Dishes

B

billw

Guest
#1
With analogue transmiissions I believe a solid dish was considered to be superior
to a mesh or petallised dish.Also a well engineered and consequently more
expensive dish would minimize sparklies and produce a better picture.
I understand that digital transmissions either produce a perfect picture or none at all.
If this is so is the type of dish at all relevant now?Surely a cheap dish provided it is
properly aligned would be adequate for most installations.
Also the sze of the dish would only enter into the equation if one required access to low
powered or difficult to identify satellites?
Your expert opnions would be appreciated.

Regards,
Bill.


i
 

rolfw

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#2
You have the basics correct Bill, it can still be advantageous to use an over spec' dish to eliminate signal drop out in bad weather though.

There are some dishes that perform better than others, therefor enabling the use of a smaller than recommended dish where environmental impact is critical. I would still recommend that you go for the best quality dish that you can afford, also the best LNB with the lowest noise figure.

As you say there are some satellites that require larger dishes for adequate reception, some because the transmission power is low and others because of the limitations of their footprint. If you are setting up for multisat reception, the dish will need to be of sufficient size and quality to pull in the weakest satellite.

Rolf
 

Channel Hopper

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#3
No matter how good the material - mesh will always be inferior to solid. There are factors of nearly 20% in terms of gain and other contributing noise elements associated with mesh at the hightr frequencies that make going for a nice non seethrough dish a viable alternative for weak signals.

For electrial purposes a grid or mesh has additional trouble with second reflections and absorbtion near the rim, especially when there is a layer of water on the surface, so rain will degrade the performance far quicker than a solid. Very relevant when you need the best possible C/N ratio for digital reception.

All the fibreglass antennas , Prodelin, Fuba, Channel Master and Sharcstar are additionally composite with mesh inserts, though the reflective fabric is more secure against outside elements such as gravity - sagging , and wind loading - distortion, so they provide a better alternative to mesh on its own. One last advantage however is the lowering of the thermal noise generated within the fibre glass, so unless your metal dish has cooling on the rear of the reflector, its quite possible the composite antenna will be better than a solid metal unit on the hot days.

Nothing like metal though on cool early mornings.
 
B

billw

Guest
#4
Dear Channel Hopper.
Thank you for for your explanation of dish technology.Is it possible to arrive at
a Best Buy for dishes - the advantages of metal compared with hybrid variants
on a cost analysis basis?
What do you think of the latest influx from Eastern Europe?Have they made the
same kind of metamorphosis that Skoda has made?.

Regards

Bill..
 

s*t*a*r*m*a*n

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#5
To use an analogy or is it a metaphor?

question :)
what happens when you pour water into a colander (sive)

answer :) the water drains out through the holes and some remains.

same thing with signal hitting a satellite dish. Solids are always best for capturing more signal. A solid dish of same size as a mesh dish will collect more signal. Perhaps this is not noticible on say astra 28 east satellite in uk as an example due to the signal being quite good (sky dishes point to this and they are quite small)but you would notice it more on other satellites in the arc and definitly in poor weather conditions :)
I had a much larger dish than my triax solid dish i have at the moment. i still receive most channels i did before i reduced the dish size (it fell of the building but thats another story)
but i lost alot of eastern channels but not as many as i thought originally, not that i watch that many eastern channels anyway :)
 

2old4this

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#6
To complete the analogy:
what happens when you pour flour through a sieve?
Some of it falls through.
Some of it does not.
The particles that are larger than the holes in the sieve do not fall through.
Well it's the same with a mesh dish.
Signals that are "bigger" than the holes will not pass through. Now it so happens that microwaves are "bigger than" the mesh gaps. IE their wavelength is longer than the diameter of the gaps in the mesh. So in principle the signals are reflected from a mesh dish just as effeciently as from a solid.

To test this, go to your kitchen.
See how your microwave oven has a metal mesh in the door with holes so you can see inside?
Now stand in front of it while it boils a mug of tea (or whatever). Are you burned?
No.
Reason: the holes in the mesh are smaller than than microwave wavelength and so the radiation is reflected back into the oven instead of leaking out to cook you.

There IS however a loss of efficiency from a mesh dish. But this has to do with a more subtle effect. All those holes effectively present a rougher surface than does a smooth solid dish. So although all signals are reflected, a small percentage of them are reflected back at the wrong angle. IE there is less concentrated signal strength arriving at the dish's focal point (where the LNB sits).
Many larger c-band mesh dishes also suffer imperfect reflections due to their segmented form. They are not perfectly parabolic.

2old
 
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