[Moon] Cabling a dish

Sat Dec 26 15:11:30 CET 2015

And into the mix of considerations is the mechanical strength and 
rigidity of keeping the feed at the optimum focal point.  I think 
that consideration had many dishes using quad support legs over a 
single center support.  For the amateur it may overrule more esoteric designs.

I put up both style feeds for TVRO systems and both seemed to work 
equally well (lacking sophisticated measurements and only SNR as a measure).

Tree and building blockage were more significant considerations for 
siting the dish (a few trees got cut down).

Chasing the last 3K of Tsys becomes a point of diminishing return for 
effort.  I stopped at that point and started operating.

73, Ed

At 01:50 AM 12/26/2015, Sam wrote:
>Thanks Peter,
>I mentioned illumination in my post as I was actually more concerned 
>about the effects of scatter/reflection and an unwanted increase in 
>noise temperature than additional blockage loss, although this is 
>still important to me. With a small dish receive is usually the 
>hardest to optimise. Transmit is 'easily' dealt with by an increase 
>in power, but reducing system noise temperature is much harder. I am 
>beginning to think that my current support leg arrangement is still 
>probably the best route for the transmit and receive feeder coaxes.
>However, everyone's input (including some off list) has been 
>valuable in leading me to conclude that keeping the feeders well 
>away from the dish centre is probably better.
>I suspect Peter will cause a few of the 'community' to think about 
>their systems again!
>
>Now Christmas is out of the way may I wish everyone a happy and 
>prosperous New Year.
>
>73 de Sam, G4DDK
>
>
>
>Sent from my iPhone
>
> > On 26 Dec 2015, at 10:20, Peter Blair <g3ltf at btinternet.com> wrote:
> >
> > I dont think its quite as simple as gain loss, unfortunately. 
> Suppose that the feed and its support system introduces 0.1dB of 
> gain loss. This power has to go somewhere, a tiny amount warms up 
> the support  but most of it will be scattered.
> > Assume that half of the scattered power is towards the cold sky 
> and the other half to the hot ground ( reciprocity applies  so the 
> receiver sees that as a noise contribution)  0.1dB is a power loss 
> of 2.3% and so 1.15%of the power "sees" 290K which is ~3K
> > A good 23 or 13cm dish system with a modern feed, ( i.e.not a 
> simple open waveguide) should have a system noise temperature of 
> 40-45K or less and so adding 3 K is significant.
> > My 0.1dB is a bit of an optimistic guess and my warning is that 
> this subject is not a simple one of gain loss, and that you will 
> probably ( almost certainly) find that the feed support system is 
> producing more blockage than the feed itself.
> >
> > There is a good treatment of this subject which allows you to 
> estimate the feed support blockage effects here by Levy 
> http://ipnpr.jpl.nasa.gov/progress_report/42-113/113G.PDF ( It is 
> based on a cassegrain structure but is applicable to prime focus 
> systems as well). For my own dish I estimate that I lose about 
> 0.2dB gain and probably add about 7K to the noise temperature.
> >
> > So, minimise all the junk around the feed, preferably have the 
> supports go to the rim and minimise their cross section (personally 
> I would always go for tube rather than square section), but the 
> effect is usually more about increase in Tsys rather than gain loss.
> >
> > There is a good  reason why offsets work so much better than 
> prime focus dishes, look at OE5JFL's results!
> > 73 Peter G3LTF
> >
> > -----Original Message----- From: Edward R Cole
> > Sent: Friday, December 25, 2015 10:13 PM
> > To: Steve Gross ; 'g4bao' ; moon at moonbounce.info
> > Subject: Re: [Moon] Cabling a dish
> >
> > I concur with Steve's observations:
> >
> > I put up over 50 TV dishes in late 1980's and had some blow out a
> > panel with customer not even realizing it.  Of course sat-TV is not
> > weak-signal so there is a bit of margin.  Losing a panel would more
> > likely affect ground noise pickup (might see it more on Sun Y-factor
> > observations).
> >
> > But as regards signal blockage the cable is minimal.  The feedhorn
> > blockage is much larger area and even it is hard to measure on a
> > large dish.  My guess is smaller than 2.4m on 23cm band a offset feed
> > dish wins for sure.
> >
> > 73, Ed
> >
> > At 12:47 PM 12/25/2015, Steve Gross wrote:
> >> John it takes a whole lot of junk in the way to have any effect on your
> >> dish. You could lose 20% of the reflective surface and never notice it.
> >> I run a bundle of cables the size of your forearm down one strut 
> & it makes
> >> no difference.
> >> I've had whole panels on a tower mounted 6 foot dish blow away & I noticed
> >> it only by looking at it. Figure out how many square feet your dish is and
> >> compare it to some tiny blockage. Nothing. My dish is over 200 
> square feet.
> >> It takes a hell of a lot of junk in the way to have any effect.
> >> 73
> >> Steve N4PZ
> >>
> >> -----Original Message-----
> >> From: Moon [mailto:moon-bounces at moonbounce.info] On Behalf Of g4bao
> >> Sent: Friday, December 25, 2015 2:38 PM
> >> To: Sam Jewell
> >> Cc: moon
> >> Subject: Re: [Moon] Cabling a dish
> >>
> >> I've got a small hole in my dish and bring the feed back down the axis of
> >> the dish. It is self-supporting and I assumed the blockage would 
> be minimal
> >> as it's the diameter of the coax bundle. Certainly saw no improvement or
> >> degradation on the original way where I took the feed down a tripod leg
> >>
> >> John
> >>
> >> On 25 December 2015 at 20:00, Sam Jewell <jewell at btinternet.com> wrote:
> >>
> >> > Hi everyone,
> >> > Please excuse this question, but as I sit here, digesting my Christmas
> >> > dinner, I started to wonder about the effects of coaxial cables in
> >> > front of a dish.
> >> > Many of us run dish feeds with the power amplifier behind the dish
> >> > rather than at the feedpoint (23cm and 13cm typical). I suspect most
> >> > of us then run the transmit feeder ( and receive feeder) down one of
> >> > the support tripods to the rim of the dish and then either back to the
> >> > centre of the dish rear or maybe away at an angle to wherever the PA
> >> connection is.
> >> > Clearly the route back to the rear centre of the dish is more than two
> >> > times the dish radius and even in the case of my 2.3m dish is a fairly
> >> > long run of what might not be the very lowest loss coax size because
> >> > of weight constraints.
> >> > My question is:
> >> > Is it better to bring the transmit feeder back from the feed to the
> >> > centre
> >> > (hub) of the dish or to keep it off to one side where the effects of
> >> > blockage and illumination level are lower, but longer coax than to the
> >> > centre?
> >> > I'd be interested to know what others do before I cut any
> >> > (inadvisable) holes in my 2.3m dish mesh?
> >> >
> >> > Ohh, and Merry Christmas, if it isn't too late!
> >> >
> >> > 73 de Sam, G4DDK
> >> >
> >> >
> >> > Sent from my DDKpad
> >> >
> >> > g4ddk.com
> >> > g4ddk.blogspot.com
> >> >
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> >
> > 73, Ed - KL7UW
> > http://www.kl7uw.com
> >    "Kits made by KL7UW"
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73, Ed - KL7UW
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