[TowerTalk] resonance/swr/impedance plots
Guy Olinger, K2AV
k2av@contesting.com
Wed, 29 Nov 2000 10:27:23 -0500
The numbers are right. However, resonance is a vastly overrated concept.
The inverted L case proves it. Particularly if you are talking 80/160
where these things are more commonly used
If it is resonant, the feed impedance of the antenna, which has to have
a ground sink to match, is twenty something, possibly a good deal less.
That means that the ground needs to be near lossless at currents levels
consistent with 25 ohms and down. And a matching device is required.
Make it longer, and a simple fixed series capacitor will take out the
reactance and give you a 50 ohm feed point with no reactance, properly
tuned. Further, the ground losses have been reduced by a factor of 4
( I squared R loss) since the current at the feedpoint is cut in half.
The antenna is NOT resonant, as you point out, but it's BETTER. The
numbers may be too long for resonance, but right on for a simple,
well-designed antenna.
And if you wanted something to do with that transformer that you bought
to match 25 ohms, make the inverted L even longer, reduce the size of
the series cap to give 100 ohms zero reactance, use the transformer as a
step down to 50 ohms, and reduce the ground losses by a factor of 16
over the resonant design. They WILL hear that at the other end.
--... ...-- . ... --. .-..
Regards, Guy
Apex, NC
----- Original Message -----
From: alsopb <alsopb@gloryroad.net>
To: Jerry Devine <krr2ak@juno.com>; <towertalk@contesting.com>
Sent: Wednesday, November 29, 2000 9:12 AM
Subject: [TowerTalk] resonance/swr/impedance plots
> Jerry,
>
> To further illustrate my point. Min SWR is NOT ALWAYS the indicator
> of true resonance.
>
> Here are two tables of data generated by EZNEC for a dipole cut to
> resonance at 13.8 Mhz. One assumes a reference impedance of 50 ohms
> for SWR calculation the other assumes 200 ohms.
>
> Zsource =50 ohms =200
> ohms
> freq 13.5 z = 95.2 SWR =1.905 (complex =-33.7, real =67.7) 3.045
> 13.6 z = 81.5 SWR =1.636 (complex =-22.0, real =69.3) 2.973
> 13.7 z = 73.6 SWR =1.473 (complex =-10.3, real =70.7) 2.834
> 13.8 z = 72.3 SWR =1.448 (complex =+1.46, real =72.3) 2.766
> 13.9 z = 73.9 SWR =1.561 (complex = 13.1, real =73.9) 2.720
> 14.0 z = 75.5 SWR =1.772 (complex = 24.9, real =75.5) 2.697
> 14.1 z = 77.2 SWR =2.051 (complex =36.6. real = 77.2) 2.694
> 14.2 z = 78.8 SWR =2.384 (complex =48.3, real =78.8) 2.710
>
> The true resonant point is defined where the complex part is zero.
> That is slightly below 13.8Mhz. The z=50 case seems to track this.
> However, if you look at the z=200 case, the resonance point appears
> (from min SWR) to be about 14.1 Mhz. It "shifted" 300 Khz. The
> antenna didn't change.
>
> Minimum SWR is only a good indicator of resonance if the zsource is
> near the resonant impedance of the antenna. What does that mean in
> practice. For a dipole, not much. However, if your antenna is a beam
> with an impedance near 20 ohms or a folded dipole with an impedance
> near 300 ohms, the error in inferred resonance is substantial. Being
> off several hundred KHz is not my idea of being close enough.
>
> A case in point is the published inverted L lengths. They are too
> long. The antenna is resonant at a lower freq. However, if you look
> at the design you will see a series capacitor which brings "tunes the
> antenna to resonance". It isn't tuning the antenna to resonance at
> all. It is just canceling the inductive reactance of the antenna to
> bring the impedance down and hence provide a better SWR.
>
> 73 de Brian/K3KO
>
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>
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