At 04:14 PM 12/3/2002 -0800, Tim May wrote:
On Tuesday, December 3, 2002, at 11:09 AM, Steve Schear wrote:
In the late 70s, I was at TRW we built inflatable (beach ball) antennas
for a black project. About 1/3 of the balloon's inside surface was
aluminized and the feed was simply snapped into place at the opposite
side. The antenna could either be used hand-held or place in a ring
mount on a flat surface. This sort of approach could work well for cell
phones and WiFi cards with external antenna port.
For cell phones the entire instrument could be placed in at the
reflector's focus and operated via a mic/headset adapter (some older
Nokia models have an external antenna port behind a small rubber plug on
the rear.)
Clever, but a section of a sphere is not a section of a paraboloid, so how
did you deal with the focus issues?
Perhaps the feed part had a secondary antenna (like a secondary mirror in
a telescope to correct aberrations in the larger primary).
Parabolas are most desired when a broad range of frequencies are
simultaneously being received and need to be brought into a common crisp
focus (e.g., handling chromatic aberration in optical
telescopes). Secondaries and corrector plates, as are common to handle
field flattening, were considered too complex for the application. At
these frequencies and reflector size the difference of figure between a
parabola and a sphere is relatively small though not insignificant. I
believe the engineers were able to introduce balloon distortions
approaching a parabolic figure. In the end the engineers found other
imperfections of the antenna and feed predominated.
steve
