| Author |
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DeadlyEyes
| | Posted on Monday, June 23, 2003 - 8:35 am: | 
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A TVI tip. If you have a TVI problem pehaps it is your gound. You do ground your radio don't you?
Well here is a grounding tip. Rather than using a short piece of copper wire you might try using what I call a mobile coax trap. Get an old piece of mobile coax long enough to reach from your radio to your grounding point and two caps of say about .01 mfd or so. The value of the caps is unimportant as long as they are non electrolytic and of. Strip the coax to the center conductor and expose a bit of the shielding at each end. Solder a small value non electroletic cap between the shield and the center conductor at each end. Ground the radio using the center conductor.
The caps and the outside shielding together combine to form a kind of frequency trap. I saw this in a very old ham book and well, it worked for me. |
Kd4amg
| | Posted on Monday, June 23, 2003 - 9:45 am: |
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great idea now i got some use for that old white rg - 8 that dont work too well... thanks a lot !! |
Tech833
| | Posted on Monday, June 23, 2003 - 11:20 am: |
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A cap between the center and the shield, then ground the radio to the other end of the coax center conductor? Did I get that right? No specific length time the velocity factor? Ummm... Hams don't know everything. Hams are worse than CB antenna makers in that they believe in 'voodoo'. Even if you 'tuned' that coax, you would have a bazooka filter section and that would not be the proper way to hook that up.
There is no substitute for grounding. Even WITH grounding, if your radio is producing harmonics or spurious emissions on or near television frequencies, you will have TVI.
If you install a TVI filter on an otherwise clean radio, you will still have TVI unless the filter is well grounded. Otherwise there will be noplace for the shunt to drain. |
Bluesman
| | Posted on Monday, June 23, 2003 - 1:04 pm: |
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And always use a SOLID wire conductor for ground.. |
Kc0gxz
| | Posted on Monday, June 23, 2003 - 4:54 pm: |
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Tech833
First of all, I don't understand how a untuned random length of coax with a couple of caps are going to filter anything.
And I am wondering about your TVI filter statement. Are you refering to a "low-pass" filter? If so, am I to understand that a low-pass filter WILL NOT work unless it is grounded?
I think I may have missed something here. I'm confused. But then it doesn't take much to confuse me these days.
73s
Jeff, kc0gxz. |
Tech833
| | Posted on Tuesday, June 24, 2003 - 12:28 am: |
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Jeff,
Like I questioned above, an untuned piece of coax with a couple of caps will do nothing. Thus, my question as to what the statement was about. I was trying to understand.
A TVI filter to the CB market is also known as a low pass filter. I am saying that if you run an ungrounded low pass filter (with no drain for the shunt) then you will not completely solve TVI, if the TVI is caused by transmitter harmonics. |
Simon
| | Posted on Tuesday, June 24, 2003 - 9:09 am: |
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DE
What book did that come from?? I'd like to read the article as like Tech833 says it seems to go against all Ive learnt about filtering RF.. |
Tech833
| | Posted on Tuesday, June 24, 2003 - 10:54 am: |
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I have to filter TVI (and possible two-way, aeronautical, etc.) interference out of mega-watt ERP radio stations. If I fail, I don't eat. I know a thing or two about filtering transmitters. |
Kc0gxz
| | Posted on Tuesday, June 24, 2003 - 2:10 pm: |
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Tech833
OK. I'm convinced. You're the man. I won't be questioning your statements again.
I've been in this business a long time and if I was still in it another 30 years from now, I still wouldn't have half the knowledge that you do. I'm sure you have forgotten more than I will probably ever know. And besides, I would have a constant headache carrying around and trying to remember all of that knowledge.
In my earlier post to you about grounding the L-P filter, I hope you didn't think I was questioning your intelligents. I would never do that. Also, I AM aware of your background and education. I know the only way you and a few of the other Techs on Copper's forum could have possibly gotten to where you are today is through years of studying theory, self-disipline, hands-on, trial and error, and great sacrifice. I admire you for your accomplishments.
73s
Jeff, kc0gxz. |
Tech833
| | Posted on Tuesday, June 24, 2003 - 10:52 pm: |
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Jeff, I thank you for your kind words. Please don't feel like you can't ask me anything or question my answers. If I need to be questioned, or did not explain something in enough detail, please ask!!
If I don't know about something, I stay quiet. I like to read about the things I do not yet know. If someone asks a question and I know (for a fact) that I have the correct, accurate answer, then I open my mouth (keyboard).
You will notice, most of the time I am quiet. What does that say about me? |
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