Practical_Antenna_Handbook_0071639586

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top thus reduces high-angle radiation on both 160 and 80 m. The radial system can be
the same as that for the inverted-L.
In the early days of AM broadcasting, many stations in the United States and elsewhere
successfully employed the T antenna as a way of putting most of their radiated
signal into the groundwave; only in time did the T come to be supplanted by the purely
vertical monopole.
The downside to the flat-top is that it requires twice as much span for the horizontal
wires than the inverted-L does. However, if total available space in the long dimension
is an issue, the two sides of the flat-top portion of the T can be bent or run in zigzag
fashion. To minimize high-angle radiation, such bending should be applied to both sections
of wire as symmetrically as possible, but don’t lose sleep over it.
80-Meter Dipole on 160
Although not optimum, an 80-m dipole can be used on 160 m with some success. In
fact, there are two different ways to do this.
The first approach requires an ATU capable of tuning out a wide range of reactance
on 160 m. Assuming you have such a unit, simply match your antenna to your transmitter
(presumably designed to match 50 Ω) on 160 m by tuning the ATU for minimum
SWR at the chosen 160-m operating frequency. Even though the dipole is too short to
fully expose the high-current portions of the natural radiating standing wave, there’s
enough total wire length to get signal out of the feedline and on its way. There are two
disadvantages to this scheme:
• A suitable wide-range ATU typically costs more than most units commonly
seen in use, and may be harder to locate. The old Dentron Super Super Tuner (no
longer manufactured) is one that can match short antennas on 160, but not
without some internal losses.
• The antenna feedpoint impedance is highly reactive and a terrible match to any
available transmission line. Consequently, the line will experience very high
standing waves with concomitant voltage and current peaks and increased I 2 R
losses that are characteristic of high SWR. Open-wire line is probably the best
feedline for this application because it has less added loss in the presence of
high SWR than coaxial cable or twin-lead does, and it is less likely to break
down because of the high voltages encountered. On the other hand, if you’re
running only a few hundred watts or less, just about any RG-8 or equivalent
cable should be able to handle the voltages.
Another approach is to use your existing 80-m dipole and its feedline as a pseudoflat-top
or T-antenna on 160. This is particularly attractive if you have a “reasonable”
feedline length (say, more than 60 ft) and if the center of your dipole is substantially
higher than your radio room. If so, it’s clear that your feedline goes not only horizontally
(to get from indoors to outdoors) but up as well. In other words, there’s a vertical
component to the feedline path that can serve as the vertical section of a “poor man’s”
inverted-L.
In this case, simply disconnect the transmitter end of the feedline from its normal
connection, tie the bare ends of the two feedline conductors together, and reconnect the
joined ends to the hot (center) terminal of the SO-239 coaxial socket on your ATU or
transceiver (Fig. 18.8). Tune the ATU for minimum SWR, but it shouldn’t be excessive