[Moon] Focal point question.

Fri May 12 22:07:54 CEST 2017

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
> http://lists.moonbounce.info/listinfo/moon

Moon Info Page<http://lists.moonbounce.info/listinfo/moon>

lists.moonbounce.info

Mailing-List for Radio Amateurs interested in Moonbounce (EME) run by DF6NA Sponsored by: SM4IVE, ON7UN, G3LTF, DL1YMK, DL9KR, HB9BBD, ON5TA, WB2BYP, K1DS

>
> Moon Info Page<http://lists.moonbounce.info/listinfo/moon>

Moon Info Page<http://lists.moonbounce.info/listinfo/moon>

lists.moonbounce.info

Mailing-List for Radio Amateurs interested in Moonbounce (EME) run by DF6NA Sponsored by: SM4IVE, ON7UN, G3LTF, DL1YMK, DL9KR, HB9BBD, ON5TA, WB2BYP, K1DS

> lists.moonbounce.info
> Mailing-List for Radio Amateurs interested in Moonbounce (EME) run by
> DF6NA Sponsored by: SM4IVE, ON7UN, G3LTF, DL1YMK, DL9KR, HB9BBD, ON5TA,
> WB2BYP, K1DS
>
>
>
>
> _______________________________________________
> Moon mailing list
> Moon at moonbounce.info
> http://lists.moonbounce.info/listinfo/moon

Moon Info Page<http://lists.moonbounce.info/listinfo/moon>

lists.moonbounce.info

Mailing-List for Radio Amateurs interested in Moonbounce (EME) run by DF6NA Sponsored by: SM4IVE, ON7UN, G3LTF, DL1YMK, DL9KR, HB9BBD, ON5TA, WB2BYP, K1DS

>