The first set tunes from about 5.8
to 14 MHz, which includes the 49, 41, 31, 25 and 22 meter shortwave broadcast
bands. The capacitor is the oscillator section of a poly film variable
capacitor, and the coil is wound on a toilet paper core. The main coil,
L1, was wound on a length of 1/2 inch of the core, and separated from the
antenna coupling coil, L2, by about 1/8 to 1/4 inch on the same core.
Calculated inductance of L1 was 10.6 uH. Measured capacitance of the
variable was 10 - 68 pF. Using a 50 foot end fed antenna, stations
could be heard 24 hours a day, with the higher bands being heard during
daylight, and the two lower bands coming in at night as well.
There are 4 primary ways to attach antenna
and ground, and all work to one degree or another. First and second, the
antenna alone may be connected to either the antenna tap or to the ground
tap. Third and fourth, the antenna and ground are connected to the normal
taps as indicated, or they may be reversed. A small frequency shift was
noted when going from option 3 to 4. Use of the ground sometimes made the
signals stronger. Quantitative assessments using on-air stations was not
attempted as the signal strength varied constantly, with shifts of up to 10 dB
being noted within the space of 30 seconds. As a side note, just
connecting the antenna to the top of the tank coil was not as productive.
Selectivity, while not great, was
sufficient to separate the short-wave BC bands (the 25 and 22 meter bands tended
to run together, however), and often to separate two stations within a
band. Slow tuning, even with this selectivity, is needed due to changing
signal strengths and because of the fast tuning rate.
Using a longer antenna gave higher signal
strengths, as expected, and little or no shift in frequency was noted when going
from one antenna to the other. No MW broadcast band interference was
observed.
This is a schematic of the set:
Spacing between L1 and L2 was closer to 1/4 inch, as it turned out when my
students built this, and they seemed okay. Since the shoddy antenna I have
in my classroom is well shielded by the metal roof of the building, I used a
grid dip oscillator to check the sets for response before letting them out of
the room. With the unmodulated signal from the GDO, I could hear a few
faint heterodyne squeals from stations that were there but too weak to hear.
I made another coil on the same diameter form using #20
solid plastic coated hookup wire, winding 20 turns on 2 inches for L1 and 6
turns for L2. This gave me slightly stronger signals, as expected. I
made a breadboard attempt to use this with a Tuggle antenna tuner, but gave up
early on, primarily since this was designed to be a student radio, and for other
more technical reasons which I'll explain below.
This is
really a "first iteration" radio, designed just for shortwave use, and, while it
works pretty well, clearly has some way to go. The tuning rate, some 10
MHz in a 180 degree rotation of the tuning knob, is way too fast, but is
probably okay for this first attempt. It was clear that the selectivity of
the set leaves something to be desired, so more work on the coils is in
order. I didn't spend much time optimizing the coil tap for the detector,
just picking one that is historically useful. Tapping the detector to the
top of the tank circuit was not as productive as the tap I used. I also
didn't spend any time picking over detectors, and just grabbed the first
germanium diode out of the box. If I can get the selectivity up a bit, I
will have to get serious about band spreading the tuning. Obviously, I
need to go back and look at an antenna tuner again. One thing I realized,
which may or may not make a difference, is that at these frequencies the antenna
is a quarter wavelength long or longer, so you can't make it resonant with just
a few turns of a coil in the antenna line. However, the set is a snap to
build (especially the coil), and picks up a bunch of stations, and is fun to use
if you haven't tried the higher frequencies.
Okay, time to do some more playing around. Variation #2 was built to
try to get some more selectivity for a kid radio, and essentially the same as
the first but with the primary separated from the tank coil. This seemed
to work a bit better, and the bands separated better as well. This is the
schematic with some notes:
For more information on the general nature of shortwave
listening and the frequencies for each band, try this web site:
http://www.danatkinson.co.uk/html/sw/faq.htm