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Stopping volts going down UHF cable ??

Help !!

I have a star type UHF Aerial DA feeding TV to rooms in my place.

I also feed reverse direction from CCTV switcher via another discrete UHF cable, into the DA via splitter.

There is no feed from DA into CCTV switcher, so no loop there.

(apart from house mains that is)

When I connect up this cable, there is a noticeable patterning, diagonal lines, fed into the DA, apparent throughout system

upon checking, I find that there is a potential difference of around 48v AC, 8v DC, virtually no current, but enough to hurt a little, between UHF earth and equipment casing earth (UHF socket input).

This is presumably due to slight domestic 240v mains differences between downstairs and upstairs, fed off same junction box, house is quite large, giving the potential difference.

Is there a simple way to stop both DC and AC volts going down the UHF cable, without degradation of UHF signal.

Don't want to get into mains isolation, but is there a similar device for use at UHF?

I have tried another DA in circuit, this doesn't work.

Presumable something like a suitable capacitor in series would work.

Any suggestions ??

Temporary solution is to reverse feed CCTV via 2ghz, but this is not perfect.

TIA

Reply to
UHFPD
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A high pass filter tuned to suppress anything below 100Hz?

The other thing you might want to check is that there are no mains cable running parallel to the cable anywhere along its length and/or maybe run a separate earth lead from the DA upstairs to the same grounding point as downstairs to eliminate any ground loop.

Chris

Reply to
Chris Dugan

Earthing methods are different at RF and AF. Whereas we avoid ground loops at audio frequencies, the correct method at RF is to earth the coaxial shield at both ends: ground loops are inevitable; however, UHF equipment is unlikely to be troubled by 50Hz pick-up.

Furthermore, though you can measure voltages between the earths of different mains outlets, they should equalise when you plug-in the coax. You should find a very much smaller potential is developed if you connect (say) a 1k load resistor between them.

Are you sure the coax outer-braid connections are sound? Could it be RF crosstalk that you're seeing? Can you try it with the cables connected, but with no signal on one of them?

Reply to
Andrew Holme

Thanks for all your comments so far:

Further information: forgot to mention that the cctv o/p(s) are (2) 1v p-p video, which is then fed via scart input to old vhs x 2 to RF modulate, before being sent downstairs via the RF cable.

Upon further investigation, even altering rf o/p channel does not remove diagonal patterning from feed down UHF cable to downstairs.

This is really obvious when the cctv rf feed is fed downstairs, into the aerial DA, without the aerial feed into the DA, seems to be reduced somewhat by combining at the aerial DA i/p, with the UHF normal earial

Could this interference be similar to old radio IF's beating in the 2 x VHS recorders.??

I've not had this problem before with 2 vhs in series, is it possibly because same mains used for 4 x cctv psu, 1 x cctv quad recorder, and 2 x vhs machines, with seperate mains feed downstairs, though I don't see why it should.

The volts, AC & DC, are co-incident, but probably not related to the interference, and they seem to do no harm to aerial DA i/p, but it would be nice, and safer to my hands, to remove these.

This is presumably caused by floating or whatever RF earth o/p's on the vhs's

I have come across a circuit on the internet, which suggest 1x 10nf 250v capacitor in series with both legs of the RF cable should do the trick in removing the volts, any comments on this

TIA

Reply to
UHFPD

"The" RF cable - singular - i.e. is everything modulated onto one coax? Just checking.

This paragraph is unclear. A diagram might help.

Sort of: it could be intermodulation distortion (IMD) in the DA or the final television receiver input.

Is the DA meant to handle UHF modulator level signals? Does it have multiple inputs? You're not using an aerial splitter / combiner into its antenna input are you?

I doubt it has anything to do with the mains.

By "2 VHS in series" I take it you mean daisy-chained via UHF in/out. So you weren't using a DA then? Hmmm.... Can you post a link to spec / info on this DA.

What is the angle of the diagonal pattern? How many fringes do you get across a TV line? What channels were the UHF modulators operating on at the time? Does altering the UHF channels change the pattern?

Reply to
Andrew Holme

Both CCTV RF and Aerial connected gives less patterning than CCTV RF on it's own

Yes

How many fringes do you get

approx 30

What channels were the UHF modulators operating on at the

No

Reply to
UHFPD

PS combining both signals now I have moved the daisy chained VHS's downstairs, feeding them from the 2.4ghz link, with the house aerial does NOT give any interference problems !!

Must be something to do with the loop between downstairs and upstairs earths from mains / possible inductance into CCTV RF Cable from upstairs to downstairs, though I doubt this as the interference signal is so strong on it's own.

Possibly something to do with CCTV HD recorder video o/p, VHS video i/p's, and RF cable earth ??

Reply to
UHFPD

coax?

not

downstairs.

into

UHF

RF on

have

into its

upstairs

in/out. So

spec / info

Have you tested the cable run (inner and outer) for continuity/shorts?

Reply to
Andrew Holme

Yes..

Reply to
UHFPD

its

So

info

Reply to
Larry Oravetz

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