Topband: inverted L antenna match system

[Charles Moizeau]

Here is how I did it, along with my reasoning 
behind the
design.



I have a tall ash tree and was able to achieve an 85' vertical run with 
an easy
bow-and-arrow shot.  I elected to build
an inverted L antenna  



I knew that the current maximum in any antenna is where maximal 
radiation
occurs, and that the current maximum in any antenna occurs a 
quarter-wavelength
back from the wire's open end(-s).  I
wanted as much radiation as possible to occur from the vertical portion 
of my
antenna.  With an antenna's sinusoidal
current distribution, that meant that I should make the length of the 
antenna's
horizontal portion such that the current maximum would be positioned at 
the
center of the vertical section.



I reckoned a quarter wavelength of #12 gauge solid copper insulated 
house wire
to be about 128 feet.  So with 85' up I
made the horizontal leg 85' out.  Do the
arithmetic and you'll see that the current maximum occurs where I wanted
 it.



I could point the horizontal leg in almost any direction, but I chose to
 run it
towards the northeast.  I did this so
that the antenna's horizontal radiation would be favored by a dB or so 
from the
"elbow" of the inverted L towards the southwest.



I ran my 52-Ohm coax in a buried pvc conduit to the vertical portion's
base.  With the antenna’s overall length of somewhat greater than 1/4
wavelength (in fact about 1/3 wavelength), there is an inductive 
reactance at
the base feedpoint, so all I needed for matching was just some 
capacitance
between the feedpoint and the 50-Ohm coax. 
I used a husky transmitting variable capacitor.  I
 haven’t pulled it to measure its value,
but my eyeball guess is about 300 pF.  

 

I have thus avoided a lossy inductor anywhere 
within the
antenna, and at the shack there is no balun or antenna tuner with their 
losses.



With eight in-ground radials I could adjust the capacitor to give me a 
1.5:1
SWR at 1.810 mHz.  I added six more
radials, readjusted the capacitor to get a 1.35:1 SWR, and then brought 
that
down to 1.2:1 with 22 radials.



The antenna performs well on 160m. 
Without any real effort, this past winter I worked several 
European
countries with five Watts, and no QSOs occurred during contests.



If I had had only, say, a 40' vertical run, I would have adjusted the 
length of
the horizontal leg accordingly.  And if
I were space limited for the horizontal leg, I would have used linear 
loading
to reduce the needed length by a third or so, or would have made a bend 
in the
horizontal portion to fit it in the space available; i.e., do whatever 
is
needed to locate the antenna's current maximum at the vertical leg's 
center. 



I do worry about the emerald ash borer, imported from China in the 
wooden
packing crates used to pack stuff coming from there.  Nothing
 has yet been discovered to eradicate this pest that has
killed many thousands of ash trees in this country (including four of 
mine so
far).  Also, laying in radials that
range from 70 to 160 feet in length is very slow and tedious work when
extracting lots of rocks and going around tree trunks and under their 
roots
near the surface.

72,



Charles, W2SH