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Re: false signals-images-harmonics-whatever? FIXED !!



On Friday 04 January 2002 09:04 pm, Jens Schmidt wrote:
> Hi Tracey,
> is it a secret how long the stub need to be in Wavelength ??
> 73 Jens    ZL2TJT
>
> Tracy wrote:
> > snip ....
> > 1.  I installed a coax "T" off the helix and connected a 5 1/2" coax stub
> > off the extra end of the "T".
> > 2.  I monitored the image signal strength while trimming little pieces
> > off the coax stub.
> > 3.  After about 20 cuts, the image became inaudible.
>
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>From the foggy depths of my morning mind, I think it is a quarter-wave open 
stub, acting like a short circuit. At 70cm this calculates out to 5.4 inches, 
with a velocity factor of .80

The idea is to "notch" the 435 mhz offending signal without screwing up the 
2.4 gig desired. It's a neat trick...and you can avoid cutting by using an 
1/8 wave line with a small tuning cap across the end and then tune for the 
null. (I think it was an 1/8 wave ...it's been a long time since I did it).

You can't make a stub like this by calculation. The way Tracy did it is the 
only way that works (without test equipment), i.e., make it a bit long and 
trim back for the null. The null is fairly broad. 


Congrats on getting it fixed! 73
-- 
hasan schiers, N0AN
schiers@netins.net
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