There seem to be some real beginners following this thread. It is
worth noting? for them that what goes on when putting a transmitter's r.f. into the feed line and antenna applies exactly to what goes on when receiving on that antenna and feed line (if we are to believe simple reciprocity theory). Though it probably won't be noticed, the receiver suffers no less than the transmitter.
One could go on at length on this topic. The observations will be
halted here by saying that elderly coaxial cable is the chief offender in the ham station. Cheap used cable from the ham flea market is not the choice for those interested in the frequencies in use with discone antennas as Manfred will confirm. Also, painful though it is, what was installed as brand new, expensive and high quality, cable is none of that twenty years later. Three dB of power being lost in a run of cable across the yard and all the way up the tower will produce a most misleading impression in the mind of the unsophisticated investigator. His view of the impedance match of his antenna as seen in the station will be distorted by the fact that his instrument is contending with a 6dB loss in the reflected power displayed compared to the power being delivered.
He is, in particular, to be highly suspicious of how broad the
resonance of his antenna is as determined from readings in the station. If it is too broad, then it is too broad for some reason: there are no "special" antennas with magical properties as Manfred is telling us. For the ham operator, the feedline is the first suspect.
'nuf said.
John
at radio station VE7AOV
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On 2023-08-03 14:00, Manfred Mornhinweg 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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