Topband: Inverted L observations and question.

[Donald Chester]

>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.

Don k4kyv





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