I designed a sleeve or bazooka balun. But during simulation, I placed sleeve balun at different distances from the coaxial feed point. Then I observed that the radiation pattern of the antenna is changed when I vary this distance between feed point and sleeve balun. I want to know why this distances effects in antenna radiation pattern?
There will be currents down the outside of the coaxial cable as far as the mouth of the balun. The currents at the balun mouth are zero, or close to zero, but they will grow between the balun and the end of the cable. These currents will radiate. The currents will also be missing from the side of the antenna that is joined to the outer conductor, so it will radiate less. Both of these effects will change the polar diagram, and the impedance.
Greetings Srabonty
I did some research and crafted a small report on Sleeve Baluns back in the day. I will enclose the report and hope it attaches properly as it is fairly large.
Enjoy
Thank you Malcolm White sir for your answer. Could you kindly tell me during design sleeve balun the length of the sleeve balun t why should be lambda/4 from the antenna feed point? I am not clear about that. Can I vary the length, width of the sleeve balun for different frequencies?
Peter Nanni sir I have designed several sleeve balun for several frequencies. Could you please clear me the fact that the length of the sleeve balun why should be lambda/4 from the antenna feed point? Can I vary the length, width, also thickness of the sleeve balun for different frequencies?
The details that are needed for me to understand what is being done are not clear enough in the power-point pictures.
I can't see where there is an earth on the cable, for instance, or read the writing on the original picture.
The length of the active part of the balun must be lambda/4 to have the currents cancel and hence impede the common mode currents which try to make the cables a part of he composite antenna. Note that the length refers to the effective length, which would include any dielectric "fillers" which might be included in the balun. When it comes to the diameter of the balun, its mainly about bandwidth (loaded Q)
I'm not sure where the quarter-wave baluns are in your diagram. The insides of the cones look like they might be used as baluns. There may be a balun on the cable. If so it is probably there to stop currents going down the outside of the feed.
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