The Linear-Loaded "Crappie" 40M Rotatable Dipole
A Short 40 Antenna You Can Build for $50 or Less
the SARTG RTTY contest in mid-August 2006, my 40M performance was
terrible -- just over 20 contacts. That was using a two-element
wire yagi (inverted-V elements) up nearly 70'.
for the following weekend's SCC RTTY contest I vowed to improve
my 40M antennas. I had a week of vacation with most of the week
free to play with antenna stuff.
Tuesday, I decided on building a rotary dipole to put at the top
of the tower. When fully cranked up, that would be about 55' above
ground level. Clearly this is not optimum -- 70' is the "industry
standard" half-wavelength height for good 40M operation. But
a lot of reading on the web told me a rotatable dipole even 50'
up will be as good as, or better than, an inverted-V at the same
height. And it can be pointed to null out noise or peak a signal
(a bit, at least).
aware of the constraints I was accepting, I was ready to try something
The first step was to review all the designs out there. I looked
at the Cushcraft D40
dipole that Don Hill, AA5AU, installed earlier this year, following
the terrible damage by Hurricane Katrina last September (2005).
His is at 70', and he reports great results with it. Mine would
be at 55' and quite a bit shorter, but I hoped I'd still get good
the kind of antenna I wanted, and not being very inventive (or at
all handy for that matter), I needed a specific model to follow.
I looked at two kinds of shortened dipoles:
Two Poles and Some EMT Conduit
I examined my on-hand materials: I had two good 16.5' fiberglass
"crappie" fishing poles that I had purchased for $9.99
each from Cabelas.com in 2002 to make a Spider Beam yagi. In
2011, they're still available for $9.99. Just
look for part number CCt-165!
had removed the whippy end sections, and added a couple of feet
by fabricating a pair of three-foot long 1.5" PVC plumbing
pipes reinforced with dowel (or broom handles), and inserted into
the 1.5" end of each pole -- they fit snuggly. This gave me
17.5 feet for each side, or 35' tip to tip.
connect the two fiberglass poles, I found a piece of 1.5" EMT
thin-wall steel conduit that would snuggly accommodate the butt
end of each element's PVC extension. I cut this to about 4.5' in
used a pair of bolts on each end to secure the connection of the
EMT tubing and the inserted pole ends. It's plenty strong enough
for this purpose.
Plate on the Cheap
The "chopping board" mounting plate, with U-bolts for
the mast and around the EMT tubing.
a mounting plate on the tower's mast, I used two white chopping
boards, each about half an inch thick of tough acrylic. Got them
for $1 each at the Dollar Store -- quite a bargain.
chopping boards were bolted together for a 1" thick mounting
plate. Half-inch holes were drilled for the 1.5" U-bolts --
two around the mast, and two around the EMT tube of the antenna.
length of the antenna was now about 38' tip-to-tip. Pretty
short for a 40M dipole. But I was going to load the antenna with
mid-element coils, per GM4JMU's
design. I spent a morning making the coils according to the
design (40 turns on a 1.5" PVC pipe 6" long), and by evening
had the whole thing ready to test in its stand in the back yard.
Start With Mid-Element Coils
my Palomar RX Noise Bridge ($5 used from a local ham -- and once
I figured out how to use it, it's fabulous), I determined that the
antenna was too short.
lengthened the elements with alligator clips so wire hung over the
ends. After an evening and a morning of experimenting in the test
stand, it was resonant at 7.040 Mhz with 2' more wire on each end
-- or total length of 43' or so.
didn't want a dipole up on the tower with extra wire flapping off
the ends. I know I could make this work with shorter elements by
changing the coil diameter or adding more windings, but this meant
a lot of work and it was a crap shoot (purely guesswork).
have no grid dipper or analyzer to give me an idea of where the
coil I'm building is going to resonate. I wanted something even
simpler -- it was already Wednesday and my week was running out.
Nov. 2008 -- I plan to try the GM4JMU mid-element loaded
design again. From what I've read, the inductance loading can provide
more bandwidth than linear loading, which is useful for contesting
when running CW and SSB.)
Course: Linear Loading with Twinlead
Schematic of the dipole. Linear
loading twinlead nearly as long as the radiating elements.
Maybe I am breaking laws of physics here.
