[TowerTalk] Modeling question - for the experts!

[jimlux]

On 6/2/16 6:39 PM, john at kk9a.com wrote:
> So we agree that the 15m SWR of a 40m dipole will be over 2:1.  If it is not
> high impedance (100+ ohms) as I stated causing this, what is?
>

Well, to me, when someone talks "high impedance" in connection with 
dipoles, I think in terms of kOhms... (like a doublet that is a 
wavelength long).

100 ohms (compared to 72 ohms for an ideal dipole) isn't really "high", 
it's a 1.4:1







> John KK9A
>
>
> To:	towertalk at contesting.com
> Subject:	Re: [TowerTalk] Modeling question - for the experts!
> From:	jimlux <jimlux at earthlink.net>
> Date:	Thu, 2 Jun 2016 15:52:34 -0700
>
>
> On 6/2/16 8:17 AM, john at kk9a.com wrote:
> A full sized 40m dipole will have a high impedance on 15m and the SWR will
> be over 2:1 so the 2.7:1 SWR on your beam indicates that it is resonant.
> Is it hurting anything?  Your model should show the interaction using the
> stock 40M4LLDD antenna and spacing to other Yagis.
>
>
> I don't think the Z is high..
>
> I thought that 40m dipoles are close to resonant on 15m... 3/2 wavelength
> (21 Mhz vs 7 MHz)
>
> A quick model shows the SWR around 21 MHz of a dipole 1" aluminum, resonant
> at 7.15 MHz, 30 m above the (moderate) ground, is 2.4:1 at 21.8 and 22.33
> Mhz (lowest is 2.2:1 at 22.1)
>
>
> on 40m, the lowest VSWR is 1.44:1 at 7.15 and it's pretty broad: 1.7:1 at 7
> MHz, 1.75 at 7.3MHz
>
> If you want a dual band 40/15, you might tune the dipole a little low (since
> the BW on 15 is narrower than on 40)
>
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