All posts by Tim
Below are all of Tim's postings, with the most recent are at the bottom of the page.
To MikeP, re: amplification, log periodics have less gain than banded "contract" yagi aerials and so a masthead amp is generally recommended, some coming with them built in. They have the benefit of wideband reception and also a tighter focus on the transmitter with better rejection of "noises off", but need an amp to produce the same output as a yagi. I agree about "digital" aerials, all signals are transmitted in analogue form ultimately.
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To MikeP: I have a Vision log periodic V10-040, the predecessor to the one you link to, see a picture here (not mine): http://www.satcure.co.uk/…shed . See here for an explanation of the gain differences between log periodics and more commonplace aerials: http://www.satcure.co.uk/…raph - see how even a "Rolls Royce" log periodic's output is much lower than a Yagi's. The point of the amp is not to amplify the received signal and noise (providing little benefit as you say) but to help overcome losses by boosting the signal for a noisy, lossy and often long downlink cable, which can be very lossy at UHF frequencies, when using an aerial producing less gain. On aggregate this arrangement with a log periodic produces better results than a Yagi. If the downlink cable is short, like the shed installation, then it is unlikely to be of much benefit.
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To MikeP, you are right that amplifiers amplify the signal and noise, even with a little more noise added by the amp itself, but when amplifying the signal prior to a lossy link, e.g. a downlink cable or even a mast-to-antenna link like Mendip to Chippenham amplified to 100kW (still with noise beforehand or course), it boosts the signal over the noise of the path so that the ratio at the receiving end is improved. Perhaps it wasn't expressed so clearly but I'm sure that x years of experience will not be lost on this point. So my advice to Dr. Harvey in Chippenham is still to use an amp unless the downlink is short. Too many parts of the town have antennae on such long masts that are testimony to the shadowing of the intervening terrain around Corsham and nearer to be able to have total confidence in not using one. A signal path predictor which also takes the earth's curvature into account can be found here: Terrain - shows radio profile between two UK sites to optimise you DTT, Freeview, DAB or analogue TV reception .
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David, there is an outside chance you are seeing co-channel interference from the Huntshaw Cross transmitter, which reuses UHF channels 52 and 56, the ones you are having problems with. The transmitter is more or less behind Mendip from your location though much more distant. You may be able to confirm this by getting your equipment into a portable state and testing it from a location where Huntshaw Cross is not in the frame to see if it behaves the same. Neighbours are likely to be suffering the same issue.
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No COM7 or COM8 (UHF chs 33 and 35) at the moment (~3am), are they off at night time for engineering work? COM7 was fine earlier today. I see that they will disappear at the end of March this year, perhaps that is related. Other muxes e.g. the man HD mux (ch58) are fine.
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StevensOnln1: both muxes were back again this morning (of 18/1/18) and are still ok now at 4am with nothing done to my setup to bring that about - the STB signal level meter showed zero when they were out and now it back to the good level they have always had in the past. Thanks for the information about the COM7/8 changes, this information seems to be in conflict with the chart above regarding what happens after 31/3/18 where COM7/8 currently disappear.
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Friday 17 April 2015 12:51AM
COM8 (ch35) now generating a receivable signal, no scannable channels as yet.