Rich,
Two possiblities:
1. As Rik pointed out, it is common practice to provide a DC ground path for
MW towers. This would be done with an inductor, and it may not have enough
reactance at 136 kHz.
2. If the tower has obstruction lights, their wiring was decoupled (at the
original MF) by series inductors (RF chokes) or by a transformer with a wide
air gap between the primary and secondary windings (an "Austin"
transformer). If the decoupling was done with chokes, they again may not
have enough inductance at 136 to avoid a big transformation of the base
impedance.
If the above suggestions are ruled out, then Jim's comment about the low
base resistance should be noted. You could transform the low resistance with
a transformer, as he described, or just ground the low end of the loading
coil, and connect the transmitter to a tap. This actually forms an "L"
network, and is the simplest way to feed a short vertical where a 50 ohm
match is required. Lacking an impedance bridge, it would be helpful to have
an SWR indication to know where to place the tap.
John Andrews, W1TAG
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