On 18 dic, 09:05, "Robert Lacoste" wrote:
Dear all,

For a quite specific application I'm looking for an antenna design with
ideally the following characteristics, which are, I know, contradictory :

- 2m band
- TX (low power, less than 1W) & RX
- ultra compact, ideally able to fit in a 12-15cm x 5 cm x 5cm volume
- as omnidirectionnal as possible on the horizontal plane
- more or less vertical polarisation
- raisonnable gain for its size (-5 to 0 dBi ?)
- low dependency to nearby grounds (would be installed either at ground
level or on a wall)
- could be narrow band (say 1MHz) but in that case with stable tuning...
- low cost

Not trivial, isn't it ? For the moment my best match is a dual helix antenna
(two colinear helix, each made with a L/4 long wire, driven differentially),
but performances and size are not optimal.

Any good idea ? Would small magnetic loops be usable in such high
frequencies and small size ? Has someone already experimented other good
designs ?

Thanks,
73s,
Robert

Hello Robert,

Loop antennas (from strip material) are an option when you accept a
figure-of-eigtht radiation pattern in the horizontal plane. When your
link budget has sufficient margin, a figure-of-eight pattern can be
accaptable for indoor applications as propagation can take place via
reflection.

For a strip material loop that covers 0.12*0.05 m^2, you may expect
about 17 pF tuning capacitance. That capacitor must be able to handle
some kV (at 1W input power). matching can be done, for example, with
a gamma match or just an inductive coupling loop.

Your may add capacitive end-plates to your helical dipole. The actual
wire length will reduce, hence giving better efficiency and some
bandwidth increase. With a good match, -5 dBi must be possible with 1
MHz BW (VSWR=2, in open air).

I have serious doubts about efficiency and tuning stability when in
close proximity of materials. As fields close to the antenna are very
strong, comparing to full size antennas with same input power, I
expect significant loss in wall building materials and detuning. So
the match to the transmitter will vary significantly.

You should use all the volume you have to get reasonable efficiency
and useful bandwidth, so you will very likely not find a ready-to-
build design that fits your needs.

Best regards,

Wim
PA3DJS
www.tetech.nl
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