I remembered the K4VX
design I'd read on the web, using 450-ohm ladder line as a linear
loading method. I had miles of very good (big wire) 300-ohm twinlead
picked up cheap when Radio Shack dropped "Radio" from
their inventory and, in my opinion, became just "Shack"
-- and then in Canada dropped the whole name to become The Source
by Circuit City. But I digress (I am bitter -- can't even get a
chunk of RG8X locally any more).
decided to give this 300-ohm twinlead loading a try. The original
design called for 22'6" elements and 11'10" linear loading
sections of ladderline. I figured seeing as my wire elements
were going to be just 19' long, I better start with about 16.5'
of 300-ohm twinlead.
the coil-loaded experiment, which took more than a day to get together
and adjust, the entire W4VX system took less than half an hour to
build. It's SO EASY!
the Sweet Spot
SWR plots for the Crappie 40M Rotary Dipole.
was very close in my guesswork. The RX Noise Bridge measured the
antenna's resonance at 7.460 Mhz. Too short.
I added 16" first, then up to 24" of twinlead to each
end of the loading sections. (Total twinlead length on each side
was now 18.5').
Resonance smack-dab in the CW/DX RTTY band -- 7.035 Mhz, which was
my design goal. (Blue line) SWR 1.1:1
and below 2:1 all the way up to 7.120 Mhz. Quite wide-banded considering
Details: Making It Weatherproof
step: making the thing robust. I taped the wire elements (#18 insulated
copper) to the 300-ohm twinlead, then taped and cable-tied them
to the fiberglass poles. Resonance went way off due to closer coupling
of the wire with the linear loading twinlead, so I re-adjusted the
300-ohn twinlead lengths (shortened them by a couple inches) and
resonance was back at 7.035 Mhz.
wire taped to TV twinlead.
took some doing to make the thin wire connections with the SO239
connector nice and strong. Not sure it is going to hold up in the
weather, but I soldered everything, tape it up, and tied it in as
best I could.
antenna bolted onto the tilt-over tower's mast very easily. I left
it about 1.5' down from the very tip, and added fishing downrigger
stainless steel guys from the pole to just past the midpoint on
each element. (I worried this would throw the antenna out of resonance,
but it had no impact). It keeps the fiberglass from supporting its
own weight -- eliminating the 14" of tip sagging that became
apparent in the test stand.
With Tower Height
the tower was fully extended, (red line)
the SWR flattened out a bit, and climbed to 1.4:1 at 7.035 Mhz.
I can live with that. (The resonant midpoint moved up 20 Khz to
7.055). With the tower nested down, dipole at only 35', the SWR
is one or two tenths lower across the board.
time the tower's tilted over (next summer I hope), I will retune
the loading sections by taking out a couple of inches to bring the
"in the air" resonance point down 20 Khz.
Tried the antenna in the SCC RTTY contest August
26, 2006, and had a hard time working anyone with any antenna, so
the verdict isn't in yet. It's a lot quieter than the vertical.
Made about 30 out of 40 contacts with it that night (the remainder
using a ground-mounted vertical).
post an update once I've had a chance to try it out when conditions
are better than terrible.
Finally ordered some new 100' coax cables, freeing
up some good RG8U from other antennas in the yard. Replaced the
run of RG58 from the shack to the base of the tower, where it barrel-connects
to the RG58 going up to the Crappie dipole. The old cable was the
problem and now this antenna really works well, considering its
short, very linearly loaded elements and the low height of 55'.
with the very thin-guage copper wire, the antenna handled the amplifier's
1000W in ARRL DX CW and CQWW WPX RTTY without a problem. I didn't
stay at 1KW for long, as I didn't need that much power to run stations
when I wanted. Typical power was closer to 500W.
the dipole aimed at 100 degrees (to most of the U.S.) in NAQP RTTY
to work about 85 of the 102 contacts on 40M. Rest were on the ground-mounted
worked lots of European stations on the second night of ARRL DX
CW, after the cable was replaced. Not as many as the previous year
using the delta loop array, but still darn good for a simple, low
what I worked from 4:56 p.m. (Pacific) to 5:53 p.m. on the second
night of ARRL DX CW on 40M with the dipole:
British Columbia, this is a pretty good array of 40M DX using a
simple shortened dipole. South Africa was an especially great surprise.
Highly recommended, if you can't put up elements closer to full
A roundup look at the antennas currently in use at the
Wire Vertical Phased Array Project
Simple "from the book" phasing lines, a relay switching
box, and two easy 68' wire verticals with raised radials gives me
a competitive contest signal covering most of the horizon on 80M
Hex Beam Project
Read the construction details for a homebrew hex-style beam. This
is a super performer -- if you want all five upper HF bands, or
don't have the wingspan room for a spider beam or other large-format
Beam Group on Yahoo
A spider beam hot spot. See what other homebrewers
(and spider beam kit builders) are doing to get great signals on
20m, 15m and 10m.
Spider Beam Site
Information about the spider beam from its inventor, Con DF4SA.
See how he has used this outstanding antenna to win CQWW contests
from Portugal. The
spider beam is a serious antenna that, in my opinion, is destined
to be one of the most popular homebrewed tribanders.