If you are getting excessive interference with the 25" mono then an 18" will most probably be just as bad. There are two types of interference that cause problems with large coils, one is Mains interference which generally is closely associated with power lines (ones in the air hanging off poles but also buried power lines too) and the other, which is 9 times out of ten more problematic due to its ever changing distances from the source, is Sferics EMI. The problem is the power lines act like a massive collector (fence lines can also cause this) which then channels the Sferics down the lines channelling and concentrating it, especially during stormy seasons. You can tell the two types of EMI from each other by listening to the audio/threshold of the detector, power line/Mains interference is pulse like with a speeding up or slowing down pattern depending on the demand made on the line, whereas Sferics is all over the place and generally associated with the inclination of the coil as the coil is swept along (usually the threshold sounds reasonably stable when the coil is held stationary).
With Mains EMI the only way to get rid of it is to remove yourself from the immediate area of go to a dedicated DD coil and use Cancel mode which then comes at a depth reduction compared to traditional methods, performing an auto Tune will help somewhat then fine tuning with the manual control, with Sferics distance from the source helps a lot but the distances are greater and reduce slowly over quite large distances sometimes, best method is to also perform an auto tune then fine tune the manual control with the coil held parallel to the ground, when detecting try to swing the coil in a VERY controlled manner paying particular attention to keeping the coil as flat as possible. In both instances using the Quiet Audio filter in combination with low Gain levels(10 or less on a GPX 5000) will help a great deal with the instability.
Hope this helps
Jonathan Porter
