Hi Kenny,
You set your curiosity on a real toughie. Tread where angels fear to go,
and all that.
The N4KG feed is extremely situational in its efficiency, or some
efficiency, or no efficiency.
It can be modeled in NEC 4, the version of NEC which allows you to have
conductors both above and in the ground. You need to explicitly model the
base of the tower down into the ground to see all that is going on,
including driving RF power into resistive ground, which is why you need to
model it. The model must also contain explicit description of the tower and
all the aluminum up the tower.
Then it becomes easy to see what is going on. And why it works over here,
and can be a dreadful failure over there.
At the point where the coax shield is attached to the tower, the current
will attempt to go both ways. The current off the braid will be divided
between going up the tower and going down the tower by Ohms law for
impedances at the operating frequency. The up and the down paths are not in
series, they are in parallel. And The down current will be driven into the
ground which appears as this big ugly lossy resistor.
If the path up the tower, a combination of wires on the tower, yagis yada
yada looks like a 150 ohm part R part X very miscellaneous impedance, AND
the path down the tower into the ground looks like thirty-something ohms
resistive, most of the RF current goes into the ground, wasted. Noting that
the division of power between the two is proportional to the square of the
division of current, in that case less than 10 percent of the power goes up
the tower and the thing performs like a dummy load. Even if you do some
stuff to compensate, every time something is done on the tower, you risk
undoing the compensation.
You might try to fix that nasty tower-base-to-dirt resistor by putting down
a good radial field, to reduce the resistance of the down path to 1 or 2
ohms. But then you've just done the hard part of setting up a stoutly
performing loaded tower, and the gamma/omega match actually can be adjusted
to present a 50 ohm load to the coax. The whole idea of the N4KG scheme was
after all, not to have to do ground radials.
All in all, this is why you see and hear of many gamma and omega-matched
towers that perform well, AND 24 years after the N4KG article you have to
ask around, and discover only scattered instances of one working well,
There is a way to accomplish what was intended by the N4KG method, if you
can't actually produce the tower plus antennas above the feed that actually
IS an electrical quarter wave:
1) use an inverted L with the bend supported by the tower as the directly
powered radiator. This takes the natural wide variation and uncertain
behavior of tower top stuff out of play.
2) use an FCP for your counterpoise. Plus and minus 33 feet on 160. This
gets you out of the need to find a 125 foot radius for (in the KG case) for
four resonant 1/4 wave elevated radials.
3) have an isolation transformer to a) insure that the initial direct
driving power only goes to the L/FCP and nothing else, b) the L/FCP is
completely isolated from any conductors except the supported tower [see 4)].
4) now, since the L will induce the fool out of the tower which is just a
few feet away and parallel to the L's vertical wire, counter the induced
voltage in the tower with an intentionally placed horizontal shorting wire
from the vertical wire to the tower.
5) even if you can only reduce the current in the tower to 1/3 of that in
the L/FCP, by the square law, that means more than 90 percent of the
radiation is from the L and less than 10 percent of the power is subject
to reduction by being driven into the ground at the tower base.
This scheme is explained on k2av.com, in the section "65+ Tower L Bend",
and illustrates what goes on with the RF current down at the base of the
tower, which is the same issue with the N4KG feed. Even if you could care
less about an L/FCP, the issue at the bottom of the tower is the same for
both, and the documentation for a floating L/FCP supported by a tower will
explain the issue and why you can't ignore it. .
Doncha love this stuff?
73, Guy K2AV
On Mon, Dec 3, 2018 at 1:02 PM Kenny Silverman <kenny.k2kw@gmail.com> wrote:
> I meant to ask if the antenna efficiency (dB) was better with either feed
> method.
>
> Regards , Kenny K2KW
>
> > On Dec 3, 2018, at 10:15 AM, Kenny Silverman <kenny.k2kw@gmail.com>
> wrote:
> >
> > Has any analysis even done comparing the N4KG reverse-fed tower feed
> method with elevated radials vs a shunt or omega feed with ground radials?
> >
> > This is a tall tower and I think it will be close to self resonance or
> longer.
> >
> > Regards , Kenny K2KW
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