Stopping volts going down UHF cable ??

[UHFPD]

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

 

[Chris Dugan]

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

 

[Andrew Holme]

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.

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?

 

[UHFPD]

Help !!

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

 

[Andrew Holme]

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.

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

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

This paragraph is unclear. A diagram might help.

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

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'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.

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?

 

[UHFPD]

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



This paragraph is unclear. A diagram might help.

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

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?


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.
Yes

What is the angle of the diagonal pattern? 45 degs

How many fringes do you get

across a TV line?

approx 30

What channels were the UHF modulators operating on at the

time? Does altering the UHF channels change the pattern?

No

 

[UHFPD]

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

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 ??

 

[Andrew Holme]

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 ??

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

 

[UHFPD]

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

Yes..

 

[Larry Oravetz]

Make sure the signal levels of all channels are the same level, the aurial
carriers are not too high , bandbass fitler the offending channel and then
make sure the cable "tilt" is nearly flat before amplifying any of them.