Hi All,
If you do not want to learn how the sloper system works, don't read
this.
> Normally I don't say much on these reflectors because there are
> too many
> "experts" standing by waiting to tell me the theory of why my antennas
> don't work. Here are my experiences with 80 Meter quarter wave
It is easy to see why your slopers DO work.
> slopers. I have a 70 foot Rohn 25 tower with an 8 foot mast. A
> CushCraft D3W is attached to the mast at the 77' point. A TH7DX is
> attached to the mast at the 72" point.
First, you have big antennas and are on 80 meters. That means the
antenna makes a reasonable "counterpoise".
My four 80 Meter slopers are
> attached to the tower at the 60' point.
Second, you have FOUR slopers all attached at the same height.
Each of the unused slopers will act like a "counterpoise" for the
single antenna being fed. Even if you did not have a large yagi and
even if you were on 160, the extra three slopers makes a huge
difference.
Each sloper is 67' of
> insulated copper wire oriented NE - SW -NW- SE. I switch between wires
> using a DX engineering antenna switch box. Each sloper is attached to
> assorted supports on the bottom end, but most are in the 10-15' height
> range. At the base of the tower I have a #2 wire radial ring, which
> I've attached about 50 quarter wave radials, and just laid them on the
> ground. In about 2 years the grass grows over them and they disappear.
Third, you have a ground system to increase efficiency.
> In my case there is nothing magic, just hook the sloping wire to the
> 60' point on the tower. The center conductor of the coax is attached
> to the sloping wire and the braid is attached to the tower. One trick
> I've found effective is to have the upper insulator no more than 3/4"
> from the attachment point. The closer the better. This antenna
> functions like a off center fed vertical, in t+hat the tower is the
> main radiator.
I doubt the tower is the main radiator in your case, because the
unused slopers and the yagi all "sap" current from the tower. If
current divides equally between all five (the tower, the yagi, the
three unused slopers) the sloping wire would have five times the
current and several times the radiation as the other five things
being driven.
NOW..the problem you are having with 160 meters is
> somewhat more complex, in that you don't have the height to effect a
> 50 ohm match as easily as 80 Meters. The trick here is to find the
> match point on the tower.
His problem is that the tower, plus the capacitive loading of the
antenna, makes the top of the tower have a high impedance for 160
meters. Think of the inductance of the tower being parallel tuned by
the large hat made by the yagi. As you move the tap point up and
down, you are tapping up or down on that inductance in that nearly-
resonant circuit.
It is exactly like the problem shunt feeding a tower. When the
tower is nearly resonant, with the capacitance of the large "hat"
made by the yagi, the tap point for a 50-ohm impedance will be
within a few dozen feet of the ground.
But that 50 ohms would add to the "antennas" common mode
impedance, and the total would be about 85 ohms. If the tower and
antenna are resonant on 1.8MHz, the tap would have to be nearly
at ground level to get a low SWR.
The way to "fix" that is to add a counterpoise system with a low
impedance at the slopers connection point, like you did with your
three unused sloper antennas. Another way would be to install a
yagi so large it has a low impedance when used as a counterpoise
on 160, that might require a large 40 or 80 meter yagi.
Once we understand how the system works, it is easy to see why
some people swear by slopers, and some people swear at them.
Some people get lucky, some people don't.
We might not all agree, but it is generally better to know why and
how something works and plan it...rather than just depending on
luck.
73, Tom W8JI
W8JI@contesting.com
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