Topband: INVERTD L Reactance
n6ry at arrl.net
Mon Oct 15 20:27:28 EDT 2007
At 12:14 PM 2007-10-15, Bruce n9bx at mchsi.com wrote:
>My inverted L is about 65 to 70 Foot Vertical, with the rest
>horizantal. I have attached the Ground Braid of my coax to an
>aluminum Plate about 2.5 feet in Diameter that has 66 holes drilled
>on the outside of the circumference. From their I have run 66 #14
>THHN ground Radials out in a symetrical pattern for about 70
>feet. The vertical Part is not in contact with any trees or
>vegitation until it changes direction for the horizantal run. My
>SWR is 1:3 to 1 when fed with 50 OHM coax, and when I use an antenna
>analyser it reads about 55 OHMS.
>My question is this, does an inverted L have a higher reactance
>value than the typical 36-39 OHMS that a vertical has? Or must I
>continue to Add readial to the Ground. I can still put down more of
>the 70 footers but I can also put down about 20 that are 1/4 Wave
>length long. I hear well but for some reason no one hears me. This
>is with 1.5KW to the Antenna. Any help would be appreciated before
>the snow Flies.
For an inverted-L with approximately equal vertical and horizontal
wires like yours, with a very good ground, the resistance should be
lower than 36 ohms. My NEC-2 based model shows about 29 ohms of
resistance at the feedpoint at resonance (the frequency where the
reactance is zero).
Even though your radials are about 1/8 wl, with 66 of them, the
ground losses should be pretty low. You might pick up a dB or two if
they were all 1/4 wl, but that shouldn't keep you from being heard.
You state that the vertical part doesn't contact the trees. Does
that imply that the horizontal wire DOES make contact? The open end
of the horizontal wire is a high voltage/high impedance point. If
there is enough capacitive coupling to the trees, it could introduce
losses, raise the feedpoint impedance, and distort the azimuth pattern, too.
73, Terry N6RY
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