[TowerTalk] Inverted Bobtail Curtain

[w8ji.tom]

> When you invert a Bobtail Curtain antenna, you are moving the 
> HIGH  CURRENT part of the vertical element next to the ground.
> The phasing wire will act as ONE radial for the outer elements
> and two radials, for the center wire.  NOT  a good ground system.
> You would be better off with phased verticals and LOTS of radials,
> preferably 30 or more, under each element.

Even if you install it right side up, you will STILL have appreciable earth
losses. The VERY strong electric field at the tips of the hanging elements
will displace charges in the lossy ground, and the result is wasted power.

Earth losses from the **vertical elements alone** are nearly the same
either way, but I agree with Tom the overall system losses would be worse
with the Bobtail inverted! My reason is different than Tom's, however.

Losses would be much worse because the single wire acting as a transmission
line is near lossy earth, and it's radiation does NOT cancel in the near
field. That means the single wire feeder would heavily couple power into
the soil a few feet away. At least if that single wire feeder is 1/4 wl
above earth, it has a fair chance for out-of-phase currents and voltages to
cancel the induction and radiation fields before coupling to earth. Invert
the antenna, and you add substantial feeder losses to an already
inefficient system.

All verticals, even 1/2 wl tall verticals or vertical dipoles, have
appreciable loss when one end of the antenna (EVEN the voltage end) is
placed within a moderately large fraction of a wavelength of earth. It's no
different than the Inverted Vee dipole thread on here, where the conclusion
was sticking the outer ends of the Inverted Vee dipole near earth kills
gain (efficiency).

Years ago I tried a Bobtail on 80. It was directive, but almost never
better than a pair of low inverted Vees phased, even for long DX.

When I replaced the Bobtail with three base fed
verticals, each with a good ground system, they consistently hosed my
reference 1/4 wl high inverted Vees! 

Same thing with verticals, and elevated resonant radials with high voltages
at the ends. Same with the Bob-tail. Same thing with Inverted Vee Dipoles.

No free lunch. No magic.
 
73 Tom

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