Forward scatter radio - UK Vintage Radio Repair and Restoration Discussion Forum

Hybrid tellies · 12th Mar 2007, 1:22 am

I came acrosss this old service manual for a 1960's 405 line television. It mentioned that one of the filters on the input to the tuner was a rejection filter for forward scatter radio signals operating between 30-40Mc's being picked up on the aerial lead and fed into the IF amp.
I am a bit curious about this, I knew that they used powerful UHF transmitters for forward scatter links to offshore oil & gas rigs, but I would have thought 30-40Mc's was to low a frequency for such communications.
Also back in the early 1960's were there any oil/gas rigs in the North Sea.

Trish · 12th Mar 2007, 2:05 am

My immediate thought on this would be concerning the various over the horizon radar systems that were developed in the 60's. Especially our favourite. The Russian 'Woodpecker'

http://www.answers.com/topic/over-the-horizon-radar

Skywave · 12th Mar 2007, 9:29 pm

The scattering effect is caused when radio signals pass through an ionised layer of the atmophere or are reflected by the ground. It is used at VHF and UHF.

In the late '50's & early '60's, studies in the UK & the USA established that signals due to incoherent scattering caused by random fluctuations of the refractive index in the troposphere and lower ionosphere could be used for medium-distance propogation in the VHF bands. A series of experiments by the US Bureau of Standards confirmed this effect.

Ionospheric scatter can be detected at about 600 - 1200 miles from the transmitter. At 50 MHz or so, some 50 kW of transmitter output power is typical with high-gain receiving and transmitting aerials. The 30 - 40 MHz region is unusual for this type of propogation, but not unheard of.

Tropospheric scatter communication is theoretically possible up to about 700 miles, (400 miles being typical), depending on transmitter erp, the mode of modulation, and if this is digital, the bit error rate that is acceptable. Transmitters in the 1 to 10 kW range are typical, but with frequencies in the 400 - 1,000 Mhz + region. There is usually a wider range of frequencies that will successfully support tropospheric rather than ionospheric scatter.

It's all a very fascinating branch of radio communications - the above merely scratches the surface.

Al / Skywave.

Hybrid tellies · 13th Mar 2007, 8:56 pm

Thanks, I new about UHF forward scatter radio coms, but did not know about low vhf forward scatter.
I suppose forward scatter radio is now a thing of the past thanks to satellite coms.
Yes Trish I do remember hearing that woodpecker thing as it moved up and down the HF bands. Indeed if it was close to 10.7Mhz it would break through (via the IF) and be heard on many FM receivers.

12th Mar 2007, 1:22 am	#1
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,967	Forward scatter radio I came acrosss this old service manual for a 1960's 405 line television. It mentioned that one of the filters on the input to the tuner was a rejection filter for forward scatter radio signals operating between 30-40Mc's being picked up on the aerial lead and fed into the IF amp. I am a bit curious about this, I knew that they used powerful UHF transmitters for forward scatter links to offshore oil & gas rigs, but I would have thought 30-40Mc's was to low a frequency for such communications. Also back in the early 1960's were there any oil/gas rigs in the North Sea. __________________ Simon BVWS member

12th Mar 2007, 2:05 am	#2
Trish Retired Dormant Member Join Date: Apr 2005 Location: Reading, Berkshire, UK. Posts: 95	Re: Forward scatter radio My immediate thought on this would be concerning the various over the horizon radar systems that were developed in the 60's. Especially our favourite. The Russian 'Woodpecker' http://www.answers.com/topic/over-the-horizon-radar

12th Mar 2007, 9:29 pm	#3
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Forward scatter radio The scattering effect is caused when radio signals pass through an ionised layer of the atmophere or are reflected by the ground. It is used at VHF and UHF. In the late '50's & early '60's, studies in the UK & the USA established that signals due to incoherent scattering caused by random fluctuations of the refractive index in the troposphere and lower ionosphere could be used for medium-distance propogation in the VHF bands. A series of experiments by the US Bureau of Standards confirmed this effect. Ionospheric scatter can be detected at about 600 - 1200 miles from the transmitter. At 50 MHz or so, some 50 kW of transmitter output power is typical with high-gain receiving and transmitting aerials. The 30 - 40 MHz region is unusual for this type of propogation, but not unheard of. Tropospheric scatter communication is theoretically possible up to about 700 miles, (400 miles being typical), depending on transmitter erp, the mode of modulation, and if this is digital, the bit error rate that is acceptable. Transmitters in the 1 to 10 kW range are typical, but with frequencies in the 400 - 1,000 Mhz + region. There is usually a wider range of frequencies that will successfully support tropospheric rather than ionospheric scatter. It's all a very fascinating branch of radio communications - the above merely scratches the surface. Al / Skywave.

13th Mar 2007, 8:56 pm	#4
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,967	Re: Forward scatter radio Thanks, I new about UHF forward scatter radio coms, but did not know about low vhf forward scatter. I suppose forward scatter radio is now a thing of the past thanks to satellite coms. Yes Trish I do remember hearing that woodpecker thing as it moved up and down the HF bands. Indeed if it was close to 10.7Mhz it would break through (via the IF) and be heard on many FM receivers. __________________ Simon BVWS member