> 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
