Topband: inverted L question

[k6se@juno.com]

Bruce, K1MY/7 wrote:

"I am working on plans to put up an inverted L for 160 and have a few
very basic questions.

The L will be 100 feet vertical and about 100 feet horizontal or so."

"1. Is longer than 100 feet horizontal better?"
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No.  The longer horizontal section is, the more high-angle radiation
there is -- usually undesireable for DX work.
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"2. I understand that I need to add capacitance, some have suggested a
vacuum variable at the base."
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An EZNEC model shows that if the inverted L is 100' vertical and 57'
horizontal, at 1830 kHz the impedance at the base will be 50 + j174.1
ohms over a good ground system.  A series capacitor at the base of about
498 pF will cancel the inductive reactance, giving a perfect match to
50-ohm coax.
==========

"I just finished reading the Feb 03 QST evaluation of antenna tuners.
Will an antenna tuner in the shack accomplish the same as a box at the
base of the tower and am I reading the "product review" correctly that
they Ameritron ATR-30 is the best of the lot for this purpose?"
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An antenna tuner at the transmitter end of the coax only allows the
transmitter to see a match into the feedline.  The tuner must be placed
at the feedpoint of the antenna to truly "tune" the antenna.

In the case of the inverted L I modeled, it is not neccesary to use an
antenna tuner.  Merely attach the antenna end of the 50-ohm coax directly
between the base of the inverted L and the ground radial system because
the antenna will already be a close match to 50 ohms with the 498 pF
series capacitor.

The RF voltage across the capacitor will be about 780 volts with 1000
watts at 1830 kHz applied to the antenna.

73, de Earl, K6SE