Practical_Antenna_Handbook_0071639586

408 p a r t V I : a n t e n n a s f o r O t h e r F r e q u e n c i e s
where b is the distance (usually half the total boom length) from the mast to where the
elements farthest from the mast are attached, and e is half the average length of elements
nearest the ends of the boom. Only elements directly grounded to the boom
should be considered as fully contributing to the top hat effect.
Note If it is to ever be used as a radiating monopole, any guyed tower must have at least one
insulator in each and every guy wire attached to the tower, including guy wires attached at
guy stations farther down the tower. With the exception of any initial lengths established in
the top set of guy wires for top hat purposes, the distance from the tower attachment point
to the uppermost insulator in each guy wire should not exceed 3 ft or so. Even though the
tower itself may be grounded at its base, failure to insulate each and every guy wire near
the point where it attaches to the tower completely changes the electrical characteristics of
the structure, complicates calculations and assumptions, and may well destroy the overall
efficacy of the tower as a radiator. Even if you believe you have no intention of ever using
your towers as low-frequency antennas, interests and plans can change—insulate your guy
wires when you erect your towers—it’s a whole lot easier to do it then than it is to do it later!
As discussed in the chapter on verticals (Chap. 9), if a grounded vertical monopole is
increased in length somewhat beyond l/4, its elevation pattern is “squashed”; that is,
radiation (and reception) at low vertical angles is enhanced at the expense of the higher
angles. For most intended uses of an efficient transmitting vertical on 160 or 80 m, this is
desirable—hence, the suggestion that it’s okay to let the total equivalent electrical length
exceed l/4. When this is the case, the feedpoint impedance at the base of the antenna will
increase and the reactive component will appear as a series inductance, which is quite
easily canceled with a transmitting air variable capacitor in series with the feedpoint.
When the base of a vertical is insulated from ground so that it can be fed directly
from the transmission line, the technique is called series feed, and it is most often employed
in conjunction with wire verticals. But for mechanical simplicity and low cost,
towers and rigid 160-m verticals in most amateur installations are grounded at the base.
For these, shunt feed must be used. Here’s how it works.
In general, the impedance at any given point on a grounded vertical monopole increases
with height up to l/2. Numerous ways of connecting a feedline to such a structure
have been employed over the years. Figure 18.2 shows one of the simplest: an
unbalanced output ATU feeds a wire connected to the side of the tower at an appropriate
point. Electrically, this is half of the delta match often employed to provide a balanced
feed to the center of dipoles and Yagi driven elements. The “correct” height for the tap
on the tower depends on the design and tuning range of the ATU; the objective is to find
a height where both the resistive and the reactive parts of the impedance seen at the
lower end of the wire by the ATU are within its tuning range.
Similar to the sloping wire or half-delta feed of Fig. 18.2 is the shunt feed of Fig.
18.3. This is known as a gamma match; it is basically the unbalanced form of the T-match
often used to feed HF and VHF Yagi driven elements. Physically, the only difference
between this and the delta match is that the gamma wire or rod is held a constant distance
from the side of the tower, allowing the ATU and gamma rod to be placed very
close to the tower. The gamma match is adjusted to provide minimum SWR on the
transmission line at the center of the operating frequency range by alternately changing
the tap point on the vertical and the spacing of the gamma rod from the radiating element
while retuning the series capacitor.