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Old 17th Jun 2020, 11:29 am   #1
Restoration73
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Default "Forward Scatter" (Thorn 850 Service Manual)

I have just been looking through some manuals. The Thorn 850 description
of the 13-Channel VHF Tuner Unit says ;

"The Aerial Filter incorporated provides high rejection (approximately 50dB overall) at the TV sound and vision intermediate frequencies to prevent interference from 'Forward Scatter' transmitters operating on 30-40 Mc/s."

What would cause this interference. I am aware that scatter was used for communications e.g. to oil rigs, but that was at a much higher frequency. Did anyone experience this problem back in the day ?
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Old 17th Jun 2020, 5:08 pm   #2
jayceebee
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

I have a vague recollection of the term scatter interference on the North East coast which the source was allegedly RAF Boulmer. An in-line filter in the aerial lead fitted by the field techs apparently solved the problem, l’m not aware of what the on screen effect was.

John.
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Old 17th Jun 2020, 5:42 pm   #3
G6Tanuki
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

The original "Chain Home" RADAR operated between 20 and 40MHz; I wonder if there was still a legacy CH transmitter (not yet replaced under the ROTOR plan) operating at the time that service-bulletin was issued?
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Old 17th Jun 2020, 7:20 pm   #4
Restoration73
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

The 850 manual is 1963-64, and there is no mention of this in the manual for the next
Thorn 900 chassis, which is 1964-5. It was only 20 years after the war.
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Old 17th Jun 2020, 9:28 pm   #5
Graham G3ZVT
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

I remember the reference to "Forward Scatter" in the 850 manual. I had, perhaps erroneously, assumed it was experimental ionospheric sounding activity, perhaps inspired by the IGY, a few years earlier.

I wonder what the last manual to reference "Shortwave Diathermy" as a potential problem?
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Old 18th Jun 2020, 6:02 am   #6
Mr Hoover
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

There were a lot of E. European and Soviet Union
signals in the 30 to 40 MHz band in the
late 1950s and early 60's.
Due to the sunspot
maximum in late 1957 they could put
a very high signal strength into the
UK in the afternoon period.
As the sunspot number declined
Going into the 60's it was less of
a problem.
I remember next time round(peak is roughly
every 11 years) in the late 60's
hearing Russian voices counting
numbers interfering with a fringy
Crystal Palace Ch1 signal.
Anything in that band ended up being
referred to as Forward Scatter.
There was even mention of a "radar"
in Iceland giving trouble.
Tried a quick Google search,
couldn't find very much
mentioned about the
specific problem.
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Old 18th Jun 2020, 8:41 am   #7
Restoration73
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

You have triggered a memory. Around that time I also heard Russian voices on our
Thorn 900 TV, it had a push button tuner so could tune all of Band I. I had thought this
emanated from one of the boats known ostensibly as "Russian Trawlers" which were
often around the Cornish coast then.
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Old 18th Jun 2020, 8:52 am   #8
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

I was just reading the notes relating to the Pye PTV 14" portable of 1957.

'If R.F. interference is experienced from aircraft or marker beacons it can be attenuated by adjusting the core of L5. This filter circuit covers 33-40Mc/s.'

It must have been a problem in certain areas. John.
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Old 18th Jun 2020, 9:55 am   #9
Mr Hoover
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

The BBC were concerned enough in
1947 to do tests with an aircraft
putting out "GH" pulses around 40 MHz.
See the link below.
https://www.bbc.co.uk/rd/publications/rdreport_1947_07

There's also another
report, 1947_03
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Old 18th Jun 2020, 10:02 am   #10
Mr Hoover
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

Quote:
Originally Posted by Restoration73 View Post
You have triggered a memory. Around that time I also heard Russian voices on our
Thorn 900 TV, it had a push button tuner so could tune all of Band I. I had thought this
emanated from one of the boats known ostensibly as "Russian Trawlers" which were
often around the Cornish coast then.
Sometimes "The voice" sounded a bit slurred,
refreshment must have been required
to read out all those numbers for long periods.
There were also multitone teletype signals at
times making chirping noises.
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Old 18th Jun 2020, 5:13 pm   #11
Pfraser
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Default Re: "Forward Scatter" (Thorn 850 Service Manual)

I read an old DX-TV reference; would have been in 'Practical Television' or its successor, to forward scatter propagation of signals from the Soviet space launch site.

I've also seen a reference to UK usage of forward scatter to enable communications with Malta.(!) Presumably a 'happy medium' between HF propagation vagaries and the cost of nascent satellite comms.
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