Ham Radio Forum

hi. 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 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 don't 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

A half-wave horizontal dipole high above ground would have greater radiation efficiency than either the existing antenna or the existing antenna cut to quarter-wave resonance. However, it would have to be about quarter-wave above ground to be highly-efficient and its maximum radiation would be up toward the ionosphere rather than over the surface of the earth like you want.

Because of these considerations I think it would be best to make your existing L antenna quarter-wave resonant. The vertical conductor should have a large diameter for high efficiency, because the RF current in that element will be high. Also, it will be very important to have a good ground radial system extending in all directions from the base feed-point, because the same current that flows into the antenna feed point will flow through the ground system resistance. Unless you already have an excellent ground system, you probably can gain more from improving the ground system than from changing the antenna.

thank you for yor answer.
i have 3 questions
1) For local strong signals T antenna or inverted l antenna is better? (i dont want to transmit by using ionosphere) .And which will be the best lenght of my antenna: 1/4 , 5/16, 3/8 or 1/2 wavelenght?
2) T antenna requires radial system?
3) how many radials i must install to have a positive result ???

note: i feed directly my antenna by using this coupler http://www.geocities.com/tzitzikas_ee/antenna.gif without using 50ohm coupler and 50ohm coaxial feedline cable.

1. T or L makes little difference. Is the transmitter located close to the center of the aerial? If so, use a T, if located nearer one end, use an L.

In both cases, it is the length of the vertical radiator that makes most difference, as the horizontal section is (or should be) acting as a capacitance hat. For MW, low-angle radiation, the vertical mode of polarisation is the only way to go, without building towers 100 metres high! Use a çaged'' vertical section - see:

http://www.smeter.net/antennas/160-mete ... rted-l.php

for an idea. It is simple and light and will improve efficiency markedly.

2. Either antenna requires a good radial system. Ideally, you need a multiple of wires around 1/4 wave long. In practise, as this would be much larger than the antenna horizontal section, it is probably impractical, if you don't have the space. You don't mention the location, but that will make a lot of difference. I would recommend at least 1/4 wavelength (~50m). If the ground is quite conductive (i.e. marsh, lowlands with a high water table), you can use shorter radials but you need a lot of them. In Salonika, you are near sea level I presume?

3. Stories abound of 30+ radials, but more than 8-10 may be a cause of diminishing returns (i.e. more than 12 will result in less than a 10% improvement). Some say that the number of radials is more important than the length - this only applies over highly conductive ground. If you are in a desert or on top of a mountain, fewer radials with a longer length is better than 30+ short radials. The issue with radials is to transfer as much 'return' energy to ground as possible. If the ground is highly conductive, a lot of short radials will have a lot of capacitance to ground, which is good. In dry ground, it doesn't matter how much capacitance to ground the radials have - the ground isn't going to conduct anyway - so use <real> (i.e. 1/4 wave) radials.

Your L-tuner may require configuration to work OK if you lengthen the antenna beyond 1/4 wave. Physically short (i.e. less than 1/4 wave) antennas have a low feedpoint impedance and your components are working with that now. Should you extend the total wire length to 1/2 wave (in the next town...), you may need to move the cap to the antenna side - see

http://www.smeter.net/feeding/impedance ... tworks.php

for more info on L-tuners.

Your plate load needs to be around 6500 Ohms. With your existing antenna and 12x 55m radials on average soil, the feedpoint impedance is around ~30 Ohms with a reactance of ~140 Ohms. to tune with an L-tuner from 6500 Ohms would require 243pF of capacitance, and 35uH of inductance, using the same network configuration you already have. BTW, this is so close to 50 Ohms AND at only 1.4MHz, you would get very little loss with a few metres of RG-58 coax. This would help to get the high current portion out of the shack and closer to a good place for radiation.

I hope this helps you. Please post a reply and let us know how you get on. Remember to test any changes you make against a 'standard' receiver located at some distance (2-3km is good - more may be affected by sky waves).

73s de VK2JDP

I just recently suggested to a friend who has an inverted "L" on 160 that he migh try using a cage arrangement for the vertical portion of the "L". Now I'm thinking about the effect of making the first 1/8th wave of the dipole element of a 3 element 40m beam to increase bandwidth.

Thoughts welcome,
dave
wa3gin

if you have the space ,bild up a 'kage' dipole using 4 swg12 ,solder them only at feed point and at the ends....its heavy but realy improves bandwidth...but lowers the eficiency...for larger bandwidth use the TTFD configuration .....mery CHRISTMAS ..73..