November 2000 QST

WORKBENCH
PROJECTS AND INFORMATION FOR THE ACTIVE AMATEUR
The Doctor is IN
QDave, WD8DK, asks, “I am using a G5RV on 80-6 meters.
How efficient is this antenna on 6 meters I have been
told that it is very inefficient on this band. In fact, I have been
told that a 1 /2 wavelength dipole is more efficient than the
G5RV on 6 meters. Any comments”
AOn 20 meters, where the G5RV was designed to operate, it
boasts a little gain over a conventional half-wave dipole.
Given a reasonably efficient feed line (450-Ω line) and a good
antenna tuner, there’s no reason why the G5RV can’t be at least
as “efficient” as, say, a coax-fed dipole in the HF bands.
However, on 6 meters the G5RV acts as a long-wire antenna,
with an azimuthal pattern with multiple, very narrow lobes. The
narrow lobes are what give it gain, but also what make its
performance compared with a regular garden-variety dipole
inferior in direc-tions other than the ones it favors. The EZNEC
plot shown in Figure 1 assumes that the antenna is mounted as a
flat top at 50 feet above average ground. The G5RV has
significantly more gain than the simple dipole, but it achieves this
mainly in four, narrow-beamwidth directions. For the rest of the
azimuths, its pattern has nulls that the dipole covers well.
Any multiband antenna is a compromise, but most of us can’t
have five or more dipoles hanging in our backyards. On 6 meters
I would recommend a separate antenna designed for that band.
There are a couple of inexpensive 6-meter wire antenna designs
on the ARRL TIS Web site at http://www.arrl.org/tis/. Go there
and click on “Antenna Projects,” and then “Other VHF Antennas.”
QI have a 10-year-old Realistic 13-inch color TV that I use
with my ATV station. Recently the TV went completely
dead. It won’t turn on when I press the ON button on the front
panel, or when I try to turn it on from the remote. I checked the
power supply fuse and it is okay. The power supply appears to
be working as well. Any ideas
TVs operate in what you might call a “sleep” mode.
AMany That is, there are circuits within the TV that are active
continuously—even when the rest of the TV is off. Usually the
primary microprocessor is always active, waiting for the
command to switch on the rest of the set. If the microprocessor
isn’t responding to manual or remote “on” commands, the
microprocessor could be defective. If you have a volt-ohm meter
and a schematic diagram, measure the voltage at the V cc
pin of the
microprocessor. Is it receiving power from the power supply
(probably 5 V) If so, find the microprocessor pin that produces
the output signal to turn on the rest of the TV. Do you get a
reading at this pin when you press the TV’s “ON” button If not
(and I suspect you won’t), the microprocessor is probably dead.
On the other hand, if you do get a reading, it’s time to troubleshoot
the rest of the circuit that is responsible for switching on the set.
This is likely to include a couple of switching transistors and
possibly an optoisolator.
QLast night I heard a strange CW signal on 6 meters. It was
hissing and buzzing, but I was still able to copy. To my
astonishment, I learned that the station was 500 miles away
from me. Was this sporadic E propagation
AMy guess is that you heard auroral propagation. The clue is
your description of the signal as having a hissing or buzzing
characteristic.
Those of us who reside at the higher latitudes are occasionally
treated to the visual spectacle of the aurora borealis, better known
as the “northern lights.” (Yes, there are “southern lights” as well,
visible occasionally in South America and Africa.) The aurora is
caused when the Earth intercepts a stream of charged particles
Figure 1—This is an EZNEC plot of a G5RV antenna on 6
meters compared to a dipole cut for 6 meters. The solid line
represents the G5RV pattern while the dashed line represents
the dipole. Notice that the G5RV is creating numerous narrow
lobes of radiation.
48 November 2000
Figure 2—The auroral “curtain” can function like a giant
mirror in the sky, reflecting radio signals over substantial
distances.