RG59 cable acting strange - Page 1
Forum section: electronics / repair

author: CaptDon
author_rank: Super Contributor
timestamp: July 09, 2020, 09:15:26 pm
content: We moved into a new house. The CATV wiring was a mess. So I tookout unused runs, splitters etc. I now have one splitter, feeds a run tothe living room and a run to the bedroom. The problem, the run to theliving room looked terrible on low band VHF channels 2-6!!! Very snowyand lots of crosstalk like the 'window screen' effect. Made a bunch ofmeasurements, found all to be in a normal expected range. Not shorted,not open, F connectors secure and no leakage current using Hi-Pot at500vdc both polarities. Didn't measure exactly, but the cable was maybe15 feet long. Replaced with new piece and problem is 95% gone but stillnot as crisp as the higher channels. Feed into splitter is perfect, tried usingliving room T.V. in the basement. Tried different splitters, exact sameproblem, unhooking bedroom feed doesn't seem to make any difference.If their was an open and we had only capacitive coupling that wouldexplain the higher channels looking better but that was not found usingmy ohm meter. It almost acted like there was a quarter wave stubkilling the lowband signal. Anyone else ever had a weird problem likethis??? Is there such a thing as a resonant length acting like and opencircuit or band reject filter??? This one has me puzzled.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 03:10:39 pm
content: What is the manufacturer and part number of the coax that you installed?

author: CaptDon
author_rank: Super Contributor
timestamp: July 13, 2020, 01:56:38 pm
content: I haven't had a chance to look closely at the coax I installed,but what I really should have noted was the markings on themalfunctioning piece I threw away!!! It measured perfect inevery way but for some reason showed a huge loss end to endin VHF channels 2-6 when installed between the splitter andthe T.V. The new run had the aluminum foil shield and thealuminum braid that can't be soldered. Used 'average' qualityF connectors. Picture is way better now with just a hint ofcrosstalk/noise. Wish I still had the piece I took out for moreinvestigation/testing.

author: tkamiya
author_rank: Super Contributor
timestamp: July 13, 2020, 05:34:34 pm
content: Try removing the splitter. Join cables to route A and route B one at a time with barrel (both side female) connector and how the signal look.Unless horribly abused, it is rare cables go bad to that degree. It sounds like to me, the splitter doesn't have the necessary bandwidth.

author: TimFox
author_rank: Super Contributor
timestamp: July 13, 2020, 08:33:09 pm
content: Also, is it possible that you mis-connected the splitter? A good "hybrid" splitter will isolate two ports from each other (both coupled to the common port). If you try to drive one of the two isolated ports, then there will be signal at the common port but very little signal at the other isolated port.

author: tkamiya
author_rank: Super Contributor
timestamp: July 13, 2020, 08:42:37 pm
content: It's odd that loss is larger on lower end of frequency. Only thing I can think of is, possibly a hidden filter, or passband of divider is not sufficient. I vaguely recall seeing there are now two types of splitters. Ones made for on-air and ones made for catv.

author: richnormand
author_rank: Unknown
timestamp: July 13, 2020, 10:23:29 pm
content: Similar issue in the days of analog TV fed with an outside antenna for VHF and UHF. UHF was OK but low end was crap but only on one TV (TV was know to be good)Suspected the cable to that one TV. Bought enough wire spool to do the run from the basement to the TV via the stairs and it was still bad. Now, the second floor TV was also bad with the new cable run present.....Unhooked everything and did direct antenna runs to each TV with the test wire spool. All OK.Turned out, once opened, the divide by four splitter was not a simple resistive splitter but had an array of tiny coil/transformers around ferrite rings. One of those had a clear split. Changed the splitter and all was OK.In addition any unused output on your spitters should have a proper terminator.Splitter ferrite must have acted as a loss or notch filter and was clearly dependent on the impedance presented by the other ports. It would have been so nice the have an SA or a VNA at the time, like I do now

author: tkamiya
author_rank: Super Contributor
timestamp: July 13, 2020, 11:40:11 pm
content: One time, I put a splitter on bench and swept across the band. NO isolation. I couldn't even open the device, so I wouldn't be surprised if there's nothing in it!

author: duak
author_rank: Super Contributor
timestamp: July 14, 2020, 10:23:52 pm
content: Richnormand may be on to something. A two way splitter went blinky on me too. If it had a cracked core, that would go a long way to explain why the lower channels were bad - the inductance went away.

author: Whales
author_rank: Super Contributor
timestamp: July 14, 2020, 11:25:27 pm
content: I'm a bit daft here: surely high-frequency signals in coax would hate passive splitters, various lengths of coax and various intermixed loads at different points (eg TVs plugged in at different spots)? Won't you end up with different channels filtered out depending on what position you plug your TVs into?

author: TimFox
author_rank: Super Contributor
timestamp: July 15, 2020, 01:55:45 am
content: A proper passive splitter will send half of the power from the input to each of two outputs. A hybrid splitter also isolates the two outputs from each other. A good one will do this over a wide bandwidth. Used in the opposite direction, they can combine two signals into a single output without coupling of the two signals to each other.Here is a catalog page from a leading supplier of splitter/combiners:https://www.minicircuits.com/WebStore/Splitters.html

author: duak
author_rank: Super Contributor
timestamp: July 15, 2020, 05:33:03 pm
content: Tim twigged a memory of an article that touches on back to back splitters:https://broadbandlibrary.com/rf-signal-level-2/