And what's more with the discone, both the elevation angle of radiation
(and reception) and polarization change with frequency. It may look good
on the Smith chart, but in reality, neither the discone and the bicone are
very good antennas, even compared to a dipole or LPDA.
In the past, we have used the bicone in EMC testing for regulatory
purposes. However, when commercially available LPDAs with extended
frequency ranges and methods of calibrating the antenna factor hit the
world, the bicone was rapidly abandoned. Even in most cases where I used
the bicone for regulatory testing, we put it to bed ar roughly 300 MHz. It
is not a particularly "good" antenna.
Dave - W?LEV
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On Thu, Aug 3, 2023 at 9:00?PM Manfred Mornhinweg <manfred@...> wrote:
Over the years I have built several biconical and discone antennas. These
antennas do have a wider bandwidth than a standard dipole, and also a lower
impedance, but they are far from covering several bands and everything in
between, which is what some people claim.
For example, I made a biconical antenna to use as the driven element of a
digital TV antenna, operating in front of a large reflector. I used a 30¡ã
angle, solid cones, properly trimming the length, and this gave just the
required 500-700MHz range between the 3:1 SWR points. A wider angle lowers
the impedance too far below 50?.
I have read that a slightly larger bandwidth can be obtained by making a
full wave biconical, and use a very wide angle, wider than 90¡ã. It should
also give some gain. The impedance of a full wave dipole is very high, but
making one this fat, it comes down into a usable range. But my practical
tests turned out not very brilliant. I never got a usable match to a coax
cable.
Planar phased array antennas often use sets of fat full wave dipols, with
a matching circuit consisting of wuarter wave sections, all made on a
single PCB. These seem to work, at least some of them, but only over a
pretty limited bandwidth.
A biconical antenna is a balanced antenna. When feeding it with a coax
cable, a suitable balun should be used, even when the antenna is placed
vertically. Without a balun, running the coax (and a metal mast!) inside
the lower element should be better than placing them outside, but there
will always be interaction.
A discone is also closer to a balanced than an unbalanced antenna!
Dissolving the solid cones and discs into sets of spokes, the behavior
changes very much! Some authors claim that using 8 spokes is the same as a
solid cone, but this is not true. Just try it!
In any case, the claim that a discone or biconical antenna is almost a
DC-to-daylight antenna is simply not true, even in RX, let alone in TX!
Which of course doesn't hinder manufacturers to keep claiming this, and
book authors to keep copying and pasting that claim without having ever
built such an antenna themselves.
Manfred
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*Dave - W?LEV*
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Dave - W?LEV
