l/2 horizontal dipole vs inverted L 160m band

[tzitzikas]

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

 

[W5LZ]

tzitzikas,
The simplest solution would be standing the whole antenna vertical. But, that's just not the most practical thing to do. Even if it were possible, it would definitely change your present matching system, and probably require a larger radial field.
Changing to a 1/2 wave antenna isn't really going to be of much benefit considering your purpose of use. Using a vertical, loaded, 1/4 wave antenna might. Would still require changing the matching and radial field though.
Your radio set up is basically the same as is used by commercial broadcast stations here in the USA. Very few, if any, use a full 1/4 wave length antenna. Most are much shorter than that and a reduced efficiency is accepted.
I know it is possible, but not if it is practical for you. You would have to decide that.
Good luck!
- 'Doc

 

[tzitzikas]

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) do you know about T antenna (3/8 wavelenght)??? it's better than inverted L???????

 

[W5LZ]

tzitzikas,
1) Radials, or the 'other half' of the antenna.
Increasing the number of them typically increases efficiency. There comes a point where the amount of 'increase' just doesn't warrant adding more. (Or, you get to the point where you are just tired of adding more radials and quit .) A typical number to 'aim' for is 120 of them. That is quite a project, and I've never even come close to it, just got tired of adding them.
More radials will change the input impedance of an antenna so be prepared to change your matching device.

2) 'T' antennas, 3/8 wave length.
Can't answer that question, I've never 'played' with that type of antenna. I can tell you that it will require a very good ground system/radial field. I have 'played' with an antenna that might be thought of as a 'T'. Shorted the feed line of an 80 meter antenna to 'load' it on 160 meters with a tuner. Did it work well? No. But it did 'work' to some extent, nothing close to 'good'. If I remember correctly, I may have been heard -some- place, maybe?

Your present antenna is probably more effective than you might think. Certainly nothing wrong with wanting to make it better, just don't forget how it's placed now. You may want to go back to it .
Good luck.
- 'Doc

 

[tzitzikas]

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