Inverted-L design and matching
Inverted L's can be nice antennas if you know how to build and how
to match them. Due to its nature the inverted L is a more or less
vertically polarized radiator. Basically: the longer the vertical
section the more dominant is vertical polarization. Also the feed-
point impedance becomes higher when you increase the height. I once
designed inverted Ls using an earlier version of MININEC which gave
the results shown in the table below. The overall (total) length was
modified to give a (nearly) pure resistive feedpoint impedance.....
Even with a (poor) ground system consisting of 50 radials each being
only 50 meters long the measured values came in very close to the
calculated ones (I only had to add the earth losses!).
A simple L/C-network was used to match the antenna to a 50 ohm coax
cable.
50 ohms o-----o----- Lr -----o antenna, < 50 ohms
I
I
-------
------- Cp
I
I
-------
Dimensions given are in meters. Values of Cp and Lr are calculated for
"n o" ground losses (= ground system with zero ohms) and 1.83 MHz.
I-------- The inverted L --------I I---matching section---I
vert. horiz. total imped.(ohms) Cp (nF) Lr (micro-H)
10 31.5 41.5 6.9 4.3 1.5
12 29.4 41.4 9.2 3.7 1.7
14 27.4 41.4 11.9 3.1 1.85
16 25.3 41.3 14.5 2.7 2.0
18 23.3 41.3 17.3 2.4 2.05
20 21.3 41.3 20.0 2.1 2.1
22 19.2 41.2 22.7 1.9 2.15
24 17.2 41.2 25.3 1.7 2.17
26 15.1 41.1 27.6 1.55 2.16
28 13.0 41.0 29.8 1.4 2.13
Look at the value of Lr! It varies very little for a height between
14 and 28 meters. Make the antenna longer to resonate around 1.72 MHz and
remove Lr !! This is the equivalent length! Then you only need a capacitor
between feedpoint and ground to get a close match to 50 ohms. Very simple!
It can be a lot of fun designing aerials - and make them work. But my back
still hurts from putting the radials into the ground.....
73, Peter/DJ8WL - ONE-SIXTY ? FOREVER !
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Message sent at 00:59 (= UT+2) on 30.09.96
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