> I had an idea for the antenna for the transmitter I am building.  It would
be
> like a Yagi antenna, from the standpoint that it would have one or more
> reflectors, a driven element, and some directors, but it would consist of
> circular loops instead of beams.  The idea is to use a non-conductive
> fuselage tube, with a full wave loop as the driven element glued to the
> exterior or interior of the body tube.  (I prefer the exterior, using
> something similar to that conductive tape they used to use on store
windows
> as part of the alarm system.)  Above the driven element would be a ring of
> the same stuff for the reflector, below it would be rings for the
directors.
>
> As I haven't actually built the rig yet, what is the list's opinion of the
> efficacy of this setup.  The idea is to build the antenna so it radiates
> straght down behind the rocket.


An entirely workable idea in theory but probably limited in practice by the
points I raised for a straight Yagi. Mainly, element size limited by body
size and probable presence of metal in the path. The actual design would
need to be specific to a given rocket design due to dielectric effects of
body material.

A Yagi like antenna with closed square loops that is used world wide is the
"cubical quad" (invented at HCJB) which uses typically two or three
elements. It has square(ish) loops rather than straight beams. Another
variant is the Delta Loop which uses a 3 sided element (slightly curved out
of triangular shape by construction methods. Both are used at lower
frequencies to reduce the sheer span of a low frequency Yagi (7 odd  metre
elements at 20 metres/14 MHz band. They also have some usability advantages
in some circumstances.

Such an "end fire" multi ring aerial would be possible but probably most
useful only for well controlled near vertical flights where the beam is
pointing straight down.
Where you want to use the transmitter for location purposes or the flight
path curves markedly you are probably better off with a more omnidirectional
design with the aerial gain made up by the use of more fancy ground based
equipment - eg a long yagi of not too high gain (and directionality) at some
distance to one side which roughly tracks the rocket.

For anyone keen on antenna design a good US reference is "The ARRL Antenna
Book" - 700 odd pages long and includes a CD for a cost of about $US30.



regards

                Russell McMahon

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