A shortened vertical on 160 need not mean very narrow bandwidth. While
searching my antenna books for ideas on a new 160 vertical last month, I ran
across a Doug DeMaw design called the "W1FB 160 Meter Monopole" in Bill
Orr's "W6SAI HF Antenna Handbook" (page 6-9 of the second edition, 1998).
The text talked about a 2:1 SWR range of 165 KHz, so I decided it was worth
a try. The design is a "fat" vertical, using two #14 wires spaced at one
foot, with a loading coil near the top, and a unique folded top-hat design.
The loading coil is 80 microhenries, and an additional base loading coil of
around 30 microhenries is used to tune the antenna to resonance, and to
provide a point at which to connect the feedline for a 50 ohm match.
I had an old length of B&W coil stock in the junk box, and it measured right
around 80 microhenries. I enclosed the coil in a clear plastic tube with
removable endcaps, sold at Lowes home improvements for holding info sheets
when attached to a "home-for-sale" sign. The base loading coil was mounted
in a 6x6x4 waterproof plastic electrical box. For the elements, I used dark
gray insulated stranded #14. The separators were made from 1/2" white
plastic waterpipe, and I used one every 8 feet.
I found that using the antenna length in the article produced far to high a
resonance, and I ended up adding about 8 feet to the three-wire top loading
section above the coil. Adding length below this coil to the two wire
section produces much less change in the resonant frequency per foot of
wire. Of course, you could also just increase the value of the coil, but
new coil stock is a bit expensive these days. When tuning the antenna,
there's considerable interaction between the base coil tap when adjusting
for resonance, and the 50 ohm tap for the feedline. I used the MFJ 259B to
make short work of these adjustments. Now to the good part: With no radials
at all, the 2:1 SWR bandwidth was indeed in the neighborhood of 160 KHz.
With eight 125 foot radials, the 2:1 bandwidth is 80 KHz, so, with the
resonant point set at 1.835, I can cover from 1.800 to 1.875 with no
problem. There would have been more radials, but the antenna was finished
just as the sun was setting on the first day of the CQ 160 CW contest.
Contest results were very encouraging, with 48 states worked (where in the
heck was SD and KL7?) along with dozens of DX multipliers, while running 800
watts to the antenna. Multiple contacts have been made into the middle
east, with good reports. In short, I can work everything I hear with this
vertical, so now the challenge is to hear better on 160.
If you happen to have a nice 75 foot tree in your yard, this makes a nice
"stealth" antenna. If you paint the spreaders flat black and use the gray
insulated wire, the antenna is not visible from 100' away. A shorter
support tree means a larger coil at the top. Back in the late 70s (or early
80s?) there was an article in QST describing a toploaded radiator called
"The Minooka Special". That vertical, which I also had good luck with, was
only 35-40 feet high and used a top loading coil of closewound enameled
copper magnet wire wound on a section of PVC pipe. I had no problems with
that coil, also running 800 watts output. Of course, the bandwidth on that
antenna was substantially less than this W1FB design.
K8AC
Floyd Sense - Angier, NC
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