Guys,
It seems that you have missed exactly what this antenna is.? The cage acts like a capacity hat, adding enough impedance at the ends to bring the antenna to resonance.? I think you may also be mixing terms when talking about efficiency.? It in the above example of tuning the input to a transmission line so that maximum power is delivered into the transmission line does not mean that power is actually radiated.? Lossy parts of the antenna system, ground included, do not radiate.? There are literally thousands of articles on using capacity hats at the ends of radiators to bring the antenna into resonance but the part of the radiation resistance equation that radiates is still very small compared to a full size radiator.? So while you may approach 98% efficiency for the transmitter load, you may not be radiating that power into space.? It is possible for that power to be dissipated in wire losses, or ground losses or even in directivity sending the power to somewhere is it not useful.? When you think about shortened antennas, consider that the field generated by a short wire is going to be considerably smaller than a full size wire.? In the case of the described antenna above, I have no doubt that the antenna will present a matched load at the transmitter and it likely radiates some signal.? However, look at the wire in the cylinder and realize that the resistance of the wire in the cylinder is not radiating but it is dissipating power just like any other resistance.? I suggest that that the cylinder radiates very little as the radiation of the wires folded end to end likely cancel each other out. That does not make it a bad antenna!
There are several antenna design in the Antenna handbook that play with black magic to get the antenna to radiate on the low bands, 160 and 80.? Some actually will use large diameter coax for the radiator or will use pipe to play with the L/D ratio or one really unique design uses a combination of lengths of coax to resonate the antenna in two closely spaced frequencies to broaden the antenna bandwidth.
I would like to point out that the handbook also has some great drawings on the current distribution in antennas.? Take note of the vertical that has a large inductor at the base or center and see the disturbance in the antenna current.? Visualize the current generating a field and you can see that these antennas may still deliver full power at the input of the transmission line, but all of that does not translate into current in the radiator.
As to the use of baluns and their mention in the handbook, of course baluns are mentioned for use with dipoles.? The dipole fed at the center or even off center are balanced loads.? If the antenna is fed with coax, then an imbalance is produced.? That translates to feed line radiation which may (most often does) affect the radiation of the antenna due to fields cancellation.? If one uses a balun to couple the balanced load with the unbalanced transmission line, balance is restored and radiation of the feed line is minimized.
As to comparing with antenna performance with dipoles or an isotropic radiator, that method is valid.? We know what an isotropic radiator field will look like and what load it presents.? To compare the antenna above with a full size half wave dipole at the same location and height would be useful for someone considering the design to match criteria at their QTH.? All of us have used various antennas for years.? We all have had contacts confirmed from DX stations, many with QRP, but that does not mean the antenna is better than others.? It means you can use that antenna to transmit to someone in that location sometimes.? Remember the old adage, "you can get a wet noodle to radiate, it does not mean you should or would use that all the time".
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Al Skierkiewicz
WB9UVJ
Al WB9UVJ