[Moon] Cabling a dish

Sam jewell at btinternet.com
Sat Dec 26 11:50:17 CET 2015


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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>    dubususa at gmail.com
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