[Lowfer] Re: 600MRG> Link to some darn good antenna designs...

[Johan Bodin]

Hi

Bob Roehrig wrote:

> He noted that increasing the top loading increased the efficiency, =
which
> is what I have been playing with. I have gone to extremes by =
decreasing
> the L/C ratio using a fixed capacitor of several hundred pf off the =
end of
> one of the top hat wires to ground. (The original loading coil brought =
the
> system to resonance at 600 meters with just the original 6 foot long =
hat
> wires). I haven't received any real signal strength data from afar to
> verify this so I plan to take some field strength measurements from a
> few miles away soon.

Don't be too optimistic. When the antenna structure is very small =
compared
to the wavelength, the current flowing in the antenna wires can be =
considered
to be in phase at all points (the current amplitude rolls off toward the =
end points,
however).

If you connect a capacitor between the top of the antenna and a wire =
that
goes to GND, that wire will carry a "physical" current that opposes the
radiation from the main vertical element. With a capacitor big enough
(which is a dead short for AC) you'll end up with a short circuited =
antenna
that doesn't radiate at all. Connecting a capacitor in parallel with the
loading coil does almost the same "trick". Less L is needed for =
resonance
but the penalty will be more reactive power circulating in the coil - =
causing losses.

Try to get as many "vertical ampere*meters" (or feet :-) as possible =
through
a copper conductor UPwards (which radiates) and let the current return =
to
earth through "space capacitance" DOWNwards (which doesn't radiate
because it doesn't move any charges (electrons) vertically).

73
Johan SM6LKM