i have a transmitter about 150 watt (output an 813 tube 1400vdc) at 1431khz (salonika greece) and the antenna that i'm using is an inverted L antenna (with a less than 5l/16 length ,about 20m vertical segment and 40m horizontal segment). the coupler that i'm using now is l-type . at http://www.geocities.com/tzitzikas_ee/antenna.gif you can see a schematic of my coupler.i am using 3 l/4 length radials and 2 l/8 length radials. the ground system is an copper tube 3m approximate which is burried vertically to tha ground.the distance between transmitter and ground system is about 10m
i would like to improve the efficiency of my antenna. i have 3 thoughts
1) to cut the antenna at l/4 length and to use an 50ohm coupler ( after the variable coil i must add a variable capacitor 0-1500pf with the one point grounded)
2) to use an l/2 horizontal dipole with 50ohm feed line (coaxial RG213 cable) and to use a coupler like one i described above (thought 1)
3) to keep the antenna that i have now.
i dont want to transmit by using the ionosphere cause my transmitter uses a frequency in the band mw, and at the night my frequency is filled by stronger signals. i would like to have a local strong signal.
which of the above antennas i have described is the best???????
thanks

Tzitzikes wrote:
"Whitch of the above antennas I have described is the best?"

At 1431 KHz, you need vertical polarization to avoid ionospheric
propagation.

You can turn your inverted L into a T by connecting its vertical part to
the exact center of the horizontal part. Horizontal currents in the
capacitive hat will then be equal and in opposite directions and not
rafiate. The capacitive hat should be just as effective in loading the
vertical section as the horizontal part of your inverted L is but unlike
the L, the T should not radiate any horizontally polarized energy. There
should be no change needed in your tuning coil and capacitor.

Best regards, Richard Harrison, KB5WZI

i would like to ask you 2 questions:
1) the radials increase or decrease the inverted L efficiency when i don't use 50ohm antenna (coaxial cable 50ohm and 50ohm coupler). i know that when we use an 50ohm inverted L the radials increase the antenna's efficiency.

2) i would like to ask which antenna is better for 160m? 1/4 wavelenght inverted L , 3/8 wavelenght inverted L or 1/2 wavelenght inverted L ?? thanks

Tzitzikas wrote:
"I know that when we use an inverted L the radials increase the
antenna`s efficiency."

The same is true of any antenna which works against the earth to
complete its circuit. Radials at the surface of the earth may be
increased in number until there are more than 100 where the point of
diminishing returns is exceeded. U.S.A. government regulations require
medium-wave broadcasters to use 120 radials of at least 1/4-wave length
or else prove the installed grounding system meets or exceeds the
government`s efficiency standard. These radials are separated by 3
degrees for equal current distribution. Thousands of such installations
have been documentedand and are proved nearly lossless.

Tzitzikas also wrote:
"I would like to ask which antenna is better for 160m?"

The radiation which counts is along the surface of the earth because it
determines daytime coverage. Ionospheric coverage is useful at night
subject to many variations and even interferes with the surface wave at
some distance from the transmitter.

If some of the radiatinng antenna wire is horizontal, it may introduce
radiation which escapes to the ionosphere to interfere with the surface
wave at some distance from the transmitter unless it is carefully
balanced out as in the case of the T antenna or other antennas with
symmetrical balanced capacitive loading.

First, an antenna which is self-resonant presents no capacitive
reactance, which needs a lossy coil to balance out. The shortest
self-resonant antenna works against ground and is 1/4-wavelength. That
makes them popular. It`s a matter of getting maximum current into the
antenna. A resonant antenna is unimpeded by reactance.

If an antenna is between 1/4-wave and 1/2-wave in length it can be
resonated with a series capacitance. Capacitance is usually very low in
loss so it is efficient.

As an antenna grows from 1/4-wave to 1/2-wave it produces greater field
strength on the distant horizon, not because it is more efficient but
because its power is being radiated at lower angles near the surface of
the earth and less is radiated at angles above the horizon.

Best regards, Richard Harrison, KB5WZI