Bi-cone Antenna 2nd build

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Bi-cone Antenna 2nd build

Postby Southwales » Sat Jan 27, 2018 1:22 pm

Hi, after my first bi-cone antenna build and being happy with the results so far, I want to build a 2nd Bi-cone to try and get even better range, I have limited attic space as it is a converted dorma bungalow so only 4ft height, so can not really make the top cone any bigger because of the roof pitch but I could make the bottom cone much wider, is this a good or bad idea? if good would I be better attaching the inner core or outer braid to the larger cone or does it not matter?, What part does the inner core and outer braid of coax play in receiving signals, is one an output current and one input?. Would I benefit from more vertical wires? my first one has 8 verticals on each cone, thinking of going 12 or more on this new build, can you have to many or is more always better?. Thank you for any help and advice. I am learning a lot from this forum.
Southwales
 
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Re: Bi-cone Antenna 2nd build

Postby G4RMT » Sat Jan 27, 2018 4:19 pm

Blimey!

Well, the cone elements are pretty important - The perfect cone would be made from sheet metal. Using individual lengths simply simulates the solid cone, and the more the better, but of course the law of diminishing returns applies once you get more than 10 or so. Adding more hardly changes the polar pattern. If you have four the radiation pattern is not a circle, viewed from above, there are notches. Adding more elements gradually produces the idea circular response.

To understand the way these things work requires some fairly advanced maths and an understanding of vectors and probably Smith charts - but to simplify what is happening greatly, it's all to do with impedance. I simple half wave dipole - one up, and one down, has a resonance, where VSWR is lowest that depends on the top top bottom distance. As in 300 divided by the frequency gives you the wavelength (ignoring VF). If you look at the bi-cone or the discone from the side, the angle slopes. So from the inner most point, top to bottom is very small. From the extreme points at the edge, top to bottom is much larger. At that point - the antenna wavelength can be measured, and at that frequency, impedance is close to 50 Ohms. Measure at any point and that impedance will be similar - BUT - at the different point the frequency will be higher. Try to imagine it as an infinite number of dipoles.

With a bi-cone - they are symmetrical, so the horizontal plot is a lobe going up above the horizon - 90 degrees to vertical, and another identical lobe going down, probably wasted if your interest in aviation, but handy if you live on the top of a hill with a town full of users below you. Discones, because of the horizontal disc are not symmetrical, and more goes upwards!

Hopefully you can see why the maximum length of the elements sets the lowest frequency - if the longest element is about 19", then you'll get a good VSWR at 2m, but airband will be less good, as the antenna isn't resonant there, and performance drops off. if your elements are longer - say, 3' - then the antenna will be good right down to the 70MHz ham band - and broadcast signals will romp in, possibly so well that they desense hairband, or cause nasty intermod problems.

The angle of the elements has a very complicated impact on the impedance changes and 60 degree angle seems pretty popular as a decent compromise in performance.

In a bi-cone, swapping screen and centre conductor wouldn't make much difference - there is a slight one, because often the antenna is supported on a metal pole, so having the grounded side at the bottom, puts any small compromises this causes in the place less vital. In the discone, the non-symmetrical radiation pattern dictates centre core to the top for proper performance.

Do NOT make the antenna asymmetrical. The physics of how the radiation pattern would change are probably beyond me, but go back to the side view - where the top one finishes, would still be the cutoff point as there is no 'phantom' dipole from this point.
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