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Author Topic: TVI  (Read 500 times)
KC0QEV
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TVI
« on: January 05, 2004, 06:50:59 PM »

I have problems with TVI on ch 3 (not on CATV network)and 2m. I have grounded the entire entertainment system (Sat, TV, VCR/DVD, Stereo/Amp). When I transmit the screen gets a greyish tint and the audio is weak. The TV ant is on the 40 ft level of my 50 ft tower. I have a homebrew 2m grnd plane at the very top. I have checked the coax for leaks and found none. CH 3 is the only one effected (never bothers ch 18 which I have found to be in the amateur band). My dad wants me to have this fixed before superbowl. HELP.
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G7HEU
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TVI
« Reply #1 on: January 05, 2004, 07:38:36 PM »

Hello


You need to take a systematic approach to the problem and eliminate possible causes one by one. It's either excessive spurious transmissions or poor rejection by the T.V. set-up. Or a bit of both :-(  

You are not clear about what transmissions are causing the prob, HF or VHF? More precisely what bands? Also what power levels?

In the mean time, you mention VCR and DVD players. Have you tried connecting the T.V. antenna direct to the set without those additional boxes and cables?

The coax cables supplied with those things is usually very cheap and will be poorly screened. You might find that removing them from the circuit cures the problem. You can then direct your efforts in that area with better coax, band pass filters etc.

Have a think about the above and report back. If nobody smarter has turned up here in the meantime I'll be happy to help!

By the way, I'm in the U.K. and know nothing about the Super Bowl. How long have you got before it starts? :-)

Steve
M0HEU / G7HEU.

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K0BG
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TVI
« Reply #2 on: January 05, 2004, 09:41:19 PM »

Tell you dad to buy a new TV set. A lot of the older models (anything older than 5 years or so) just don't cut it in the presents of RF. My 15 year old Sony not only gets upset when I transmit, it throws carriers all over the ham bands. My 2 year old digital doesn't even know I'm on the air.

Alan, KØBG
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WE1X
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TVI
« Reply #3 on: January 05, 2004, 09:53:49 PM »

Don't have my TV channel frequency list handy, but wonder if Channel 3 falls near the 2m band.  Even if not, I suspect the TV is getting overloaded by your 2m signal.  If that's the case I suspect there is little that can be done other than trying to locate a VHF high pass filter for the TV.

Have you tried transmitting at various power levels and noticed any variation in the TVI?  What mode?

Harry WE1X
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WB4QNG
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TVI
« Reply #4 on: January 05, 2004, 10:01:57 PM »

You might check for a 19 inch hook up cable somewhere in the system. Sometimes this will act like an antenna for two meter and give you fits. While it is probaly in the TV  I know what it is like telling a non ham that his TV or Radio is a piece of junk even though it has a perfect picture on it when your not on the air
Good Luck
Terry
WB4QNG
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WD5KCA
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TVI
« Reply #5 on: January 05, 2004, 10:08:49 PM »

No, don't recommend he buy a new TV.  My old GE set was just fine with ferrites on the cable and power cord.  My new Sony is terrible.  I haven't been able to lick the problem except by moving my antenna.

A signal on 20M turns the set off!!  (pisses off XYL)
-Harold
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AA4PB
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TVI
« Reply #6 on: January 05, 2004, 10:48:57 PM »

The problem is that you probably do not have a good RF ground at 2M. We tend to think that simply hooking up a ground wire will solve all sorts of problems. The problem is that even if you have a very good ground at one end of a wire, 1/4-wavelength away (about 19-inches in the case of 2M) it is not ground to the RF.

First, you need to determine how the RFI is getting into the set. If it goes away when you disconnect the antenna lead then it's entering via the antenna port. I've had good success with a 1/4-wave open end stub for 2M. Use 75 ohm coax. Connect the stub in parallel with the antenna wire using a "T" connector. Make the stub 1/4-wavelength long (about 19-inches) on your 2M operating frequency. Leave the far end open (no connection between center and shield). Since the far end is open (high impedance), 1/4-wavelength back at the antenna connection it will be a short (low impedance) for the 2M signal. Since the stub is not 1/4-wavelength long on the normal TV channel frequencies it does not represent a low impedance to those frequencies and permits them to enter the set normally.
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N6AJR
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TVI
« Reply #7 on: January 06, 2004, 02:28:00 AM »

with the antennas just above the tv antenna, you might be overloading the front end of the tv, ( your signal is stronger than the one the tv station is sending)and all you can do is either move the antennas apart, or lower the power.. always do the easiest fix first..
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AC4FD
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TVI
« Reply #8 on: January 06, 2004, 09:54:58 AM »

I think that the tuning stub sounds like a good Idea, but it may not work.

As you know, 2 meters is 144-148 mhz.  Channel 3 on your television is 60 - 66 mhz. So if an overload is occuring one my question why it is only happening on Channel 3.

