I don't believe you took the velocity factor into account for your 1/4 wave
stub. Probably 66-75% of the free space 1/4 wave length calculated
depending on cable used. Have an antenna analyzer? The instruction
book may give you methods to do this by trimming until at the freq you
need.
Dan KI6X
On 6 Dec 2000, Jon Ogden wrote:
>
> OK all,
>
> I've done some more experimenting with my TVI RFI problem I mentioned the
> other day.
>
> I've tried all sorts of ferrites, filters, etc. All to no avail in getting
> the last little buzz and overload conditions (ch 2 & 6) from occuring. Not
> much change in anything except for one case.
>
> Someone mentioned to put a 1/4 stub at 10 meters in the line. They
> suggested I could use one of the 2:1 splitters for CATV use and hang the
> stub off of one of those (split the input cable between the cable to the TV
> and the stub). Well, a 1/4 wavelength at 10 meters is about 103 inches or
> so. So I tried it. Nothing. However, as I moved the cable around, I
> could cancel out the buzzing sound I got while keying the TX. The overload
> picture problem was still there, but I could make the buzz go away. I
> tried shortening the stub to see if it made a difference, but it really
> didn't. I've ended up running both coaxes (the regular and the stub) into
> the TV cabinet and ran the stub once inside across the back of the TV. The
> buzz is nearly gone. You can't hear it unless you pay attention to it
> really hard.
>
> This is great, but my question is, WHY? What sort of cancellation is going
> on here? It also seems to make no difference if the stub is open or
> shorted as well.
>
> So what gives?
>
> Additionally, the overload/harmonic problems occur on channels 2 and 6
> whether or not I have the antenna coax connected. Yes, I am filtered at
> the output of my power amp. I have a 3 KW ICE bandpass filter that has
> very sharp roll off after 30 MHz.
>
> I just want to know if anyone has a clue of why moving the position of the
> stub around would change the noise I am picking up.
>
> ANY ideas?
>
> 73,
>
> Jon
> NA9D
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Member: ARRL, AMSAT, DXCC, NRA
>
> http://www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
>
>
> --
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> Submissions: rfi@contesting.com
> Administrative requests: rfi-REQUEST@contesting.com
> Questions: owner-rfi@contesting.com
>
-- Dan Violette Amateur Radio: KI6X E-mail: danki6x@pacbell.net --
--
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