[LCC] The Height of the Horizontal Leg of an Inverted L - Ted N9NB

[w5wz at w5wz.com]

This would have been a bit more timely a month ago, before the ARRL 160m 
and the Stew Perry, but still great info!
--Scott, W5WZ


The Height of the Horizontal Leg of an Inverted L - Ted N9NB

Now with the low bands having more excitement as the sun spots wane, at 
N9NB I have been
trying to improve operations on top band. Recently, I purchased a SAL 30 
shared apex array
antenna, and have been amazed at the good directionality and low noise 
reception - it’s like
having 8 small beverages that are switchable, a big improvement over the 
low dipole and
Inverted L TX antenna. I have put down more radials at the base of the 2 
towers and at the
midpoint where a 160 m inverted L exists.

Now have 30 radials at each location, and wondered what else I could do 
to improve my signal.
Well, through computer modeling, I found that raising the horizontal 
portion of the inverted L
increases radiation resistance substantially. Originally, on my 160 m 
inverted L (a vertical wire
that is supported at the mid-point of a horizontal rope pulled between 
my 2 65' tall towers), I had
the vertical element going up to about 60' and then had a 75' horizontal 
leg that sloped gradually
to about 15 ft. height in a small tree, simply because I had not given 
much thought to the
horizontal portion of the antenna.

After the ARRL 160 m contest, I decided to measure and model the 
inverted L, and found from
EZNEC and from measurement with a MFJ 259 that the radiation resistance 
was only about 8
ohms when using such a low height for the end of the horizontal leg (I 
estimated that ground
loss also caused 8 ohms of series resistance loss, as suggested by 
W8WWV, giving rise to the
16 ohms reading I was getting on the MFJ 259 at the feedpoint of the 
Inv. L).

However, by raising the horizontal leg of the Inv. L up to 45' (using a 
rock throwing method to
get more height), I was able to double the radiation resistance to about 
16 ohms (and get the
MFJ to read 24 ohms of radiation resistance). If I can get a sling shot 
or launcher to get the
horizontal edge up to 75' in a nearby tree, computer modeling in EZNEC 
shows that I can
further increase the radiation resistance to 25 ohms - that is my goal 
for the spring. If you have
surrounding tall trees, this is a way to get "free power" by raising the 
horizontal edge of the
inverted L, since an increase of the horizontal leg height increases the 
radiation resistance -
which determines the RF power emanating from the antenna. In theory, if 
I could raise the
horizontal edge all the way to where it is perfectly vertical, the 
inverted L transforms into a
standards quarter wave vertical with 37 ohms radiation resistance.

By raising the height of the horizontal edge from 15' to 75', I 
basically increase the radiated
power portion of my applied power from 50% to 75% (due to the resistance 
ratio of the antenna
to ground loss going from 1:1 to 3:1.) That is, instead of heating the 
ground with half of my TX
power as I did with the low inverted L height of 15 ft., getting up to 
75' height will cause me to
only spend 25% of my TX power on the lossy ground effect. This should 
give me a 50% (1.8 dB)
power boost for free, just by raising the horizontal edge of the 
inverted L!