phased arrays

[gcohen]

Does anyone have enough knowledge of microwave and waveguide technology to comment on the following?

I have 2 dishes with C120 mount LNBs.

A waveguide could be taken from one of the C120 mount feeds to the other feed where it would be combined using some sort of orthomode transformer in reverse.

If the signal in the waveguide had an adjustable delay then the
combined signal would be enhanced at a particular frequency (ie 10773) as if the 2 dish areas had been added together.

I presume that this arrangement would be some sort of phased array.

Would it work ?

Geoff

 

[geoffreys]

Great minds think alike, same name (Geoff) too. I am working
on receiving Astra 2D in Cyprus, but without buying for CYP3K
a 3.8m dish. We Radio Amateurs used to stack arrays (aerials) linked by a phasing harness. This way 2 or more SMALL aerials
give the same result as one megasize one. This can be done
with Satellite dishes, say 2 x 1.8m, giving an equivalent size
of a 3.4m (note not quite double size). The problem with Sat
dishes is the LNBs - the Local Oscillators would have to be
in synch. In theory this could be done by disabling the 2 LOs in
the 2 LNBs (or 4, or 6, however many dishes stacked) and feeding
them the output of a third (spare - not active on a dish) LNB's
LO. However it appears on first look at this that the usual LNBs
on the market do not lend themselves to such mods. If this COULD be done, however, we could all pull in fringe signals at a fraction of the costs!!

I also investigating another way - more when I have it. I want
my BBC + ITV channels (now on 2D) back, but without the massive
expense of a huge dish - not to mention the fact a huge dish is
unwieldy and subject to storm damage much more than small
dishes.

Comments/ideas/innovations invited
Regards
Geoff Smith
Vrysoulles
Famagausta District
Rep. of Cyprus

 

[2old4this]

Theoretically it is possible.
But it's difficult as there is always some interference, since the signal arrives at each dish at a slightly different instant in time. In fact, the phase interference that results provides information that can be used to determine stellar distances (for example) and that whole field is called interferometry. The combination of signals from several smaller dishes to "mimic" the resolution of a larger dish is achieved via computer processing.

SETI (Search for Extra-Terrestrial Intelligence) was planning to use a phased array of small satellite-TV dishes to mimic a large radio dish. See here:
http://www.setileague.org/array2k/

2old

 

[geoffreys]

Correct, in theory. In practice in our (Cyprus my location) case
there is no practical problem as the difference in distance between 2D and our two dishes is miniscule, plus the fact the
signal strength, or Sig to Noise ratio, is receivable OK. With say
two dishes you have to combine the outputs into the single
feedline to the Sat receiver. For this a phasing harness is used
and it is very easy to construct using the right type of feeder.
I won't go into detail now - but it is easy. One thing the phasing harness does is ensure the impedance remains at 75 Ohm, we
required by the receiver and to avoid standing waves.
As I said in my first message on this subject the ONLY problem
is getting the 2 LNB Local Oscillators (LOs) synchronised. This
is purely a practical problem with the hardware currently on the
market - I not sure it can be done, but maybe some other
contributor can suggest ways?

I'm sat here with a Panasonic 30 SKY receiver and no pics! Missing
Cora and Eastenders etc as much as the other half.

There has to be a technically workable solution as an alternative
tp mega-size mega-expensive dishes, we just got to get our heads together to find it. Stacked/Phased-feedline dishes are
one obvious option to investigate further, if nowt else to keep
the bank manager at arm's lenghth!
Cheers
Geoff
Cyprus

 

[rolfw]

Old Satellite posted some information on PLL LNBs, I believe that the phase lock loop technology enables very accurate frequency control.

 

[richardinalex]

Geoff

I'm relatively new at this game - I have picked up most of the 2A/2B signals now in Alexandria on a 2.4m dish (still waiting for the SKY card to check the quality though) - however I get a "no signal" message for the 2D channels.

For this stacking system to work I'd guess that you'd need to pick up some of the signal - how do you know if you're getting a sufiicient amount of signal to allow the stacking to work.

