All posts by Jim F
Below are all of Jim F's postings, with the most recent are at the bottom of the page.
Briantist: In the maths you've used:
"Distance=sqrt((ERPW)*(10^-db))/fpMinW)"
The 10^-db needs to be 10^-0.1dB. This restores the "normal" shape to the pattern.
Beyond that, I don't see how you get the pattern radius of around 53km (scaled off the Google Map sample for Redruth) using the Rx sensitivity. If I use ERP in W I get massive numbers. If I use ERP in kW it gets smaller but not nearly enough.
Ultimately, what's important is the radius for a 1kW source to give minimum receiver sensitivity. I'd guess around 50km max based on ITU-R P.1546-3 (100MHz land path, 50% time), and a field strength of 43dBV/m (near enough to your -80dBm).
The extra power from Redruth of 1.32kW then increases the max range a bit to 57.4km.
The maths then becomes:
Distance = (max range for a 1kW Tx).sqrt((ERP in kW).(10^-0.1db)), and the pattern is only a bit more "pinched" towards the centre than the regular HRP dB plot (using a 0 to 20dB scale).
Given that we've used a maximum range which is much less than the theoretical "free space" range, the "not so pinched" appearance of the regular HRP plot might actually match reality fairly well, as the ITU curves get a little closer to the "free space" value
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saltydog: there are several things to conisder / check...
Is the TV (or digibox) faulty? It does happen, even with quality brands, and under guarantee. Easiest check is to substitute a known good digibox and see how that runs.
Is the flylead from the TV (or digibox) to the wallplate OK? A duff lead can behave exactly as you describe.
Is the wallplate OK? I've seen cheap wallplates that use a printed circuit board behind the socket - the solder joints fail making the socket loose and dodgy for signals.
Does the loss of signal occur after rainfall? If so, you may be getting water in the coax cable (which gives high signal loss). Also, on your new aerial, sometimes the lid on the connector housing disappears, and allowing the box to fill with rainwater - does it look to be intact?
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Ray - you might be getting adjacent channel interference, where the analogue signals are received and are too powerful for the digital channels that are adjacent (there's a bit of overlap from the analogue which gets worse if the analogue is much brighter than the digital signal).
The only MUX that's non-adjacent is MUX B, so are programmes such as BBC4 (9), CBeebies (71) and BBC Parliament (81) any better behaved?
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John M - Malvern hasn't had DSO yet (DSO1 is 7th September and DSO 2 is 21st September), so it doesn't have any HD content until DSO2.
From where you are, The Wrekin transmitter is directly behind the Malvern relay (but 65km away from you). It transmits the HD programmes on UHF channel 30, so I'm wondering if you've tuned into that during "lift" conditions.
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David Martin - You may have an issue with earth connections from either of your TVs. The Sky+ box and the Freesat box won't have an earth connection, but the TVs probably have. Could you isolate the TV set in the bedroom (take out mains plug and aerial lead, leaving the Freesat box plugged in with F connectors attached) and see if the "light banding" stops?
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Andrew Marjoram: Durris had a "retune event" on the 15th June, where the three COM MUXs moved down to Group A.
If you've retuned but not done a "default settings" or "factory reset" or similar (to scrub the redundant channel information), you'll find the missing programmes all up in the 800s.
Film 4 is now on ARQ B MUX on UHF channel 29.
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Ray Reed: Possible options for RF interference on digital TV include things like taxi transmitters, though your aerial would need to be fairly close to any offending transmit antenna.
Power line adapters for carrying internet signals around mains wiring are another option for interference, though I've not seen any yet that interfered directly with digital or analogue TV.
Your suggestion of a spectrum analyser test seems like the next logical step to identify what's going on.
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Chris: Could you try a full reset to start with (and check that the box really has "lost" all channels) and then manually tune for channels but in a different order?
Start with just UHF channel 29 (ARQ B) only and see if programmes such as Yesterday & Film4 appear.
Then just manually add UHF channel 26 (ARQ A) and see if Pick TV & Dave appear.
Add the rest of the MUXs one by one (SDN is UHF channel 23, D3&4 is UHF channel 25 and BBC A is UHF channel 28).
Be interested to hear back with your findings.
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M Gould: Need to know which transmitter you are receiving from.
Some early Goodmans boxes won't work with offset channel frequencies, so if your tranmsitter has changed to a mix of non-offset frequencies and offset frequencies, your Goodmans box may only receive the non-offset frequencies.
My first digibox (Goodmans GDB3) behaved in this fashion.
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Saturday 11 June 2011 10:31PM
Johnny: You'll need to check your dish azimuth setting using e.g. www.dishpointer.com and your location (drag the marker to the exact dish position).
For dish elevation, an elliptical dish face will be roughly vertical (i.e. not significantly tilted upwards or downwards).
The LNB will be skewed clockwise a bit.
That would get you somewhere near, such that your satfinder meter can help peak up to maximum signal.
Make absolutely sure that there's nothing in the line of sight from your dish to the satellite (e.g. a tree) which would kill the signal.