Topband: inverted l

[Rob Atkinson]

If you have a properly constructed typical inverted L, i.e. 50 or 60
foot vertical and similar length horizontal, AND a good ground system
serving as the other half of the antenna, your feedpoint Z will be 10
to 20 ohms.  The reason you need a matching network is that most coax
(this assumes you are feeding the antenna with a coaxial transmission
line) has a characteristic impedance of 50 ohms or more.  If you found
coax with a characteristic Z of 15 ohms you probably wouldn't need a
matching network.  Let's say you don't bother with a network.  Your
line will have voltage maxima and current maxima on it and exhibit
more loss but it may be okay depending on the line used and its
length.  The other problem is whether or not your transmitter has an
output network that can handle the other than 50 ohms Z it will see at
the end of the line with this arrangement.  If all of this is a
problem for one reason or another, you need a matching network.   With
antennas that have feedpoints near the ground, you can place it at the
feedpoint which is nice.   Any inverted L with an adequate ground
system, elevated or buried, will have a pretty narrow impedance
bandwidth for matching purposes.  Anything that exhibits a fairly flat
bandwidth, say 50 or more kc, is probably not radiating efficiently.
You can improve this, but the way to do it is to widen the width of
the driven element, for example instead of using a single wire, employ
a cage.

You can raise the impedance of the L by increasing the vertical
length.  Eventually with 1/4 wave length vertical, the ideal Z will be
around 36 ohms, but then it ceases to be an inverted L.

73
Rob
K5UJ