[amsat-bb] Re: Cubesat Antenna Modeling and truth?

[Anthony Monteiro]

At 12:12 PM 5/4/2008, Robert Bruninga wrote:
>A big problem on small satellites (cubesats) is how to match
>antennas...
>...
>Has anyone had other successes?  Just thinking what we should be
>telling these students that are building cubesats.  A 6" whip
>for 70cm is not too bad on a 4" cubesat, but 2m is compltely
>different...

Hi Bob,

This problem is not new. Walt Maxwell, W2DU had the same problem
when working on antenna for big satellites at RCA Astro-Electronics.
His solution was to use ferrites to decouple the feed cable from the
satellite body. Then you can just use a directional coupler outside
the satellite to measure the return loss.

I have used this and it works pretty well as long as you keep the feed
cables at 90 degrees from the antenna E field. You can use the EZNEC
model to determine the expected E-field.

You need to make sure the ferrites provide at least 10x the impedance
of the antenna at the satellite (i.e. if you expect 35 ohms for the antenna,
you need at least 350 ohms from the ferrites.) Also, you MAY need to
break up any resonances on the cable by putting ferrites every 1/8 wavelength
on the feed cable if you cannot get the cable at a true 90 degrees.

Note that 90 degrees will not likely be directly broadside from the satellite
especially for a 4" cube on 2 meters.

73,
Tony AA2TX