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- Vintage TV & Radio Tech Forum (http://www.videokarma.org/forumdisplay.php?f=170)

- - Ground Loop "Hum" Using BT Modulator (http://www.videokarma.org/showthread.php?t=260707)

vts1134

02-16-2014 06:26 AM

Quote:

Originally Posted by N8NM (Post 3095825)

You might try disconnecting it where it enters the house and see if you get any improvement. Loose/bad cable connections can allow signals to leak, and these wouldn't have gone away when you cut power at your breaker panel.

It's kind of a long shot, but I've seen it before.

That was the first thing I tried. I run into this ground loop problem all the time at work, so is it was the first thing I thought of.

Quote:

Originally Posted by zenith2134 (Post 3095827)

Why are you attempting to broadcast on the lowest channel possible (channel 1)? AFAIK, the B.T.s cannot do broadcast at over the air 1...Even if they could, I would choose the highest channel allocation for the band..since any power-supply oscillations/anamolies would be furthest away from modulated signal space at a higher channel. I like 12 and 10 personally.
I do not care about other channel's digital transmit realm at all.......We are dealing with low power. it is an FCC "intentional RF radiator", true. But so is my garage door opener and my 802.11 N router...

Some of my sets only have three channel tuners so I have to use 1, 2, or 3.

zenith2134

02-16-2014 01:05 PM

Electronic M, do you have any factual evidence to confirm the statement regarding 'better DX ability' of lower frequency channels? It seems to me that the opposite would be true.

Electronic M

02-16-2014 04:31 PM

I've read it on TV DXing sites that the lower channels/frequencies tend to want to skip like Shortwave radio signals where as high band VHF and UHF are more of line of sight propagation. My sets are all over the house so something that is more line of sight would probably be attenuated by walls and floors.

zenith2134

02-16-2014 06:19 PM

Interesting! I never had much luck with TV DX, but have received FM DX in a big way.
When I set up my first modulator, I tried finding a 'signal propagation by frequency' chart on the net and couldn't find much info on the subject.

vts1134

02-21-2014 03:08 PM

I received my new modulator and it too has the same symptom when broadcasting on channel 3, so I gave up the dream and hardwired all of my sets. On the plus side they look FANTASTIC with a hardwired signal. I have most of them completed now, and will hopefully wire the remaining sets this weekend. I am not quite done with my modulators though :scratch2:. I started a new thread that might be interesting here http://videokarma.org/showthread.php?t=260846

old_tv_nut

02-21-2014 04:03 PM

Quote:

Originally Posted by vts1134 (Post 3096303)

This still seems to be something like a switch-mode power supply that radiates in your channel. Really, you should take a portable TV and go around your place to see where it is strongest. You may be able to locate it and fix it. Worth the effort if you don't have to hardwire more sets in the future.

vts1134

02-21-2014 07:28 PM

Quote:

Originally Posted by old_tv_nut (Post 3096310)

This would only be possible if it were A: battery powered, or B: in a neighbor's home as I have killed power to every last thing in my house except modulator, source, and monitor with no change.

jr_tech

02-21-2014 08:16 PM

Have you tried any ferrite filters on the line cord or the coax output of the B-T? They work well for keeping undesired signals out of equipment.
http://www.amazon.com/Ferrite-Core-C...ferrite+filter
not affiliated,
jr

ChrisW6ATV

02-23-2014 02:27 PM

I did not see this specifically mentioned in this topic, so here is one thing that may have worked with this problem for me: Try plugging the modulator into one of those "grounding adapters" that have only two prongs, to eliminate the ground connection from the modulator to the building. Then, use a DVD player or VCR (also with its two-prong cord) as the signal source. (Do not have the VCR connected to any cable box or antenna system, just standing by itself.) Let me know if you tried this already.

vts1134

02-24-2014 07:54 AM

Quote:

Originally Posted by ChrisW6ATV (Post 3096474)

I tried this as well.

cbenham

04-16-2014 04:22 PM

Quote:

Originally Posted by N8NM (Post 3095807)

That definitely sounds like a local interference problem. By any chance, is your house connected to cable TV?

AND, by chance is there a local *digital* channel 3 broadcasting within 15 miles of your house? If I missed this in someone else's previous post I'm sorry. I read the entire thread.

cbenham

04-16-2014 04:34 PM

1 Attachment(s)

Quote:

Originally Posted by old_tv_nut (Post 3095537)

See the section on "feeding a dipole antenna"
http://en.wikipedia.org/wiki/Dipole_antenna

I presume your matching transformer is 75 ohms (your coax side) to 300 ohms (your dipole side)? According to the above you should not be using a 75 to 300 ohm matching transformer unless you use a folded dipole.

I built a 75 ohm FM antenna out of 75 ohm coax just as given in the first figure of the wiki attachment. I did include a triple loop right at the antenna to provide some isolation between my flea power transmitter and the antenna.
It's a pretty efficient antenna, scared me when I drove around the neighborhood and could pick it up almost a mile away! I lowered the output power with a 10dB coax pad. :no: Attached is a picture.

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