One suggestion that I would have is to try and use the Tuner on your VCR instead of your Television to tune to the channel that you wish.  Maybe setting the VCR to channel 4 (so that the VCR works when the TV is on channel 4) and then changing the channel on the VCR to channel 3. I have seen this help on an older Television when I was receiving interference from a local Police or Fire Dept.

You may want to try some other experiments.  Can you locate your radio close to the Television in question with a portable Antenna and see if the interference occurs.  Or how about using a Handie Talkie.  These types of experiments may help to isolate the cause of the intereference. Maybe your radio is putting our spurious emissions in the 60-66 mhz range or which is a harmonic there of.

Good luck, and let us know if you find the problem and how(if you are able) to solve it.

73
Randy
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W9WHE
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TVI
« Reply #9 on: January 06, 2004, 05:05:28 PM »

Simple steps:

1) Disconnect the TV antenna, which should be fed with QUALITY RG-6 coax and CLEAN, corrosion free connectors. Does TVI go away? If yes, apply a high pass 75 ohm filter to TV coax just as it enters the TV. (same for VCR and everything else connected)

2) If TVI still present with no antenna, then you need to filter the AC line comming into the TV. Look at Corcom AC line filters fwith the highest attenuation for the frequency of the TRANSMITTER. (same for everything else connected)

3) External connecting cables must be shielded.

4) Apply torroids to all interconnecting cables.

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EI2IV
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TVI
« Reply #10 on: January 06, 2004, 05:31:56 PM »

Hello

I would go with N6AJR's suggestion first! It will save you a lot of time & grief if it works.I would also try another borrowed transceiver and TV set to eliminate yours as the cause, before you start searching for the cause.
 
TVI is a very comprehensive subject and all cures and preventions will not be answered here.
I would move the TV antenna as far as possible away from all your radio feed lines and antennas. this may solve the problem for you.
 
There are several causes of TV breakthrough just to list a few.
 
1 Pick up on TV antenna lead.
2 pick up on the TV mains lead.
3 Pick up directly on the TV antenna.
4 Pick up on the internal wiring of the TV set ( a real pain in the neck)
5 Direct conduction on the mains earth if shared between TV and transmitter.
 
As someone else posted just connect the TV to the antenna without VCR's etc just to see if the TV and antenna are the source.
Try some ferrite rings or and snap on ferrite chokes on the TV mains lead and the antenna lead. you will have to experiment with the amount of turns and position on the leads etc.
 
If you have a TV mast head preamp you may also be overloading that.
 
So as you can see the subject is endless, I would seriously consider buying a book on the subject by the likes of ARRL etc. also ask some of your local hams they no doubt would have had similar experiences to you with the same type TV antenna system channels etc.
they may also if you are lucky have some spare filters made up just for your precise problem!!!
 
Also do a search in Google groups for TVI there are a lot of useful hints to be found there.
 
Hope you get it sorted in time fer the superbowl!!


73

James
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KC0QEV
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TVI
« Reply #11 on: January 07, 2004, 06:13:29 PM »

I have read thru all the replies and thank you. I want to clarify that all of the divices are connected with A/V Cables and not coax. I have it set up to use the tuner in the vcr and the tv stays on the "VIDEO" ch. I am using a 5watt ht connected to the ground plane. It got worse with the ice we just got. It is also picking a 2m repeater 28 mi away on ch3. I tried another newer tv and the same thing. If someone thinks a picture of the setup would help i might be able email one. thanks and 73s Matt, KC0QEV
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G7HEU
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TVI
« Reply #12 on: January 07, 2004, 06:31:57 PM »

Hello again

Does 28mi mean 28 miles? If so, and you are sure it was as bad with another t.v. I think that thing underneath your t.v. isn't a DVD player but a 2mtr repeater.

Sorry to be flippant but at least I didn't tell you to get the old man to buy a new t.v!

There's something very wrong here. Clever guys, what do you think?

Steve
M0HEU / G7HEU.
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G7HEU
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TVI
« Reply #13 on: January 07, 2004, 06:34:07 PM »

A thought.

What makes you sure it's that other stn 28mi away?

Does the inteferance correspond with activity on that repeater? Are you certain that it's not being caused by something else nearer to home?

Steve.
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KC0QEV
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TVI
« Reply #14 on: January 07, 2004, 09:39:45 PM »

I can hear the audio of a lot of 2 meter stns on the tvs in our house. I don't think we will have a tv ant much longer. I have thought about using it on 2m. The tvs work fine when viewing satilite and we have lived with only that for 2 years. I think we may be able to do without local tv for a few more (before I move out after high school, my dad can't wait).Thanks and I hope ot learn more on the subject and maybe somebody else might get a use out of it.

73s Matt, kc0qev
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