Which is it, RG59 or RG6?
in [Repair]
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From: Michael A. Terrell on 9 Dec 2009 18:46 Ian Jackson wrote: > > In message <Q-2dnRjtm4MceILWnZ2dnUVZ_vqdnZ2d(a)earthlink.com>, Michael A. > Terrell <mike.terrell(a)earthlink.net> writes > > > >Ian Jackson wrote: > >> > >> In message <W4idnbUfmb694IHWnZ2dnUVZ_gudnZ2d(a)earthlink.com>, Michael A. > >> Terrell <mike.terrell(a)earthlink.net> writes > >> > > >> > >> > The 20 dB isolation means that the return loss is more than 20 dB. > >> > >> No it doesn't. It means that the isolation - the loss between two > >> outputs (with the input terminated) - is 20dB. > > > > Sigh. How do you think you measure that isolation? > > > Sigh as much as you like - isolation and RLR are definitely NOT the same > thing, and you don't measure them in the same way. You use the same equipment, but connect the ports in a different order. > >> >This is to prevent the local oscillator from one TV causing interference > >> >to another TV. > >> > >> True - but, more generally, it is to minimise the effects of something > >> connected to one output affecting another output. > > > > What are you connecting that doesn't have a TV tuner? FM radios are > >supposed to use a single port -20 dB tap. > > > That could make the FM level uncomfortably low. A 0 dB or higer signal will overload the front end of a FM radio. O dBM is 1000 microvolts @ 75 Ohms. FM radios are supposed to work at full quieting 10 microvolts. Some of the -20 dB taps were labeled as FM Taps and were supposed to have a bandpass filter on the output port but none of the ones I tested in the lab did. > It's better to separate > the FM from the TV signals via a bandpass diplexer filter. This is/was > common practice in Europe. Of course, in the USA, you have always had > your Channel 5 (running right up to the LF end of the FM band). No, it is Channel 6, with the aural carrier at 87.7 MHz. That is .3 Mhz below the bottom edge of the 88 to 108 MHz FM Broadcast band. > Also, > these days, you could have digital TV signals starting immediately above > the FM (and still meeting the 'no interference' regulations for Air > Traffic Control radio. Do any cable systems still actually carry FM > radio? Are you kidding? They were dropped as soon as the franchises stopped asking for FM and wanted more video channels. They gained three midband channels without rebuilding the system. > >> As well as protecting > >> against local oscillators, it also protects against incorrect loading > >> (ie if an output port is not correctly terminated in a 75 ohm load). It > >> should not matter (too much) if an output is left open circuit, short > >> circuited, has a length of cable connected to it but going nowhere, etc > >> etc. > > > > Unterminated ports are not allowed in properly designed systems. > >PERIOD. > > > They may be installed correctly terminated, but you can bet your bottom > dollar a lot won't stay that way! Not if it's maintained by morons. But then, those systems were never in spec. > >> > I was the engineer at a CATV system with over 10,000 > >> >active drops. There are hundreds of myths and half truths about the > >> >hardware used for CATV systems. > >> > > >> Was your statement above (about RLR) one of these? [Sorry. The Devil > >> made me say that! ;o))] > >> > > >> >> >I find 2 ports with -6.5dB loss and one port with -3.5dB, which makes > >> >> >more sense. (Actually, it should be -7.0dB loss). > >> > > >> While most 3-ways are typically 3.5/7/7, there are one or two around > >> with equal outputs, of appx 5.5dB (although, personally, I can't really > >> see much need for them). > > > > > > You would, if the use of the 3.5/7/7 would require a second drop from > >the line tap on the street. > > > When I said "much use", I meant "MUCH use". While a computerized CATV > system planner may flag up an out-of-spec condition, in practice, there > may be less than 1dB difference between a nominal 5.5 and 7dB. So? If the franchise calls for all drops to be above 0 dB, one dB is out of spec. -- Offworld checks no longer accepted!
From: Robert Macy on 9 Dec 2009 20:32 On Dec 9, 3:46 pm, "Michael A. Terrell" <mike.terr...(a)earthlink.net> wrote: ....snip... > A 0 dB or higer signal will overload the front end of a FM radio. O > dBM is 1000 microvolts @ 75 Ohms. FM radios are supposed to work at full > quieting 10 microvolts. Some of the -20 dB taps were labeled as FM Taps > and were supposed to have a bandpass filter on the output port but none > of the ones I tested in the lab did. ....snip... being a bit pedantic, 0 dBm in a 75 ohm system is 273,861 microvolts rms
From: Michael A. Terrell on 10 Dec 2009 11:56 Robert Macy wrote: > > On Dec 9, 3:46 pm, "Michael A. Terrell" <mike.terr...(a)earthlink.net> > wrote: > ...snip... > > A 0 dB or higer signal will overload the front end of a FM radio. O > > dBM is 1000 microvolts @ 75 Ohms. FM radios are supposed to work at full > > quieting 10 microvolts. Some of the -20 dB taps were labeled as FM Taps > > and were supposed to have a bandpass filter on the output port but none > > of the ones I tested in the lab did. > ...snip... > > being a bit pedantic, > 0 dBm in a 75 ohm system is 273,861 microvolts rms I meant 0 dBmv, which is used in TV Land. -- Offworld checks no longer accepted!
From: Robert Macy on 10 Dec 2009 14:03 On Dec 10, 8:56 am, "Michael A. Terrell" <mike.terr...(a)earthlink.net> wrote: > Robert Macy wrote: > > > On Dec 9, 3:46 pm, "Michael A. Terrell" <mike.terr...(a)earthlink.net> > > wrote: > > ...snip... > > > A 0 dB or higer signal will overload the front end of a FM radio. O > > > dBM is 1000 microvolts @ 75 Ohms. FM radios are supposed to work at full > > > quieting 10 microvolts. Some of the -20 dB taps were labeled as FM Taps > > > and were supposed to have a bandpass filter on the output port but none > > > of the ones I tested in the lab did. > > ...snip... > > > being a bit pedantic, > > 0 dBm in a 75 ohm system is 273,861 microvolts rms > > I meant 0 dBmv, which is used in TV Land. > > -- > Offworld checks no longer accepted! Understood. Thanks.
From: Michael A. Terrell on 11 Dec 2009 09:03
Robert Macy wrote: > > Understood. Thanks. I forgot to add the 'v' after 20+ years out of TV broadcasting, then spending years building Telemetry gear. :( -- Offworld checks no longer accepted! |