Electronics > RF, Microwave, Ham Radio

Digi TV distribution amp suddenly picking up TVI? Possible?


Chris Wilson:
I have worked all the HF bands with my TS-590 and a horizontal quad wire loop and ATU for years with no sign of TVI, but after a period on LF I have found I am either locking the picture frames or causing a totally black screen on all 4 TV's in the house. They are fed from a shared indoor, loft mounted, broadband high gain aerial, and a mains powered distribution amp about 10 feet from the aerial. This feeds the TV's down separate, all indoor, co-ax cables.

The amp is amplifying, virtually no channels are available with it either bypassed or turned off. Could anything happen to this kind of distribution amp making it prone to RFI? This interference occurs on all bands from  80 meters to 50 MHz, at anything over 30 Watts. Some bands seem to cause this TVI more than others, only 160 meters seems immune. I have looked at the signal on all bands from my TS-590 by transmitting into my HP-8568B spectrum analyser, on low power, via an attenuator, and no spurious signals or other anomalies are apparent. My signals, on digital modes and SSB are reported as clean and with no visible issues on SDR panadptor displays. SWR is perfect, the loop antenna shows low resistance and seems perfectly terminated.

Thanks for reading

I'm not really answering your question about what might happen inside the distribution amp, but I'd rule out all the easy stuff first.

Same ham antenna and cabling as before?  Same antenna cable routing?

Any new coax or rerouted coax in the TV distribution?

Is the distribution amp grounded? If so, check/refurbish it to make sure it is a good quality ground. 

If the amp is accessible I'd start disconnecting the legs one at a time. Maybe the ground connection on one of the distribution legs has degraded. 

How is the distribution amp powered? Can you substitute another power supply as a test? Maybe the RFI is getting into the power supply.

Chris Wilson:
Some good points there, hadn't considered the distribution amp ground. It's mains powered, with its own internal PSU. I will check if the standard UK 3 pin mains plug uses the ground pin and if it's OK. I connected the TV socket in my shack to my spectrum analyser, and the UHF signal (I used channel 23) disappears as soon as I TX a carrier. If i bypass the distribution amp and connect the TV aerial direct to the socket the lower level signal is unchanged by my TX'ing, so I am now 90% sure it's the amp or the cabling to it, which I will go through carefully. Without the distribution amp I can only receivbe the strongest signals and lose many channels. I will put the aerial on the chimney in due course, but would rather sort this new issue first. Sod's law says the amp is difficult to access as in in between supporting walls in a recess....

No changes to the TV stuff or my ham stuff, so I think something has just decided it's time to give some grief ;) Thanks again klunkerbus!

Amp power supply will not be connected to mains ground, so add a ground to the cable shield, and use the old favourite of some ferrite toroids on the cable, either those nice split ones or an old ferrite LOPT core. Likely the connections on the amplifier have corroded slightly, making a high resistance connection to the cable shield, so unfortunately you really want to undo each cable connection, clean up a little and redo them again. That should solve it.

Chris Wilson:
Some more info below, I checked the amp and its original moulded on on mains plug has a plastic earth pin. It was fine for years, rather than re-type a huge amount i copy paste from a UK low frequency group, so if seems a bit odd  that's why, my typing speed is pitiful! Thanks Sean, are you saying use mains earth for the co-ax screens, which are all going to the internal tin RF shielded box, that's well soldered together so it would be hard to see what's within? If a new amp fixes it I am still interested to know what caused this one to go funny... Cleaning (in fact re-making) all the Belling Lee co-ax plug terminations made no diffrence, and as the thing is in a warm, bone dry loft there's no sign whatsoever of dirt or corrosion anywhere. none of co-ax feeds run outside, either.


Long story, but since stopping LF 136kHz TX for a while in lieu of
some HF working all the digital TV's in the house freeze or totally
blank on all bands above top band, up to 50MHz when I transmit at more
than about  5 or 10W. I have a loft mounted UHF TV aerial feeding an
amplifier cum distribution box. It has an internal mains PSU. Without
it we receive hardly any channels. It's been fine for years, and I
have never had TVI. My HF set up is unchanged from when I had no
issues. I put the TV antenna co-ax direct into my SA and on circa 531
MHZ see a broad digiTV signal that's strong. If I TX on say 20 meters
at high power into my antenna I see zero change in the TV signal. I
have also fed the output of my HF TX (TS-590 Kenwood) into the SA via
an attenuator and the output looks spotless on all bands. However, if
I connect the TV aerial amp / distribution box up and look at the
output from it the TV signal immediately drops into the noise when I
TX.  My neighbours have no issues at all.

This seems to have occurred since I have been active on LF, and I had a
stage  where  full power would trip the main RCD in the consumer unit,
so  RF  was  getting  into  the mains. Is it conceivable something has
occurred  to perhaps a mains filter in the TV amp? I looked inside the
plastic  case  and  it  has  a small mains transformer that feeds feed
throughs  into  a  screening  can  with all the RF stuff within it. If
there is any mains input filtering it must be on the low voltage side
of the transformer. The unit has a moulded on three pin mains plug, but
with a plastic earth pin, so the unit is not grounded.

Is  it  possible to make a mains input filter for a piece of equipment
that would filter 136kHz but not affect normal 50Hz mains operation?

Has anyone experienced anything like this?

I  have  ordered  a  new amp / distribution box of known make, and will
just  try  replacing it, but would also like to know if my LF activity
could have done this?




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