Size matters (resent to tacos by aa4hs)
jeffw at softronicsltd.com
Thu Dec 6 13:01:52 CST 2007
Ah, Winter! That time of year when young men's
fancies turn to -- Low Band DXing!
Executive summary: I have a half-mile long beverage
installed and operational. Longwave and mediumwave
(AM Broadcast band) stations are being heard regularly
in my shack near Coggon. Contact me if you're
interested in hearing it some night, if you'd like to
hook up your receiver to a real antenna, or if you'd
like to build one yourself.
Here's the long form story:
Many of you are aware of my particular fondness for
the really low bands, particularly the old 500 KHz
maritime CW band (now used by experimenters) and the
Longwave broadcast band at 153 to 279 Khz. Over the
past 3 years I've logged many LW broadcasters from
Europe and North Africa using a single wire, 1/4 mile
long beverage. It brings back memories of listening
to those same stations while on-board a ship in the
middle of the Atlantic ocean in the early 90's. Last
winter, I got to listen as Radio Luxembourg played a
musical eulogy to Wilson Picket on the day of his
passing. "In the Midnight Hour" never sounded so
Reception has always been spotty here, which should
come as no surprise given the distances involved. The
beverage's performance has been stellar on the 160m
band, giving a window into Europe almost at will. At
longwave, however, 3 or 4 good band opening nights a
year were the norm.
Every year I re-evaluate the antenna situation given
the materials at hand, what worked in prior years, and
what survived the winter. A ceremonial reading of
Misek's "Beverage Handbook" is also in order. This
year I happened to be in possession of a brand new 1/2
mile roll of 14 gauge aluminum electric fence wire.
The obvious question: Should I run a single wire 1/2
mile long; or the usual 1/4 mile, but make it a 2-wire
with a phasing network.
Misek is a big fan of 2-wire "Steerable Wave
Antennas," and I was initially convinced that this
would be the best route - until the calculator came
out and some simple math showed why LW reception had
been so spotty with last year's installation.
The minimum length a Beverage needs in order to have
directivity is a half wavelength. Most texts
recommend no more than 2 wavelengths, especially for
2-wire beverages where the reflection from the far
termination is used to aid in phasing. For typical
160 and 80m ham use, 600 feet is considered to be an
Cutting to the chase, a half-wave on the longwave band
is huge. The 800m (half-mile) roll is a half
wavelength at 187 kHz - not quite to the bottom end of
the band. I'd been getting what I thought were good
results with less than a quarter wavelength antenna
for all these years, an antenna that essentially had
no directivity. A two-wire array of the same length
would do absolutely no good on the LW bands - although
it would be great fun for AM broadcast band and 160m
Having passed the maths portion of the "Practical
Beverage Exam," Geography came next. Half a mile of
unobstructed land isn't all that easy to find and use.
In this world of Google Earth and Mapquest much of the
ground work can be done indoors. I found our house on
Mapquest and switched to the aerial view and printed
the target area. I saw that by shifting the feedpoint
several hundred feet to the southwest, I could install
the antenna in the field I usually use, plus a small
portion of the field owned by the next neighbor to
the north. One phone call explaining my strange
hobbies, and permission was granted.
Some more PVC posts and a fresh battery for the
tractor were obtained and construction began the
Sunday after Thanksgiving. Last Sunday the work was
finished. Of course, it was dark by the time the last
wire was set on its post, but that meant DX should be
rolling in. Sure enough, Morocco on 171 KHz was
easily heard. France on 162, the usual bellwether of
the band, was in and out but clearly there.
Last night the longwave signals were faint to
non-existent. Just for fun, I shifted down (in
wavelength) to the AM broadcast band. At the bottom
of the band on 531 kHz was a station 15 over 9 in
Arabic - clear as a bell in AM mode. Now, usually I
have to use a SSB filter to narrow the bandwidth and
mitigate interference. Not needed with this signal.
According to the frequency charts, it was most likely
coming from a 300 kW station in Algeria. Not a bad
haul! That was my first Mediumwave transatlantic DX.
I have sound samples from prior years and will be
recording the best bits of this season. One of these
days they'll go up on a convenient web page. If not
for problems with the recording software I'd have a
sample of last night's haul.
If any of you want to come out and listen or hook up
your own receivers to the antenna you may. Or - If
you've got time on your hands and land around your
shack - give it a try yourself.
73 de Jeff W0ODS
FarmNet mailing list
FarmNet at rf.org
More information about the Tacos