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

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
when using PM, remove abc first.

On Dec 18, 8:05*am, "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

a rubber duck or just a loaded vertical should do it. add a small
ground plane or counterpoise coiled in the allotted space and you may
even get better than 0dbi.

"Robert Lacoste" wrote in message
...
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

No point using a "short" aerial as the radiaton angle will be up in the sky
and coverage absolutely rubbish.
The easiest aerial you can make is from a piece of 300ohm ribbon cable.
It's a slim jim type aerial which has a low angle of radiation and will give
a brilliant coverage compared to expensive colinear aerials.
You can just roll it up and stick it in your pocket, then when you want
to use it - hang it from a curtain rail. or a tall structure. No need for
any
ground.

In article ,
IanT wrote:

No point using a "short" aerial as the radiaton angle will be up in the sky
and coverage absolutely rubbish.

Not so, at least not for the stated reason.

The radiation pattern of an "infinitesimally short" dipole (or very
short monopole over a good ground) isn't very different from that of a
half-wave dipole (or quarter-wave whip over a good ground). There's
only a dB or so of difference in the shape of the patterns, at any
angle.

The problem with short mono-/di-poles is that you need to add a whole
bunch of loading to them, in order to make them resonant and easy to
drive. The usual methods for such loading are electrically
inefficient... much of the power ends up being burned up as heat in
the coil.

It'd help to know a bit more about the intended installation/application
of this particular antenna, in additional to the described physical
parameters.

There might be some form of slot antenna which would do the trick.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Many thanks to all for your answers, I will try some of these ideas and let
you know...
Friendly,
Robert

That pretty much confirms what I already thought about the threads.

But what about movement and animation? Would it be better to have getX as something like;

return startx timeSinceStart speed

or would it be better to have

x = speed timeSinceStart

in a thread?