Hi Gerald,
I guess this is something I'm not clear on. By my calculations, an antenna
having an impedance of 40+j10 or 40j10 would have 41.23 ohms.
If SWR is the ratio of the two impedances (coax and load), then 50 ohms / 41.23
ohms would give an SWR of 1.21. It would take 30 ohms of reactance combined
with 40 ohms of resistance (40+j30 or 40j30) to give 50 ohms. So, it would
seem to me that the 40ohmsatresonance antenna would need to have 30 ohms of
reactance combined with it in order to achieve an SWR of 1.
At least, that is what I would expect. Am I missing something in my thinking?
Thanks,
Blair, NP2F
 Original Message 
From: TexasRF@aol.com
Date: Thursday, November 13, 2008 12:33 pm
Subject: Re: [TowerTalk] wire antenna question
> Blair, that is not quite right. Using a 50 ohm swr indicator, the
> lowest swr
> is shown when the reactance is zero.
>
> Using a transmission line program, 40 ohm load 40+j0; swr=1.25.
> 40+j10=1.37;
> 40j10=1.37.
>
> So, when you tweak your dipole for the lowest vswr at a given
> frequency, you
> are in essence resonating the antenna at that frequency.
>
> 73,
> Gerald K5GW
>
>
>
>
> In a message dated 11/13/2008 11:00:20 A.M. Central Standard Time,
>
> blair.balden@wmich.edu writes:
>
> Thanks to everyone who responded. I feel much better about what
> I've been
> doing. Terry, yours made such perfect sense, I thought I should
> have known
> it already! Because the dipole's resonant Z is usually higher
> than 50 ohms,
> getting minimum SWR would mean I'd cancelled out all the
> reactance. If the
> resonant Z were less than 50 ohms, I could get a lower SWR by
> introducing a
> reactive component. But, I suppose it would be rare to have a
> dipole with a
> resonant Z below 50 ohms.
>
> Thanks again to all,
> 73,
> Blair NP2F
>
>
>
>  Original Message 
> From: Terry Conboy <n6ry@arrl.net>
> Date: Sunday, November 9, 2008 10:54 pm
> Subject: Re: [TowerTalk] wire antenna question
>
> > At 07:07 AM 20081109, Blair S Balden NP2F wrote:
> > >When I put up dipoles, whether they be "straight" or inverted
> > >V's, I've always started out with the wire a little longer
> than
> > the
> > >formula length of 468/f. Then I've trimmed for as close to a
> 1:1
> > >match as I can get to my 50ohm coax, as indicated by my SWR
> > >meter. I can usually get a 1:1 SWR.
> > >
> > >These antennas usually work very well, but I realize that a
> > resonant
> > >dipole will not have exactly 50 ohms resistance. So, I must
> be
> > >cutting these antenna somewhat off resonance. I also realize
> > that a
> > >mismatch at the antenna (SWR of greater than 1) results in loss.
> > >
> > >What I'm not sure about is whether it would be better to get
> the
> > >antenna resonant at the desired frequency and accept the
> > mismatch,
> > >or whether I should continue to go for the best SWR
> (minimizing
> > >losses due to the mismatch).
> > >
> > >BTW, my rig does not have an internal antenna tuner, and I am
> > >currently without an external tuner for it. So, I'm running
> it
> > >without a tuner (if this matters). Also, I have not been
> using
> > any
> > >kind of matching device at the feedpoint, just direct
> connection
> > >from the coax connectors to the wires.
> > >
> > >If someone on here can give me some insight as to whether I
> > should
> > >try to go for resonance instead of low SWR, please respond. I
>
> > would
> > >truly appreciate any help with the question.
> >
> > In your case, resonance and 1:1 SWR are exactly the same
> > thing. Resonance just means that the antenna impedance is
> purely
> > resistive. Resonance isn't necessarily the point of highest
> gain:
> > many antennas actually have higher gain when they are much
> longer
> > than the resonant length (e.g. doubleextended zepp).
> >
> > Indeed, a thinwire straight (nearly) halfwave dipole in free
> > space,
> > or at certain heights, will have a resistive component of the
> feed
> > impedance of about 72 ohms. However, at your mounting height
> and
> > with your surrounding conductors, the resistive part of the
> > antenna
> > impedance is 50 ohms. This is actually pretty common, as you
> > have
> > discovered. (Because of the bending, Vshaped antennas
> typically
> > have a feed impedance under 72 ohms, even in free space.)
> >
> > Even if your antenna did have a feed impedance of 72 ohms at
> > resonance, which would give a 1.44:1 SWR, you can only worsen
> the
> > SWR
> > by changing the length of the wire, which mostly just adds or
> > subtracts reactance.
> >
> > With a 50 ohm line, the ONLY way you can ever get a 1:1 SWR is
> if
> > the
> > load Z is 50 +/ j0 ohms. (A Smith Chart would never lie.)
> >
> > In your situation, there really isn't anything else you can do
> > with
> > trimming to get higher efficiency. You already have "nirvana".
>
> > Life
> > is good. Be happy. As they say in Seattle, "keep clam".
> >
> > 73, Terry N6RY
> >
> >
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
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> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
>
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