[Antennas] Tuning Jpole question

2003年1月25日 19:17:44 -0800 (PST)

Hi,
Your SWR is not all that bad really. Not something to
fret about as much as they say unless you start
getting up to 2:1 on VHF.
You will find on VHF and up, (shorter frequencies)
changing feedpoint and length of elements just a
LITTLE can really effect SWR. That is because of the
shorter wavelength compared to say HF freq.s.Basicly,
1 inch can be a big factor when 19 inches is a 1/4
wave, like with 2 Meters.
Anyway, I always tune my 2 Meter antennas for the
LOWEST SWR at 146.000. Most of your transmitting will
be done + or - that a little so from there your SWR
curve should be O.K. (within acceptable levels).
With your 1.1 to 1.7 you are covering 
FOUR Megahertz !! Like I mentioned earlier, that's a
lot of difference in wavelength as far as VHF goes.
i.e. @ 144 mHz, 1/4 wave = 19 1/2 inches
@ 148 mHz, 1/4 wave = 18.98 inches.
That is ONE INCH difference in electrical length right
there. Don't expect to get a really flat SWR covering
that much bandwidth with a (any) VHF antenna, unless
it a factory made job.
I hope this helps explain.
73
Steve [email protected]
--- Daniel Boyer <[email protected]> wrote:
> I just put up my first elevated external
> antenna... A 3/4" copper
> pipe Jpole for 2m operation... I tried about 15-20
> different position
> for the coax connection points and got it set at the
> point which gave
> the best overall swr. The results are that I have
> an SWR of <1.1
> measured at 144.010mhz and an SWR of 1.7 at 147.990.
> If I move the
> contact points up, the swr goes up across the board,
> move the contacts
> down and the swr goes up across the board. My
> question is what should I
> do to move the low swr more towards the middle of
> the band or just lower
> it overall? Do I need to chop some pipe off of the
> main element or off
> of the stub or something similar?
> Thanks
> Daniel
> KC9DAG
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