Topband: Ground mounted 1/2 and 1/4 wave verticals (was GAP)

[Donald Chester]

Tom is  correct in that a half wave vertical wouldn't have zero current at the base feed point; I should have said "near-zero", since if it actually had zero current there would by definition be zero power transfer, the radiator would be an infinitely thin wire and the SWR would have to be infinite with no loss (including any loss from radiation).  I think that's what Capt. Lee had in mind when he touched on this topic in his Vertical Antenna Handbook, but he was citing his response to a correspondent who had proposed that the current at the base of the 1/2 WL vertical was zero. Lee should have stated his case more precisely for the benefit of the readers of his book.

The same is true with tuned feeders. At voltage and current nodes along the line the current or voltage is never actually zero, since in every case all lines have some loss.  A tuned feeder looking into an open or short circuit would have near-zero current and voltage points, but never actually zero, since the VSWR can never reach infinity, and if that were possible, the current and voltage at the loops would have to be infinite.

The finite physical width of the vertical radiator, plus the energy loss due to the radiation resistance assures us that the base impedance would never be infinitely high nor would the base current ever actually reach zero, and the VSWR along the radiator would never reach infinity.

But doesn't this merely confirm the notion that the half-wave vertical needs some kind of ground system to work against, and in addition to minimise ground loss when the base of the radiator is in the vicinity of the lossy earth?  A 2m or CB ground plane, with 3 or 4 radials, 20 ft or more in the air, has very little ground loss. At the opposite extreme, a ground-mounted broadcast tower with buried radials or radials lying on the surface, needs a large number of radial wires to divert the rf return paths away from the soil and thus keep ground loss to a minimum. In the case of elevated radials, you are moving between those two extremes; it logically follows that as the ground plane is raised above the earth from the surface to a significant fraction of a wavelength, proportionally fewer radials are needed to maintain ground losses at an acceptable value.

The old WWV site on the east coast used vertical centre-fed dipoles mounted on wooden utility poles.  I never read whether or not they deployed radial systems at the bases of the poles, but I suspect not.  However, the lower ends of those vertical radiators were some distance above the ground surface.

I have a nearly complete set of the 1930s IRE Proceedings, and this has prompted me to dig them out and review some of the articles.  I would be interested in looking at any actual experimental data compiled to quantify ground losses, using physical antennas fed with physical rf watts with data collected using a variety of physical ground planes.

Every engineering text I ever read on the topic of broadcast antennas always emphasised the point that a substantial ground plane was essential, regardless of the height of the tower, and I don't recall ever seeing anywhere that this was done merely to satisfy the FCC.


Don k4kyv