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
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
**************Get the Moviefone Toolbar. Showtimes, theaters, movie news &
more!(http://pr.atwola.com/promoclk/100000075x1212774565x1200812037/aol?redir=htt
p://toolbar.aol.com/moviefone/download.html?ncid=emlcntusdown00000001)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|