[TowerTalk] Modeling question - for the experts!

[jimlux]

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)