I recently reviewed the antenna factors for a commercially made
"overlapping bow tie" style antenna and find the factors to be rather
high (for example, over 30 dB at many frequencies over 300 MHz).
However, it is still a very simple antenna to build, and maybe your
scanner will be sensitive enough to compensate for such high losses.
Good receivers can detect signals as weak as -15 dBuV.
To improve the antenna factors, I would suggest that the overlapping
bow ties should be combined with an ordinary corner reflector. I
would expect the gain to improve (up to 10dB best case) at
frequencies over 50 to 100 MHz for a reasonable size reflector, plus
you would benefit from attenuation of unwanted ambient signals from
the rear and sides of the antenna.
The design requirements are:
S = the spacing from the apex of the reflector to the antenna
f = frequency (lowest for this case) in MHz.
Rule is: minimum S >= 0.25(300/f)
maximum S <= 0.7(300/f)
/
/ <-- reflector
/
/
O <-- S --> @ <-- antenna (in this case, our bow tie antennas)
\
\
\
\
The reflector is expected to be a full wavelength (300/f) long, but I
suspect you can shorten this to some degree.
For 50 MHz, minimum value of S is 150 cm, length is 600 cm.
For 100 MHz, minimum value of S is 75 cm, length is 300 cm.
For 200 MHz, minimum value of S is 38 cm, length is 150 cm.
For 300 MHz, minimum value of S is 25 cm, length is 100 cm.
This antenna is simple to build, but it is too large to be built for
frequencies approaching 30 MHz. You could still incorporate a large
bow tie to capture such frequencies, however.
I believe the angle of the reflector should be about 90 degrees.
A simple BALUN for this antenna would be just a few common ferrites
over the coax near the antenna end of the cable. Still, be careful
how the cable moves when moving the antenna.
Try to use very low loss coaxial cable such as Belden 9913 to improve
overall antenna factors.
Other literature may be more helpful, such as the ARRL publications
on this topic. Good luck.
Eric Lifsey
National Instruments
