David,
the capacity hat does cancel some radiation. Think of RF current going up
the mast and down the top hat wires. The magnetic fields partially cancel.
Without the capacity hat wires the 55' mast exhibits a radiation resistance
of ~5 ohms. With the capacity hat it is also ~5 ohms. So, nothing is lost
there. Where the capacity hat comes into play is to increase bandwidth.
Without the capacity hat, and using 3 ohms for GND + coil loss, the 2:1
VSWR bandwidth is ~20kHz. With the capacity hat it is ~40kHz.
The capacity hat improves the SWR bandwidth while increasing radiation
efficiency a bit.
I think the antenna radiation efficiency might be improved with a
capacity hat that does not extend down as far. A smaller capacity hat will
give higher radiation resistance along with greater loading inductance.
*I can run some models if you'd like to find an optimum top hat size based
on a coil Q of 400(?) and for various GND loss resistances. *
Dave KH6AQ
On Sat, Dec 5, 2020 at 3:37 AM David Aslin G3WGN <david@aslinvc.com> wrote:
> I'm building a contest station and the time has come to make a choice of
> 160m TX antenna. Looking at DXE Thunderbolt I note that the capacity hat
> extends to a significant distance down the vertical section. It's my
> understanding that will shield some of the radiation from the upper part of
> the vertical. Alternatives like a vertical tee would not have this issue.
> Is my understanding correct?
>
> I'm considering a commercial antenna as high strength telescoping antenna
> tubing is unobtanium in UK. My QTH is an exposed windy location so
> strength matters!
> 73, David G3WGN M6O
>
>
>
> Sent from my Galaxy
>
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