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Old 31st Mar 2019, 11:46 pm   #21
Granitehill
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Default Re: BT microwave network history

Regular tide-related fading was par for the course on the Braewynner to Thrumster 4GHz link which was on my patch in 1970. The description is in the website -
http://www.dgsys.co.uk/btmicrowave/sites/6.php
The link was 55 miles long and 50 of that was over water! Quadruple diversity was essential to get an acceptable reliability. You could watch the received signal levels in each of the four diversity channels rise and fall gradually between the high and low tides. The sheer length of the path was definitely pushing it, and there were times when the whole lot would fade to noise anyway. The equipment was a custom designed one-off, and it was very complex and fault prone, given the multiple diversity switches and (obviously) valve based technology. I think it may have been one of the longest over-water Line Of Sight links ever - anything further, and you're in troposcatter territory. The transmitter output was only one watt per channel, so there wasn't much margin in hand in the first place...
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Old 1st Apr 2019, 12:14 am   #22
pip5678
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Default Re: BT microwave network history

You can't blame the planner if the gasometer was down when he did the survey!
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Old 1st Apr 2019, 3:54 pm   #23
Biggles
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Default Re: BT microwave network history

We had a diversity microwave link at about 1.5GHz that went from Lancaster to Barrow in Furness across Morecambe bay. Two receivers used to be fed from two seperate antennas so when one faded due to the tide, the other took over. The supervisory system had to be locked out for the receive alarms otherwise you got lists of "faults" on the printout. Trouble is when there was a genuine link fault, you weren't alerted to it. One other thing was when they were planning wind farms in the early days, we were kindly contacted by them to ask if our links would be affected by the wind turbines. Difficult to assess really, as the reflections would be many and variable. In the end they didn't seem to create any problems.
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Old 3rd Apr 2019, 7:42 am   #24
kernowcam
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Default Re: BT microwave network history

Fascinating thread.
My dad was heavily involved in the build of Goonhilly. I believe a lot of their signals went on the microwave towers.
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Old 3rd Apr 2019, 11:10 am   #25
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Default Re: BT microwave network history

Called Arthur apparently and now privately owned: https://space.blog.gov.uk/2017/07/24...r-55-years-on/
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Old 3rd Apr 2019, 11:28 pm   #26
m0cemdave
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Default Re: BT microwave network history

This all reminds me of working on an event in the City of London. It was a linkup between the London and New York offices of a major financial trading house - the NY end was in the WTC which indicates how long ago it was.

As is often the case in the Square Mile, there was a building site across the road with some new architectural monstrosity in the early stages of construction.

I was doing the audio (we had installed speakers and video monitors all over the trading floors), not the link.

Some microwave engineers installed a satellite dish and equipment on the roof of the building early in the morning and got it all lined up and tested and hooked into our systems.

All was going well until near the end of the CEO's presentation when the link suddenly failed and I had to switch to the landline, audio only, backup.

The Production Manger rushed up to the roof to see what was going on. She returned to tell us that a large cantilever crane had arrived in a side street sometime during the day and had just erected itself. Directly in line with the dish...
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Old 3rd Apr 2019, 11:43 pm   #27
Refugee
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Default Re: BT microwave network history

I once read a crane story where it cut the TV signal off.
Nobody cared in the daytime so the crane driver just got asked to leave the jib down at night.
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Old 4th Apr 2019, 10:46 am   #28
kernowcam
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I was telling dad (87) about this thread and how the signals over water were affected. I asked him if he could guess why and quick as a flash said tide going uP and down. He experienced this at goonhilly in the 60's with a test microwave link to st just!
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Old 4th Apr 2019, 4:29 pm   #29
Nuvistor
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Default Re: BT microwave network history

Quote:
Originally Posted by Refugee View Post
I once read a crane story where it cut the TV signal off.
Nobody cared in the daytime so the crane driver just got asked to leave the jib down at night.
The signal wasn’t lost at one area I worked in, strong signal area, but it made the picture unwatchable with ghost images.
Same cure, jib dropped at the end of the day.
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Old 4th Apr 2019, 7:45 pm   #30
duncanlowe
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Default Re: BT microwave network history

A friend many years ago work for GEC / GPT on the PABXs. However they were also sometimes used for very small public exchanges. They kept having trouble with once such installation up somewhere in the isles. There was a public phonebook on one of the smaller islands, with a radio link to the island where the exchange was sited. The link would intermittently go down but it had always come back by the time they got up there. It didn't happen very often but they could never figure out what the cause was because everything always checked out 100%. ISTR they ended up leaving someone there for an extended period to try and figure it out. After some time, the link went down so the guy went out to investigate, to see a rather large ship moored in the water between the ends of the link!
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Old 4th Apr 2019, 8:09 pm   #31
Biggles
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I remember reading the spec supplied with our microwave link network and in the part dealing with predicted outages, they were never proven to be 100%. Some of the longer distance links (40km+) understandibly were less reliable, but surprisingly the very short hop high capacity links still were under the 100% mark. I have been on microwave link planning courses before, albeit a long time ago, and I suppose there are so many variables that mean a link can never be bomb proof. We used to expect the long links, which often were across high ground to drop out occasionally, but all panic was let loose when one of our main bearers went down, sometimes for a few minutes, then bounce back unexplained. We always used to keep our equipment cabinets locked, just in case a bored and inquisitive site sharer was poking around and switching things off! The other problem is aerial riggers occasionally disconnecting the wrong dish when there are about twenty on the mast to choose from and the cable i.d. markers are missing.
Alan.
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Old 5th Apr 2019, 11:42 am   #32
Pamphonica
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Default Re: BT microwave network history

I didn't work on links in Post Office Telecoms, but we used the West Coast microwave network for the Manchester-Glasgow links on the Expereimental Packet Switching Service (EPSS) in 1974-1978.
In bad weather, we got drop-outs on the 48Kb/s (yes, Kb/s not Mb/s or higher!) circuits we had on that link. I was told that we were getting over 40dB extra loss on the RF path, compared to normal conditions!
-Jeremy
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Old 5th Apr 2019, 12:09 pm   #33
Radio Wrangler
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Default Re: BT microwave network history

The US navy missile test station at point mugu (why do I always think of Mr Magoo?) had a microwave link to an observation station. It was all low down and across water. It was considered a world record holder for fading.

Geoff at work was visiting them when we were considering designing a fading simulator. The naval people very kindly gave him two large cans containing 16mm cine film of the front panel of an HP MLA during the link having one of its interesting period. The trace on the CRT looked like a roller coaster ride.

Each film can had a huge round sticker proclaiming POINT MUGU MISSILE TEST CENTER and US NAVY. How he got them across to the UK, I don't know.

David
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Old 5th Apr 2019, 12:39 pm   #34
AC/HL
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Default Re: BT microwave network history

The "beam" produced by the parabolic dishes in a microwave link isn't parallel as usually illustrated, but slightly elliptical. The mid point is more susceptible to intrusion than the ends, and presumably the same applies to the reflections that cause the problems with links over open water.
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