Nicolas,
Your results actually look pretty good. An SWR of 2.4:1 represents a loss of only 17%.
Small changes in the spacing of the turns on your antenna should enable you to put the minimum SWR at your desired frequency. If that does not quite do it, make the antenna wire a little shorter to raise the frequency of best match.
As for the ferrites, I usually save ones cut off of old computer accessory cables (monitors, power supplies). They are often large enough to allow an SMA connector to pass through. You can also get split ferrites which can clamp over cables with connectors that are too large to pass through. There are various ferrite types, but the ones used for interference suppression on cables seem to work fairly well for this kind of use.
--John Gord
