Offset dishes

Steve Stras

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Hello everyone

I have a question regarding offset dishes. While I was in the UK over Christmas I installed a dish and decoder for my dad so he can watch saturday afternoon football, the system is Total TV pointing at Eutelsat W3C. My mother is particuler sensitive (difficult) about large sat dishes on the house so my dad bought a cheap 65cm model. Once it stopped raining (which I think was just 1 day during the week I was there) I installed the dish and pointed it at W3C without problems using a meter.

But, the dish simply isn't big enough to provide any insurance against rain fade, a bit of drizzle sees everything on the Europe A spot beam disappear along with HD channels from the B spot, the SD channels are ok though. They are located about 30 miles north of London.

My parents are now in Australia for a couple of months so my dad asked me if anything could be done while he's away without upsetting my mother too much by installing a large dish (anything greater than 65-70 is too large apparently)

I read about offset dishes, which from what I understand give a greater gain because the LNB arm is not blocking part of the signal. So, I thought I would buy Gibertini OP 65, which is an offset dish, I can pick this up for about €40 here. This model is actually 71.5cm x 67cm as opposed to 60x65 for the existing dish. I'm not really sure how to caluclate the surface area of a dish as they aren't round but the Gibertini has to have an increased surfaced area approaching 20%.

Therefore, is 20% more steel on an offset dish (with an invacom lnb)going to give me going to give me a significant increase in performance ?
 

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Almost all dishes are offsets - here in the UK prime focus dishes are most unusual and generally only very big ones. So if the dish has one arm at the bottom of the dish its an offset.

At the end of the day the only thing that will give more signal is a bigger dish - I dont have any problems with the 16E stuff on a 1m dish here in southern Scotland.

You also often find the tuners on service specific receivers really arent very sensitive, meaning a bigger dish is needed outside the main service area.
 

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Steve Stras

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Ok, so i've misunderstood, basically anything with the LNB arm at the bottom is offset, which is nearly everything. I'll try with the Gibertini and see what happens, maybe the extra 20% will be enough.

The Total TV receivers seem quite poor quality, they hang a lot and require reboots all the time. When the rain came I was still able to lock the relevant transponders on my meter (albeit with a drop in signal quality) but the receiver would not lock onto them until the rain stopped.
 

Steve Stras

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Thanks Michka, I'll have a look at those for some ideas
 

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If you put a bigger dish up spray paint it the colour of the wall behind using car primer paint.
 

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<sup>Or paint the wall behind it the same colour as the dish...</sup>
 

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Penta 85 is good. Doesn't look that big and there's a smaller 67cm version. They are quite effective for their sizes.
 

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Ive got a Penta 85. I dont find it any better than a bog standard 80.
 

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An Inverto Black Ultra LNB might also help a little, the singles especially allow for alot of adjustment having such a long neck.
 

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An FR penta85 should perform as well as a decent 1m dish coupled with a high grade LNb.
 

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If the cable run is particularly long then cable quality can play a rôle. Also - are the connectors watertight?
 

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I'd have to agree with satelliteman. I have a FR 85 as part of my multisat set up. It's centered on 5°E and using an Inverto Black Ultra, it pulls in the Nordic beams without a problem and even allows for some rain margin. I also use the same dish to receive 16°E on an offset non descript LNB. While it could be better (and would benefit from another Black Ultra)it performs really well. To the eye it is obviously larger than a 60cm dish but because of it's shape it seems to be asthetically more acceptable.
 

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Guys, thanks for all the suggestions. I will go back to the UK next month so I will have a think about what to do but it seems I'll have to get a larger dish and and paint it (or paint the wall behind it !) I may also try putting something at ground level in the garden and then surrounding with potted plants and trees, of which my parents seem to have thousands of..

Rick, I'll start with your suggestion of using a black ultra LNB as I already have one. I suppose you're saying because of it's long neck it may be possible to slide it up or down the LNB holder to make sure it's 100% focused on the dish ? At the moment the dish has an Invacom 031 which I think is a fairly good LNB.

Hairy Badger, I think the cable is of average quality and runs for 25m, though using the meter I can see I'm losing about 15% of the signal over the cable run. With reference to my comments about locking the signal on the meter but the receiver failing to do so, this is at the receiver end of the cable run. Clearly, the tuner in the receiver is not very good. I would like to get rid of the Total TV receiver completely because it's rubbish but the card is tied to the box so I don't think I can...
 

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Beauty really is in the eye of the beholder. I find the Pentas repulsive to look at.

I also find it difficult to believe that they can outperform for their size. To my mind, any empty space where a straight edge goes could be be usefully filled with metal to reflect satellite signals.
 

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Steve Stras said:
Hairy Badger, I think the cable is of average quality and runs for 25m, though using the meter I can see I'm losing about 15% of the signal over the cable run. With reference to my comments about locking the signal on the meter but the receiver failing to do so, this is at the receiver end of the cable run. Clearly, the tuner in the receiver is not very good. I would like to get rid of the Total TV receiver completely because it's rubbish but the card is tied to the box so I don't think I can...
OK - you're already going down the Black Ultra line. This has high gain so hopefully will be enough to overcome the signal losses in the cable (not sure what 15% means in practical terms but doesn't seem to be excessive). People generally have mixed experiences with separate amplifiers, especially inline "no power" ones....
 

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PaulR said:
Beauty really is in the eye of the beholder. I find the Pentas repulsive to look at.

I also find it difficult to believe that they can outperform for their size. To my mind, any empty space where a straight edge goes could be be usefully filled with metal to reflect satellite signals.
The slight loss in area will have an impact on overall incoming signal, however the rigidity of the dish - The Penta does have deep side walls - means that it will be less affected by strong side winds pushing the reflector off line.

Plus the boom arm is held somewhat more securely in place than, say a Gibertini, and the LNB is therefore less likely to see background noise.
 

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Plus the boom arm is held somewhat more securely in place than, say a Gibertini, and the LNB is therefore less likely to see background noise.
But this will surely be negatively impacted by the use of an LNB which expects to "see" a regular shaped dish. I'm sorry, I still don't buy it.
 

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I see you are still suffering, Paul!
 

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The illumination of the dish will be kept within the confines of the oval part that sits within the pentangle shape

The bits of the reflector that sit outside this can be discarded as they are probably less than 5-8% of the overall effective area. Having a generic feedhorn on the LNBF on a standard shaped dish is going to compromise far more than the Penta dish with its apparent cut-outs

Cassegrain/Grgorian dishes work better than single reflector systems not as a result of higher gain (larger area) but by a lower signal to noise ratio at the feed. The Penta shape goes some way to ensure extra rigidity - it's more difficult to bend the reflector out of shape compared to a standard offset design. The Penta is a good compromise bwtween offset vertical (as per the Gibertini where the maximum area of the dish is 'seen' by the satellite, with an associated loss at the feedhorn) and the offset horizontal (as per Zone 1 and 2 dishes) where the feedhorn is matched to the reflector for improved narrow aperture - beamwidth - and cross-pole isolation, at the expense of satellite illumination.

I did have a load of data on the Fracarro design on the office PC when I was an agent for them, but I doubt I can get it off the drive without proper invasive scanning as it nipped up a few years back.
 
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