Hi Gerald,
I guess this is something I'm not clear on. By my calculations, an antenna
having an impedance of 40+j10 or 40-j10 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 40-j30) to give 50 ohms. So, it would
seem to me that the 40-ohms-at-resonance 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;
> 40-j10=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 2008-11-09, 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 50-ohm 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. double-extended zepp).
> >
> > Indeed, a thin-wire straight (nearly) half-wave 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, V-shaped 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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