[Paparazzi-devel] Re: Paparazzi-devel Digest, Vol 44, Issue 16

[Helmut Wabnig]

On Thu, 22 Nov 2007 12:00:04 -0500, you wrote:

Look here:
I will upload some infos on 869MHz Antennas
on http://radiation.weblife.at/ in a few minutes.

Just today I made the prototype of such a copper foil antenna.

The antenna consists of a vertical radiator and two radials
which extend downwards under 45 °.

The wavelenght in air is 300/869 = 345 mm.
The radiator must be quarter wavelength which is 86 mm in air.
Due to dielectric coupling from the carrier substrate the 
wavelenght shortenes about 1/ (SQRT(epsilon)) of the carrier material.
This is the reason why GPS antennas are so small. They use materials
with an epsilon up to 30. The antenna shown in the blog is made on 4mm
"plexi glass". On the model airplane I would mount it on the rudder in
vertical position.  Because we do not know the exact epsilon of your
specific airplane model, we can only estimate the exact dimensions
and you may want to trim the antenna when it is ready mounted.

You will note that the vertical radiator is shorter than the slanted
radials. This is to make the bandwith larger and still get a good 
SWR. The impedance of the antenna is most influenced by the
angle of the radials, not by their length, while the resonant
frequency is  strongly influenced by the vertical resonator length
and a little lesser by the lenght of the slanted radials.

How do we trim the resonant frequency once the antenna is mounted?
More on this later, I have some ideas but must work them out.

w.


>Today's Topics:
>
>   1. Antenna for 868MHz (Steve Joyce)
>
>
>----------------------------------------------------------------------
>
>Message: 1
>Date: Thu, 22 Nov 2007 15:35:08 +0100
>From: "Steve Joyce" <address@hidden>
>Subject: [Paparazzi-devel] Antenna for 868MHz
>To: <address@hidden>
>Message-ID: <address@hidden>
>Content-Type: text/plain;      charset="us-ascii"
>
>Greetings,
> 
>I'm just getting started with paparazzi, and working with the data link.  I
>have some Aerocomm AC4868-250 modules and was wondering if anyone has some
>advice about antennas.  The usual 1/2 wave dipole is a little clumsy for the
>airborne side.  An off-the-shelf plug-in unit is preferred, but I am also
>interested in home-brewed copper foil type solutions (I saw some examples on
>the wiki).  For now I am happy with a reliable link within visual range, but
>I know the Aerocomm is capable of much more.
> 
>Also it is safe to power this module from the 3.3v line on the tiny13?  Any
>other tips on module setup would be appreciated.
> 
>/Steve
>