Topband: Inverted L observations and question.
k4kyv at hotmail.com
Tue May 23 15:57:15 EDT 2006
>From: "Tom Rauch" <w8ji at contesting.com>
>I know that's another one of the very popular myths about
>antennas, but bandwidth is not a reliable direct indicator
>of efficiency. The only time a bandwidth change indicates an
>efficiency change is when loss resistance of the system is
>the only thing changed.
I use a full-size quarter wave vertical tower mounted on a base insulator,
with 120 quarterwave buried radials. Additional top loading is provided by
an 80 mtr dipole attached at the 119' level, with the open wire line going
down through the interior of the tower. When the vertical is in use, the
dipole feedline is disconnected and left floating. There is no direct
electrical contact between the tower and dipole antenna components. Due to
the top loading, the base impedance of the tower measures in the range of
200 ohms (with some + and -j, depending on the frequency), instead of the
expected approximately 40 ohms.
I feed the vertical using a simple one-section L-network at the base of the
tower, adjusted so that the SWR is exactly 1:1 at 1900 kHz. At both 1800
and 2000 kHz, the SWR is approximately 2.5:1. I just locked down the
L-network adjustments, and load the transmitter anywhere in the band by
using an additional tuning network between the transmitter and the 50-ohm
feedline. When I built that antenna nearly 25 years ago, I hadn't expected
that much bandwidth, but the tuning hasn't changed in a quarter-century, and
I don't suspect any hidden losses. I am grateful for the wider bandwidth,
since I don't have to make any adjustments on the L-network that is located
at the base of the tower, 140' from the shack.
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