Ground Loop "Hum" Using BT Modulator

[cbenham]

Originally Posted by old_tv_nut

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. Attached is a picture.

[old_tv_nut]

Just so we are all thinking of the same thing, is the following correct?

1) coax output of modulator -> 75/300 ohm transformer -> TV monitor 300 ohm input terminals: picture OK
2) coax output of modulator -> any kind of antenna; some kind of antenna -> TV antenna input: hum bar

Is that right?

[vts1134]

That is correct, except you can also add coax output of the modulator to coax splitter (main splitter for my house) to coax input on a modern flat panel in the "picture ok" camp.
I've uploaded a video to youtube to show what it looks like on my test monitor https://www.youtube.com/watch?v=bBi1uZ7j0ng.

[old_tv_nut]

Same hum bar on any other TVs using an antenna? How do you know it's not your TV?

[vts1134]

Should have also added coax output of modulator -> any kind of antenna; some kind of antenna -> TV antenna input on modern flat panel: hum bar
Same problem on all TVs in the home, vintage and modern alike.

[zenith2134]

Hiya John--sorry to hear of the continued woes. My humbar was different than the one on your video link. Perhaps it is not a transmission issue, but a circuit issue in the B-T. It would be interesting to see whether or not a different B-T unit would have the same fault, in the same room and set-up. Could the interference be external? (doubtful I feel)

[old_tv_nut]

Just had a chance to look at the video - something in a power supply somewhere is oscillating on your channel frequency. probably radiated, but might also be carried on the power line.

1) can you see it in the "snow" when your transmitter is off?
2) if you have a small portable TV with rabbit ears and can see it on that, go snooping around your house for where it is strongest.
3) you said you tried turning things off - how about light dimmers, compact fluorescent lamps, anything on the AC line with a clock display that runs even though the main function is off (like a microwave oven?).

Edit: how about your furnace?

[vts1134]

Originally Posted by old_tv_nut

Just had a chance to look at the video - something in a power supply somewhere is oscillating on your channel frequency. probably radiated, but might also be carried on the power line.

1) can you see it in the "snow" when your transmitter is off?
2) if you have a small portable TV with rabbit ears and can see it on that, go snooping around your house for where it is strongest.
3) you said you tried turning things off - how about light dimmers, compact fluorescent lamps, anything on the AC line with a clock display that runs even though the main function is off (like a microwave oven?).

Edit: how about your furnace?

I actually killed power to my entire house at the breaker box with the exception of the dryer's 110 outlet. To this outlet I plugged my monitor, my modulator, and my source. If there is interference from another device it is either battery powered, or at my neighbors house.

Originally Posted by zenith2134

Hiya John--sorry to hear of the continued woes. My humbar was different than the one on your video link. Perhaps it is not a transmission issue, but a circuit issue in the B-T. It would be interesting to see whether or not a different B-T unit would have the same fault, in the same room and set-up. Could the interference be external? (doubtful I feel)

This is the worst I can get the hum bar to look. I adjusted antenna placement in transmitter and receiver and stood between the two to get it to this point for the sake of showing it on video. In my frustration I bought another modulator last night. There happened to be one on eBay for $39 including shipping. We'll see if it exhibits the same symptoms.

[old_tv_nut]

Let me repeat,
1) can you see it in the "snow" when your transmitter is off?
This would prove it's from something else.

[jr_tech]

This is really a WAG, but I see diagonal lines in the "hum bars". Counting the lines as best I could, there are about 70 or so across the screen. 70 times 15750 (the horiz line rate) gives about 1.2 Mhz. Perhaps this is an indication of another strong signal, like a radio station contributing to this mess?
jr

[lnx64]

My same model Blondertongue also needs new caps. Mine are buldging actually.

[N8NM]

That looks a lot like an interfering over-the-air signal. I'm not familiar with the BT devices; is it possible to change it's channel and see if anything changes?

[vts1134]

I can't really tell if the problem persists with the transmitter off, I don't see the hum bars in the static, but I'm not sure I would see them if they were there. I did change the broadcast channel from 3 to 2 and the symptom disappeared. It will be interesting to see if the new modulator has the problem with channel 3 or not. Does any one know if you can get these modulators to broadcast on channel 1?

[N8NM]

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

[vts1134]

Originally Posted by N8NM

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

It is. No analog stations left though so I simply have it for interwebs.