Topband: Multiple drops and vertical antennas

[Tom Rauch]

I've received a few e-mails on this. The topic of multiple drops comes up
over and over again. Some of this is on my web page under radiation
resistance and radiation topics.

Multiple drops actually have been shown to NOT reduce ground loss over a
single conventional system that covers the same net area of ground. The key
here is "the same net area of ground".

As a matter of fact, it is easy for them to be worse if the current
distribution in the ground system is accidentally made uneven.

The wive's tale that multiple drops help ground losses comes from two
sources. The first is if we have a very poor ground system on an antenna,
and add a second poor ground system on a second identical antenna, we reduce
losses. That's because the in-phase radiation does not reduce system
radiation resistance, but current divides between two lossy INDEPENDENT
ground systems. The second one being added as an additional NEW ground
system. Naturally if we make the ground system larger, losses will decrease!

The second source of this wives tale is habitually misusing or misapplying
"radiation resistance" to mean feedpoint resistance of one conductor in a
multi-conductor feed system. Factually, the radiation resistance given by
IRE standards does not change.

If we use the same physical area to install one large ground, the system
winds up better than two independent smaller poorer grounds. The same would
apply if each vertical element had loss, such as by a lossy loading coil or
wire losses.

It is more a matter of convenience or fitting a complex site than any actual
gain in performance for a given amount of area, materials, or labor.

If we just made the single system use the same materials used in two
separate parallel systems we would come out the same or better.

This has been the result of direct comparisons of VLF systems, and it
certainly would carry over to low-band antennas. In a few VLF systems
engineered to use multiple grounds and drops a substantial amount of signal
was gained by converting to a conventional radial system covering exactly
the same physical area. Modern or up-to-date engineering textbooks at best
say the entire issue is "confusing" or unproven, and at worse give examples
where the multiple ground theory has been proven invalid in practice.

73, Tom W8JI