[CQ-Contest] Stubs

Tom W8JI w8ji at w8ji.com
Thu Jul 19 19:36:47 PDT 2012

> On 7/5/2012 11:11 AM, Joe wrote:
>> The Stubs on this page
>> http://www.k1ttt.net/technote/k2trstub.htm
> More on these stubs. Say one station is on 15 and another is on 40.
> There is no way for the 15 station to block the 40 station signals is
> there. All one can do is to insert a 15 stub on the 40 station to try to
> block any signals true?
> Or is there a way to block with a stub the 40 meter signals without of
> course also killing 15 because of the odd multiple thing?

The correct combination of series lumped component and 1/4 wave 15M stub 
could make an open on 15 and short on 40.  RG6 it would take a ~500 pF cap 
in series with the center conductor on a 15 meter shorted stub to notch 40 
out. You'd have about 1.5 ohms across the line on 15M because of stub losses 
on 40.

The mechanism would be the inductive reactance of the 15M stub on 40M would 
be series tuned and canceled by the 500 pF cap. On 15M, the shorted 15M stub 
would look like 6700 ohms, the cap can be ignored because the capacitor's 
reactance is a fraction of 6700 ohms.

On 15M, it would still appear open....about 6700 ohms. With 1500 watts on 
15M you'd have about 11 watts of heat in the 15M stub, with most of the heat 
near the shorted end.  That might overheat the stub.

Stubs are not particularly high Q. A good tuned circuit is really worth 
looking at. You could do the same thing with two capacitors and an inductor 
with a Q much less than 100! That's a pretty poor inductor. Maybe some old 
scrap wire on an old Oatmeal box. :-)

73 Tom 

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