Re: GPS 'GR Correction' Myth.
- From: "Paul B. Andersen" <paul.b.andersen@xxxxxxxxxxxxxxxx>
- Date: Wed, 10 Aug 2005 22:55:55 +0200
sue jahn wrote:
"Paul B. Andersen" <paul.b.andersen@xxxxxxxxxxxxxxxx> wrote in message news:dda558$9cp$1@xxxxxxxxxxxxxxxxxxx
bz wrote: > "sue jahn" <susysewnshow@xxxxxxxxxxxx> wrote in > news:42f6a640$0$18637$14726298@xxxxxxxxxxxxxxx: > >>"bz" <bz+sp@xxxxxxxxxxxxxxxxxxxx> wrote in message >>news:Xns96ABB83E8FE46WQAHBGMXSZHVspammote@xxxxxxxxxxxxxxxxx >> >>>"sue jahn" <susysewnshow@xxxxxxxxxxxx> wrote in >>>news:42f676d5$0$18642$14726298@xxxxxxxxxxxxxxx: >>> >>>>"bz" <bz+sp@xxxxxxxxxxxxxxxxxxxx> wrote in message >>>>news:Xns96AB9347A69D3WQAHBGMXSZHVspammote@xxxxxxxxxxxxxxxxx >>>>> >>>>>The doppler equation doesn't have, nor does it need, a term for >>>>>signal polarization. >>>> >>>>No... but you mentioned phase. That can cause some envelope >>>>delay effects. Video has to be pre distorted for really big >>>>multi element television antennas to correct for it.
This is of course utterly irrelevant.
>>>Reference?
Sue is really great at giving irrelevant references. :-)
>>[PDF] UHF Wideband Slot Antenna >>File Format: PDF/Adobe Acrobat - View as HTML >>... any UHF channel from 14 to. 69; additionally, it offers essentially >>zero group delay across ... all frequencies in the UHF TV band from 470 >>to 806 MHz, ... www.tcibr.com/PDFs/888banwebs.pdf - Similar pages > > > Wide band antenna, allowing multi channel and digital broadcast. Nothing > about predistorting video. > >>List01 >>FM Estereo y Enlaces Estudio Planta *TELEVISION: VHF UHF ... Opción >>PCP-S, Video Group Delay Pre-corrector with white level control and >>regenerator synch ... www.fmelectronica.com/htm/01.html - 43k - Cached - >>Similar pages > > > Transmitter specifications. Nothing to indicate why such a precorrection > might desireable. It could be to correct for transmission line effects or > to correct for narrow bandwidth antennas distorting the signal phase near > the band edge.
Actually you can find the possibility for group delay correction in that specification, look for "Video Group Delay Pre-corrector"
But it does not say why it is needed.
The spacing and phasing dimensions in antenna array are in *wavelengths* so its gain and other parameters vary over the passband.
Right.
The far field perfomance of a structure exhibits group delay just as a filter would. The measurement is one of the trickyest to do in commissioning a television antenna so it is usually done only once and the required video correction is carefully documented in case the corrector has to be replaced.
Right. I am not saying otherwise.
But there will _always_ be a filter between the transmitter and the antenna, or you will not be allowed to go on the air. ITU, The International Telecommunication Union, demands it.
This filter is a bandpass filter - often with notches in the stop-band, and it will have a rather nasty group delay, worse than the antenna.
But you are right. The correction filter will have to correct for the combination filter and antenna.
But since the filter dominates, I didn't mention the antenna. (Actually I did initially, but deleted it to keep it short.)
But thanks for the correction anyway.
The reason is that in a conventional TV-transmitter system, there is always (specified by ITU!) a bandpass filter between the transmitter and the antenna, to suppress spurious signals. Since this filter has to handle all the power, it is built with wave guide resonators (very high Q) to keep the losses small. This filter is big and clumsy, and is therefore made quite simple. That is, it will not be phase-linear. To correct for that, there is a phase correction filter in front of the power stage, because it is easier to make a complex filter at the lower power level. This filter is called "Video Group Delay Pre-corrector".
In a really big transmitter the output filter can look like this: http://gallery.bostonradio.org/2004-05/viacom-tower/100-01688-med.html
A more "normal" one can be seen here: http://www.wcov.com/technical/transmitter/transmitter.html The 7. picture from the top. "It filters out all the bad stuff so that our signal consists primarly of entertaining and informative program fare." :-)
In small transmitters, the filter is small: http://www.transmittersrus.com/taylor2w_vhftv.htm - even if it is bigger than the transmitter. :-)
And all this is utterly irrelevant to the doppler radar.
Move your television from cable to antenna and think about that while a few airplanes while some air and automotive traffic rolls your picture arround.
I din't get the point with that remark. Video group delay pre-correction is utterly irrelevant to doppler radar.
Paul .
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- Re: GPS 'GR Correction' Myth.
- From: Paul B. Andersen
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