Take back 40 Meters from foreign broadcasters
By O.E. Gardner, W9RWZ
Powerful foreign broadcast stations discourage much nighttime Ham activity
on 40- meter phone. Although they are strongest late at night, they seriously
interfere during the early evening hours when the short-skip distance
is still suitable for stateside activity. This low-cost, easy-to-construct
receiving antenna can change each interfering broadcast station into two
almost private channels (USB & LSB), for those who care to experiment.
Hams with conventional antennas will still avoid those frequencies.
This idea has been tried by many Hams in the past, but apparently no
antenna has proved effective or reliable enough to gain wide acceptance.
Note that deliberately transmitting on foreign broadcast frequencies
is permitted because U.S. amateurs have primary rights on our 40-meter
band.
Plotting this antenna on ELNEC or EZNEC reveals why it can work so effectively.
Two vertical loops in the same plane, separated by 1/8 wavelength and
fed with 135 degrees phase difference (over real ground), yield deep nulls
in both vertical and horizontal polarization at low and medium elevation
angles. The cardioid pattern maintains a narrow cone-shaped zone that
nulls out incoming waves of changing polarization, and probably explains
why this system is able to maintain its deep nulls for extended times.
Obtaining this improved performance requires rather precise control
of phase and amplitude, along with good stability over times suitable
for QSOs. All of these are accommodated using two High-Q loops of about
4-foot diameter, made from 1/2 inch diameter surplus aluminum cable-TV
coax, where the inner conductor is used only to deliver DC control-voltage
to the varicaps. They are tuned with a varicap, which on this model, is
the internal-diode of a 400-volt power MOSFET. It performs exactly as
a varicap. Most other low-cost Power MOSFETs perform similarly.
The 135-degree phase separation is obtained by making one feed line
longer than the other. But if the feed-tap on one loop is oriented in
the opposite direction, one feed line now need only be (electrically)
45 degrees, or 1/8 wavelength longer. The longer one attaches to the loop
nearest the direction of the null. Termination impedances affect the accuracy
of these phase shifts so it is prudent to allow for some change. Polyethylene
dielectric with a velocity factor of 0.66 would be about 11 feet longer
at 7.2 MHz. Cellular polyethylene dielectric would be about 13 feet longer.
With the required phase shift provided by the feed lines, the two High-Q
loops can now be resonated near the exact frequency of the foreign broadcast
station, to maintain good sensitivity. But since great accuracy in phase
and amplitude is required, remote tuning with potentiometers at the operator
position provide the necessary tweaking of these resonant loops to trim
for small phase and amplitude changes that occur when the resonant frequency
is changed. Each loop is tuned by its individual potentiometer in conjunction
with a differential potentiometer which is common to both. One must accept
that reaction will be very sharp to get 40 to 50 dB deep nulls, and very
careful tuning will be required. It almost requires an AM carrier such
as foreign broadcast provides, and even then, rapid QSB can be tedious.
Deeper nulls can be obtained if an isolated 9-volt battery is used at
the junction box near the operator position, with the box grounded only
to the three coax shields (see figure). Using an AC power supply instead
of a battery adds stray capacitance and may degrade the nulls.
These loops were strapped to ordinary concrete blocks with rubber truck-tarp
straps, and could be easily moved around the yard. Having the loops in
exactly the same plane (as in a pair of eyeglasses) is not critical, but
the center-to-center spacing should be fairly accurate at 1/8 wavlength,
which is about 17 feet at 7.2 MHz.
Performance was very good, consistently nulling 15 dB over S9 AM stations
down to the noise-floor at S1 and remaining there for extended time periods.
A test generator with an AM tone, with strength about S7 (not in the null
area) was tuned exactly on frequency with a 40 dB over S9 foreign broadcast
station (i.e. when only one loop was used conventionally). Of course it
wiped out the test generator, but when the second loop was attached (having
previously been tuned for null), the foreign broadcast was severely attenuated,
and the S7 test generator now dominated. Audio cassette tape recordings
were made to demonstrate this. A typical Ham station, not in the null
zone, would be quite easy to copy.
An interesting, less important feature, is the ability to quickly utilize
one of the loops conventionly for 160M or 80M (or both). Tuning the loop
to 160M is done easily by attaching a shunt capacitance across the MOSFET
with small alligator clips. A combination fixed/variable of about 1700
pf resonates it. The MOSFET still provides just enough remote tuning to
cover the 160M band. Sensitivity is very adequate as indicated by a winter-time
noise-floor of S2, and strong signals reading at least 20 dB over S9.
Since this article is about receiving loops only, it is left to the
reader to arrange a receive/transmit configuration. Note that the 400-volt
MOSFETs will probably tolerate strong fields from your transmitting antenna
without damage. But in the presence of very strong fields at close range,
it might be very effective to reverse the DC voltage polarity to the MOSFETs
during transmit to cause the varicaps to conduct lightly and partially
short out the High-Q loops.
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