Tree/Tom, et al:
I had a question regarding tower detuning. This past weekend
Bob, N6ET and I tried detuning the Rohn 45 tower at W6VIO that
is adjacent to our K9AY loop array. We used a technique similar
to the one Tree describes on his website (current transformer
driving an oscilloscope). The tower is 60' tall with a TH7DX on
top and a 2 element 40 (402-CD) stacked 10' above the tribander.
Computer modeling suggests that the tower is close to resonance
at 1.8 MHz. The tower has two sets of guys. The top set (@58') are
phillystran and the bottom set (@~25') are EHS and are not insulated
from the tower at all! Also the 7/8" hardlines coming down the tower
are not bonded electrically to the tower at the bottom. The shunt
arm consists of #14 THHN running from the top of the tower to
the ground and spaced about 24 to 30" from the tower. We are
tuning the shunt arm with a 330pf series capacitor. The current
transformer consists of an Amidon 2" O.D. x type 77 core split
into two halves with a 2 turn secondary terminated into an 82 ohm
resistor.
Questions:
1.) What sort of peaking should we expect to see on the scope
when the current transformer is placed on the shunt arm (We
see about a 4x increase in voltage when the capacitor is set
at about half scale [150pf] as compared to the readings we
get when the capacitor is set at either min or max capacitance)?
2.) Should we bother insulating the bottom set of guys (I am
concerned that the fact that they are not insulated is messing
up our ability to effectively detune the tower).
3.) Do we need to bond the shields of the hardlines to the tower
electrically near the base of the tower (the hardlines are physically
attached with tape to the tower legs all the way down to the base
of the tower, but are not bonded electrically)?
4.) We did not see a corresponding dip (it was very very
slight) when we put the current transformer around the leg of
the tower below the junction point of the shunt arm and the
tower leg. Why is this? Could it be that some of the current
is flowing in the other two tower legs or on the hardline
shields or down the bottom guys wires since they aren't
insulated)?
See the following for pictures of the setup (the FT-757GX was
used to drive the K9AY loop at 1825 KHz in order to
excite the tower):
http://www.dellroy.com/k9ay_1.htm
Thanks and 73,
Mike, W4EF......................................
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