[Moon] Cabling a dish

Sat Dec 26 11:20:37 CET 2015

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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> > Moon at moonbounce.info
> > http://lists.moonbounce.info/listinfo/moon
> >
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73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     dubususa at gmail.com

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