Why a new antenna design?
The original AAVSO antenna design accompaning their
Gyrator II circuit was fussy and tended to oscillate.
It was also frustratingly difficult to tune.
To avoid the problems, we switched to the MAX 275
as a bandpass filter tuned to the desired frequency
we wanted to mintor. with a simple antenna design..
With a handful of
metal film resistors, it has a very steady frequency response and a nice
high-Q, about 100 hz from the centre frequency. All of
the tuning is on a daughter card that plugs
into the main board. The frequency board has only two
10K pots that we pre-set the frequency before shipping the unit to the
school (or radio club, or person, etc.). If an area proves difficult to
monitor at one particular frequency then we can ship other frequency
boards tuned to other frequencies to try. These modules are simple to
install by the user
and do not require any external equipment such as an oscilloscope or DMM.
There was some initial concern about the small wire low-Q
loop. But we did some tests and noticed
a huge difference in the 14 AWG tuned-loop vs. the 26 AWG non-tuned loop.
When the 14 AWG wire loop was not tuned, the number of
turns of the wire made the difference in
how well we picked up the VLF signal.
With our 14 AWG tuned antenna and the SID monitor with the 26 AWG
untuned antenna, our
performance is quite good.
We expect our students receiving SID monitors to contruct their
own antenna, and we provide information on building several
inexpensive models. Given the wide variation in materials
used, we needed the SID monitor to work regardless how
the students assembled their antenna.
So we came up with our basic design. However, other
designs are certainly possible and we welcome suggestions
for improved low-cost antennae.