I'm very interested in your idea - I've just found a 3.7m dish locally made - but really don't fancy putting it up (expense & high winds + local concrete could = no roof - that wouldn't go down well with Mrs richardinalex

Cheers

Richard

 

[geoffreys]

Well of course there must be signal there (from 2D say) in the
first place or no dish will pick it up and lock-on. In Alex, as here
in Cyprus, the signal is there alright but in noise of course. A
large dish merely increases the signal to noise ratio sufficiently
to get a picture. Using 2 smaller dishes connected electrically
with a phasing harness makes the combination 'look' like a
large dish electrically. The phasing harness has to be AFTER the
freq down conversion of course, which is why at the satellite's
transmit freq both LNBs MUST convert down to give the exact
same down-converted freq; to achieve that both LNBs must have
oscillators running at exact the same freq. So f1-f2 are same in
both cases. The theory I am 99.99% sure is sound. The practicalities are my problem since to buy locally all the hardware I need to experiment would bust the household budget! And her
indoors' patience, I learnt a long time ago that is unwise.
More on this topic soon I hope, I have a friend of mine in UK with
BBC's Engineering Dept working on this - he owes me a favour
from 40 years ago which he had forgotten about.
Geoff

 

[gcohen]

Many thanks for the reponses - but I think that you are all missing my point

What I am proposing is to combine the signals before they reach any LNB.

I do not think that combining the LNB outputs - even when phased locked would give more than marginal improvement.

My idea would need some sort of low loss flexible waveguide operating at 11Ghz with some way of adjusting the waveguide length to phase match the signals - this bit could be done by some sort of tube sliding within a tube arrangement.

Do low loss flexible 11Ghz wabeguides exist ?

Geoff

P.S. for Alexinalexandria - the dishes are already lined up on Astra 2A and B

 

[meatball]

yes,

contact andrew corporation.

experts in commerical microwave technology.



regards meatball.

 

[mctighed]

Great minds think alike, same name (Geoff) too. I am working
on receiving Astra 2D in Cyprus, but without buying for CYP3K
a 3.8m dish. We Radio Amateurs used to stack arrays (aerials) linked by a phasing harness. This way 2 or more SMALL aerials
give the same result as one megasize one. This can be done
with Satellite dishes, say 2 x 1.8m, giving an equivalent size
of a 3.4m (note not quite double size). The problem with Sat
dishes is the LNBs - the Local Oscillators would have to be
in synch. In theory this could be done by disabling the 2 LOs in
the 2 LNBs (or 4, or 6, however many dishes stacked) and feeding
them the output of a third (spare - not active on a dish) LNB's
LO. However it appears on first look at this that the usual LNBs
on the market do not lend themselves to such mods. If this COULD be done, however, we could all pull in fringe signals at a fraction of the costs!!

I also investigating another way - more when I have it. I want
my BBC + ITV channels (now on 2D) back, but without the massive
expense of a huge dish - not to mention the fact a huge dish is
unwieldy and subject to storm damage much more than small
dishes.

Comments/ideas/innovations invited
Regards
Geoff Smith
Vrysoulles
Famagausta District
Rep. of Cyprus

Have you thought of making a mould for a 4mtr dish and knocking out some fibreglass dishes? I understand that the arc of the dish is critical but I feel sure accuracy could be achieved with a good template. I, like you, am loathe to spend cy£3000 without trying something cheaper. After all the dish is only a convex shape pointing in a particular direction.
It could be cemented into a brick plinth to stop it becoming a frisbee.
Regards Derek

 

[geoffreys]

Have you thought of making a mould for a 4mtr dish and knocking out some fibreglass dishes? I understand that the arc of the dish is critical but I feel sure accuracy could be achieved with a good template. I, like you, am loathe to spend cy£3000 without trying something cheaper. After all the dish is only a convex shape pointing in a particular direction.
It could be cemented into a brick plinth to stop it becoming a frisbee.
Regards Derek

Yes, I have considered it - and still have it as an option. It would need
to be metallic coated. Would be best constructed in one of those
little workshops in Limassol or similar premises.
Geoff

 

[mctighed]

Yes, I have considered it - and still have it as an option. It would need
to be metallic coated. Would be best constructed in one of those
little workshops in Limassol or similar premises.
Geoff

I think aluminium foil glued into the surface with fibre glass resin would do the trick. I was thinking of making a mould myself, but I need to find dish dimensions, namely, arc in relation to diameter and distance of lnb from the center.
Derek

 

[gwise]

I have no experience with these, but Swedish Microwave sells PLL LNBs that will lock to an external 10 MHz reference.

See http://www.smw.se/products/pll10ref.htm

These might provide down conversion to the same L-Band frequency from two dishes (for combining).

You might contact Swedish Microwave direct to see if they have other options. I am in Northern Greece and would very much like to receive the BBC FTA signals from Astra 2D.

Regards,

Gary

 

[deepbluesky]

Hello everybody
This is a very interesting topic you are talking about here. I remember around 10years ago in analogue times that in the Tele Satellite International Magazine an experiment was published about receiving US-satellites in Venezuela which would need a 20m dish approximately. They achieved this with 8 5m dishes finally. I really have no idea if this is also possible with MPEG2 signals but why shouldn't it be ?

