Practical_Antenna_Handbook_0071639586

C h a p t e r 1 8 : a n t e n n a s f o r 1 6 0 M e t e r s 413
Insulated-Base Loaded Verticals
In some situations, insulating the tower base or feedpoint from ground may be necessary
or desirable. One prime example is when an array of vertical monopoles is going to
be used to provide greater forward gain and/or front-to-side (F/S) or front-to-back (F/B)
ratios than can be obtained from a single (omnidirectional) radiating element. As discussed
in Chap. 11 (“Vertical Arrays”), getting full performance from a multielement
array requires strict control of both the magnitude and the phase of the feedpoint current
in each element of the array. As a general rule, this control is easier to maintain
when the feedpoint is not grounded—i.e., when series feed is used.
A loaded insulated-base vertical is essentially the same thing as one side of a shortened
dipole or one side of the driven element of a beam that uses loading coils or some
other technique to shorten the elements. As before, the performance of such a vertical
will be no better than the conductivity of the ground system beneath it. Unless the vertical
is mounted in seawater, radials are a must.
As with the grounded vertical, loading of an insulated vertical can take many forms;
the simplest and most efficient are capacitive top hats with or without a small amount
of base loading inductance. However, keep in mind:
• Radial wires lying on the ground or in very close proximity to ground are not
resonant wires; there is no one specific length that is optimum. In general, 20 or
more radials between l/4 and l/8 in free-space length will provide an excellent
ground for a vertical monopole, but there is nothing magic about either the
length or the number. “The more the merrier”, and if they have to be shorter
than normal to fit a backyard, so be it.
• Up to an effective electrical height of 5/8l (or 225 degrees), increased height
results in a modest increase in the low-angle radiation of a vertical monopole,
obtained at the expense of high-angle radiation. This additional gain reaches its
maximum at 5/8l, beyond which the low-angle gain gradually decreases.
Thus, a monopole having an equivalent electrical length of l/4 or 90 degrees on
160 can do well on 80 (although matching its high impedance there can be
tricky), but will suffer reduced performance on 40 unless a phasing section is
added such that the physical structure acts like a collinear array.
• In an array of multiple vertical elements, each element should have its own
radial field and the fields should be as similar as possible. If radials from
different elements overlap, it may be useful to connect them together, but it’s
not clear from the available research or modeling that the possible improvement
in ground conductivity is worth the extra effort. Radials that overlap should not
be allowed to intermittently touch or develop oxidized connections, however!
Either connect them together firmly or insulate them from each other.
Figure 18.5 shows a situation in which a 90-ft tower is insulated from ground and
operated on 160, 80, and 40 m. Here it is fed by a simple L-section or reverse L-section
ATU. Note the approximate values of the inductors and capacitors in each configuration,
and also their relationships. On 40 and 75/80 m, the feedpoint impedance of the
tower is inductive, so a series capacitance and shunt inductance are used for the ATU
(i.e., reverse L-section coupler). On 160, the feedpoint impedance is capacitive, so the
inductor and capacitor are reversed.