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Do you have any information regarding what frequency(ies) you are looking to detect? There are a couple of ways to improve the antenna type while keeping it compact. Also, the method of coupling the signal from the antenna can have a great affect on efficiency of collection.
If the frequencies are below 50 MHz, perhaps a magnetic loop antenna ( H-field) may prove very effective. These are physically quite small and very efficient for the lower HF frequencies. They are also relatively immune to urban E-field noise from power lines etc. The trade-off for this is they are very narrow bandwidth.
If the frequencies are higher, then a helical design is very attractive. These look a bit like a cork-screw and the centre pouts toward the target. The more turns in the cork-screw, the higher the gain and directivity of the antenna.
There's plenty of info available on these designs ( I think). Maybe worth considering. Any gain you get in your antenna is better than through using amplifiers, as the latter introduce additional noise.
HTH,
Brian C
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