@gwise:
There is someone in Kastoria (40.6N/21.3E) who gets the BBC and most of the hor. Transponders of 2D almost all day. The dish is a 2.40m motorised and with C/KU-Feed, so effectively you can compare it with a 2.20m. Maybe some kms further away the situation is harder and you might need a much bigger dish but if you are in N-Greece i would advise you to call Emo system communication in Katerini (Komvos Korinou just on the old road to Thessaloniki). I talked lots of years ago (1995) with them when they had their store inside Katerini and they had installed many bigger dishes (mostly for Astra 1) all around Greece so maybe they are informed about the Astra 2D in different locations in N-GR. Here's their mail:
emo@otenet.gr

What an irony, i am inside the 2D footprint in Stuttgart Germany but the wall is between my balcony and the southern hemisphere.

Hope i could help a little. Please keep on searching for a solution with the phase arrays. There are so many weak satellites and if these could be received with this method this would cause a revolution in the market )
cu ;)

 

[Channel Hopper]

2 x 1.8m, giving an equivalent size
of a 3.4m (note not quite double size).


2 x 1.8m dishes would have the equivalent area of a single 2.5m if there are no other losses involved, so the complexity of combining will not be offset by the increase in signal from the phased array

Good in thory but a pipe dream in practice

 

[gcohen]

Many thanks for your response - there is obviously nothing new under the sun !

Do you remember anything as to how the output from the dishes was combined - if whatever was done worked on analogue signals there is no reason why it should not work on the present digital signal.

So far as implimenting the idea is concerned I have found that there is no problem getting flexible waveguides to convey the 10.7 Ghz signal between the dishes but the only adjustable delay line system I have found so far is a digital box costing about $2000.

Geoffrey

 

[deepbluesky]

Ok finally i have found the article about receiving a US-Satellite in Venezuela. It's published by Christian Mass (Chief-Editor of Tele Satellite International) in 9/1993 p.178/179 (in german)
19 dBW had to be received then from G-Star 93.5W
DH Satellite from Wisconsin made the job.
They set up 8 5m dishes. Each antenna was aligned with a spectrum analyser to a max. of signal. Then the 2nd dish was connected with the first and in right phase. Unfortunately it isn't described what they did exactly. But with 8 5m dishes the signal was still too weak, didn'T reach even the FM treshold. So another 8 5m dishes were installed. Then it was ok. So they avoided the not possible installation of a 40m (!!!) dish.
After that they tried the same in Wisconsin. But Brazilsat could not be received because strong signals from the neighbour satellite Spacenet (4° close to Brazilsat) made the reception of the wished sat impossible.
Ok this all only for C-Band. Upon that time they hadn't had any luck for KU-frequencies.
Since then i don't remember any article i have found for this.
I would advise to contact Christian Mass. As mentioned in the new issue of TSI he will travel in winter to Southern Brazil where Klaus Schumacher has built up his own 8m dish for receiving Astra 19.2 analogue and the Hotbirds on 13E also only analogue. Maybe he knows more about it.
Here's his mail:
drdish@drdish.com

P.S.: Maybe also in Namibia/South-Africa could be anyone who tried this since as far as i remember around 95-97 there was someone receiving Astra 19.2 analogue with a 12m dish.

Chris

 

[easysat]

Perhaps it's possible to combine the bit streams from several receivers (each with their own dish and LN after demodulation but before error correction. This would avoid the phasing problem from different LNBs. Some sort of logic that looks at several streams and decides whether a bit is a 1 or 0 by majority voting would output a single stream for error correction and channel decoding.

 

[Channel Hopper]

Unfotunatley the error correction of the two streams has no 'majority' vote it can rely on (the Space Shuttle has three computers for this purpose)

Anyhow the reasons for relying on a sinple QPSK system leaves the need for a more robust method of transporting data by digital packets wide open. It works for low signals with little error in the first place. Once the problem with fringe reception crops up QPSK falls over quickly

 

[PaulR]

Quote... Perhaps it's possible to combine the bit streams from several receivers (each with their own dish and LN after demodulation but before error correction. This would avoid the phasing problem from different LNBs. Some sort of logic that looks at several streams and decides whether a bit is a 1 or 0 by majority voting would output a single stream for error correction and channel decoding.

I don't think this would work as you would only get a bit stream if you had enough signal to start with, ie a big dish or phased arrays first.

PaulR