Jeff: please sweep the other stubs you mentioned, so
we have something for comparison. Might as well do all
the varieties possible, and tabulate the results at
both 900 and the FM band.
Considering how easy it is to make such traps, perhaps
a short article could come out of it? You could add it
to your busy schedule.
Bob M.
======
--- Jeff DePolo <[EMAIL PROTECTED]> wrote:
>
> > My coax is RG-8M, similar to RG-8X. I couldn't
> find a
> > published velocity factor for M, but thought I
> remembered
> > .87. RG-8X is .84 (not .66). I cut a piece off an
> old scrap
> > cable (with connector) to 41 inches, measured from
> the tip of
> > the PL-259's center pin to the cut point, as a
> starting point
> > for trimming. When I stripped and shorted the end,
> (now about
> > 40.75",) and got it on the T-connector, I found it
> was
> > already showing a "center" of 146.15 MHz. Even
> with a .84 VF,
> > this is much closer to a half-wave than a 1/4- or
> 3/4-wave.
>
> Paul,
>
> Curious who makes the RG-8M you have.
>
> Anyway, 84% Vf would be typical of a very
> low-density foam dielectric,
> something like LMR-240. Regular RG-8X with a solid
> poly dielectric is
> around 66%. Regular poly "foam" is 78%. So, my
> guess is that you are
> really still longer electrically than what you had
> originally estimated.
>
> > If the VF of my cable is .84, that's still 214
> degrees of
> > electrical length at 146 MHz, very close to 180
> degrees, and
> > nowhere near either 1/4- or 3/4-wavelength.
>
> It's 34 degrees longer than a halfwave, or 56
> degrees short of a 3/4 wave,
> assuming it really is 84% Vf. If it's 78% Vf, then
> you're up to 230
> degrees. If it's 66%, then you're at 272 degrees
> (3/4 wave).
>
> > I just tried it...99 MHz, R=1, X=0. So, the coax
> and PL259
> > together have a velocity factor of .66? Wow...if
> that's the
> > case, it is indeed a 3/4-wave stub, and I should
> be able to
> > cut the stub waaaaay back, until it's 1/4-wave in
> electrical
> > length, and find a length which yields similar
> results. Hmmm...
>
> Bingo.
>
> In the interest of science, I just sacrificed a 1/2"
> Heliax jumper to make a
> quarterwave shorted stub for 900 MHz. The length of
> the connector itself,
> plus the added length of the port on the tee,
> resulted in the quarter-wave
> point being just a hair longer than the connector
> itself. At 900 MHz, the
> insertion loss was negligible (about 0.04 dB). The
> return loss was very
> good as you would expect (> 30 dB). In looking at
> the sweep, as you get
> appreciably low in frequency, it starts to look like
> a high-pass filter.
> The -3 dB point was 184 MHz. At the top end of the
> FM band at 108 MHz, it
> was down about 6 dB. At the bottom of the FM band
> it's only a little more,
> about 7.5 dB. At 20 MHz it's down about 20 dB. So,
> a shorted quarterwave
> probably isn't the best solution if the goal is to
> attenuate the FM band
> energy.
>
> In comparison, a run-of-the-mill 1/4 wave bandpass
> cavity (TxRx 4" variety)
> set for 1 dB insertion loss at 900 MHz had 43 dB of
> attenuation at 108 MHz,
> 44 dB at 88 MHz. In fact, it was better than 40 dB
> everywhere below 870
> MHz.
>
> I haven't swept an open half-wave at 900 to see what
> that looks like at FM,
> nor an open quarter-wave at FM. If you want me to,
> let me know and I will.
>
> > Jim, I may owe you the beverage of your choice at
> a future hamfest.
>
> If you're in a beverage-buying mood, a bottle of
> Macallan Fine Oak will do
> for me, preferably 25 or 30 year :-)
>
> --- Jeff
____________________________________________________________________________________
Never miss a thing. Make Yahoo your home page.
http://www.yahoo.com/r/hs
