optimised SM6FHZ 
Kumar. f/D is 0.375
I do have however a known, constant, x axis error in the dish in that I did 
not allow for the mesh thickness when I made the rib jig  and I dont know 
what effect that has on these values, it may be why the 6cm one is so 
different.
73 Peter G3LTF


-----Original Message----- 
From: Steve Gross-N4PZ
Sent: Friday, May 12, 2017 9:07 PM
To: Charles Suckling ; "'freetalk moonnet'"
Subject: Re: [Moon] Focal point question.

Hello Charlie

An interesting observation. The focal point on 1296 is closer to the dish 
than 2304. That may be a clue that you are on to something The last time I 
worked with it the choke ring was off on 13 cm. I don't recall if I ever 
peaked sun noise on 23 cm w/o the choke. Hmmmm. That's where I will go next 
time I did put the 13 cm feed on the same spot as the 23 cm feed at one time 
and worked some guys w/o any adjusting. I'm picking up a 300 watt 13 cm SS 
amp in Sweden next week so hearing my echoes will be easier although that's 
a very tough way to adjust focal point. Current power 140 watts. Thanks for 
taking the time to help me Charlie.I never stop learning. I'll bet the 23 cm 
FP is right and the 13 cm sun noise peak is bogus. What dost thou think?

73

Steve N4PZ

PS. My dish has a .3 F/D ratio.



________________________________
From: Charles Suckling <charlie at sucklingfamily.free-online.co.uk>
Sent: Thursday, May 11, 2017 10:58 PM
To: 'Steve Gross-N4PZ'
Subject: RE: [Moon] Focal point question.


Hi Steve



I have a theory as to what may be happening.



When you optimise a feed for best sun noise, you are actually optimising for 
best G/T ratio on receive, not necessarily for maximum gain.



If you move the feed closer to the dish than the true focus, the gain will 
reduce but the spillover will also reduce and hence less pickup of ground 
noise.  If the spillover effect is larger than the gain loss, then the feed 
will end up closer to the dish than the true focal point.



At 1296, the loss in gain with having the feed closer to the dish will 
possibly be less than at 2304 as the phase errors introduced by a certain 
physical movement toward the dish  will be less at 1296 than at 2304 because 
of the longer wavelength at 1296.



An alternative, but somewhat more difficult,  procedure is to optimise feed 
position for maximum echo strength. On TX, having the feed at the true focus 
will maximise erp while on receive you are still  playing the G/T game, so 
in this case the gain will have two contributions to the overall equation 
and you may end up with the feed closer to the true focal point than using 
sun noise alone.



Just a thought.



73



Charlie



________________________________

From: Steve Gross-N4PZ [mailto:n4pz at live.com]
Sent: 11 May 2017 23:51
To: charlie at sucklingfamily.free-online.co.uk
Subject: Re: [Moon] Focal point question.



Hello Charlie

Sun noise and a General Radio 1236 noise meter. Obviously moving the feed in 
and out to find the peak. After each change make sure Make sure the dish is 
still smack on the sun.

This has me stumped Charlie.

Steve N4PZ



________________________________

From: charlie at sucklingfamily.free-online.co.uk 
<charlie at sucklingfamily.free-online.co.uk>
Sent: Thursday, May 11, 2017 10:48 AM
To: Steve Gross-N4PZ
Subject: Re: [Moon] Focal point question.



Hi Steve

How did you determine the best focal point?

73

Charlie G3WDG

> Thanks Craig
>
> Strange. Seems it has something to do with the actual focus happening
> inside the pipe. Mine is 5 cm farther out on 13 than 23 cm. I tried to
> double and triple check it and that's not my error as far as I can tell.
>
> A bit puzzling.
>
> 73
>
> Steve N4PZ
>
>
> ________________________________
> From: craig <cats at amplex.net>
> Sent: Wednesday, May 10, 2017 9:24 AM
> To: Steve Gross-N4PZ
> Subject: Re: [Moon] Focal point question.
>
> Steve :
>
> I noticed that on my 5.6m antenna. The 2304 location was different than
> the
> 432, 902, and 1296 positions. Just figured that it was an anomaly of the
> dish.
>
> Craig, N8DJB/4
>
>
> -----Original Message-----
> From: Steve Gross-N4PZ
> Sent: Monday, May 08, 2017 1:22 PM
> To: "'freetalk moonnet'"
> Subject: [Moon] Focal point question.
>
> Hello All
>
> I have a 4.9 meter dish. I have two ve4ma feeds one for 1296 and one for
> 2304. I find the focal point for the 2304 feed to be about 5 cm farther
> from
> the dish than the 1296 feed. With or without the choke it is the same. I
> am
> not a newcomer as  have been building and using dishes both commercial and
> HB for 40+ years. I am puzzled.
>
> Help.
>
> Steve N4PZ
> _______________________________________________
> Moon mailing list
> Moon at moonbounce.info
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