Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hplsla!tomb
From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: How do I make a notch filter with cable?
Message-ID: <5170047@hplsla.HP.COM>
Date: 21 Jul 89 14:59:43 GMT
References: <4436@merlin.usc.edu>
Organization: HP Lake Stevens, WA
Lines: 26

cyamamot@nunki.usc.edu (Cliff Yamamoto) writes:
>
> (stuff deleted about trying to notch out a freq. with a stub)

>         -----------------------------------------------------------
>Antenna  ------------------------------+---------------------------- Receiver
>         ----------------------------+ | +--------------------------
>                              ^      | | | \
>                              |      | | |  \ 
>         The length of this   |      | | |   \___location of BNC "T" connector
>         "stub" is cut to the |      | | |       important??
>         wavelength of the    |      | | |
>         offending frequency  |      | | |
>                              v      |_|_| <---Short center conductor to shield
>
>The pager frequency is 152.4 Mhz.  So using the ol' ARRL handbook I get a
>wavelength of 1.96 meters (length of the "stub").
>
Yes, this will work, but just use 1/2 wavelength stub shorted as shown, or
1/4 wave open on the free end.  The 1/2 or 1/4 wave is IN THE TRANSMISSION
LINE, so if you are using std. polyethelene insulated coax (not foam), multiply
the lenght you get for free-space by about .66; see coax refs for other types
of insulation.  Note that this will have attenuation quite a ways away
from the design frequency (not a super-sharp null, though pretty deep with
good coax).  Should not be very dependent on location along the line,
if the line is operated at low SWR.