In order to verify that my current maximum in the drop wire
corresponds to minimum current at the base of the tower, I
added a current meter at the very bottom of the tower.
This allowed me to see a relative measurement of the current
in the tower.
It turns out the minimum current in the tower was pretty close
to the maximum current in the wire. However, I found the
setting for minimum tower current was slightly lower in
capacitance than maximum drop wire current.
Another interesting find was that the current probe on the drop
wire had an effect on the setting as well - along with a pretty
big impact on the depth of the null. The null was much better
without the current probe on the drop wire. I think that making
the measurement on the tower is a better place to do it.
I made the current transformmer on the tower with 4 pieces of
ferrite taped together with two windings of wire, terminated
with a 150 Ohm resistor. I had no problem getting enough of
a signal on my scope to take readings.
Pictures at http://www.kkn.net/~tree/160/
Image 028 - The current probe on the drop wire. You can see where the
wire attaches that hooks up to the other side of the cap from the drop
wire.
Image 029 - The current shunt on the base of the tower. This is about
a foot below where the wire to the cap connects (on the same leg). This
is 4 pieces of ferrite with tape holding it together and two windings
of the wire with a 150 Ohm resistor.
Image 030 - The drop wire (this is a prototype). I used electrical
tape at the top to hold the wire away from the tower (tied to the boom of
that Telrex 20). The wire is just below the feedpoint - somewhere between
35 and 40 feet. The wire is 12 gauge solid home depot wire.
Image 031 - You can see my home made balun - its a coil about 10 inches
in diameter with something like 20 turns. Also, I put some clamp on
ferrite below the coil. It is RG8X. Currently, I just have one
radial - which is going up off the top left of this picture. I will
eventially have 4 of them. Currently, the shield of the coax goes
to the radials and the center conductor goes to the tower. It
really shouldn't matter which way it is hooked up if my balun is
working.
I am going to see how frequency sensitive it is when I get some more
time.
First QSO with the antenna was 4X4NJ - so it must be working okay.
Tree N6TR/7
Boring, Oregon
tree@kkn.net
PS: Well, okay... Riki was in Utah.
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