Mike,
with the bottom Yagi only the modeled tower resonant frequency is 2.6 MHz.
With the top Yagi added the resonant frequency is 2.0 MHz. The current is
uniform along the first 48' and drops to one half for the next 16'.
It looks like a great candidate for top band with a radiation resistance of
15 ohms at 1.8 MHz.
Dave WX7G
On Mon, Nov 9, 2009 at 2:10 PM, Mike & Coreen Smith <ve9aa@nbnet.nb.ca>wrote:
> Hi Gang,
>
> Some folks mentioned that the top yagi is not "seen" on my short tower if I
> should shunt feed it.
> Is there any proof of that? Papers? EZNEC? Anecdotal evidence etc? (like
> you
> removed the top yagi and the resonant freq on 160m remained the same ...)
> I'd love to hear the comments.
>
> I think I may have another stab at shunt feeding my tower and am curious
> more than anything as to its true electrical height.
>
> The reason for my curiosity is:
>
> A) My Tee vertical slung in the trees is now working "fairly" well and the
> top of it is just about the same as the tower....though the wire actually
> slopes up at a 45º angle, so not really vertical....also the top loading
> wires come towards the ground at almost 45º.
> Certainly not an ideal "vertical". . .Picture an umbrella, opened, sitting
> on the floor, with no material, and only 2 wires. That's about the shape
> of
> it.
>
> B) If the tower is only as tall as the first (lower) yagi (& the same as
> the
> Tree Tee) I might not bother shunt feeding it.
>
> TOWER SPECS:
> 48' Delhi, self supporting tower sunk into a massive block of concrete.
> Bottom yagi is for 50MHz (elements are aprox 10' long we'll say) with a
> 30' boom.
> Top yagi is 16' above that, identical to the first.
>
> SOme have said the electrical height is:
> 48' tower + 1/2 the boom on the low yagi (15') + half of 1 element (5'),
> top
> yagi & mast invisible - so aprox 68'.
>
> Is there any chance it could be more like:
> 48'tower + 15' boom + 16' mast + 5 + 5 = 89' or am I dreaming? (I find it
> hard to believe there is no current in the top yagi or the 16' mast--?
> doesn't make sense)
>
> I keep thinking back to my Cushcraft D40 rotatable dipole, which has
> capacity hats nearish the element tips.....if the radio doesn't see
> anything
> past the capacity hat, why have 4 foot (or whatever they are) element tips
> at all?)
>
> Thanks very much,
>
> Mike
> ve9aa
>
> Mike, Coreen & Corey Smith
> 699 Rte 616 Keswick Ridge
> NB
> Canada
> E6L 1T1
> _______________________________________________
> 160 meters is a serious band, it should be treated with respect. - TF4M
>
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M
|