I own and use detectors with large coil combinations here in the UK,dont use them all the time as they are mainly site specif use machines,when and why would i use them,well currently working 3 hoard sites that have all the hallmarks of a machine scattered pot hoards ie deep ploughing with powerful tracked tractors that usually start taking the tops of of pots/containers or what ever that can indicate a scattered hoard.As celtic or romans did not have banks to deposit valuable in times of strife,rape,pillage,war etc they would basically dig a hole near a landmarked area say a tree or what ever then come back after the skermish or problem was over,of course alot of folks never did come back to recover these valuables hence we are finding hoards with detectors.
So when you start finding coins or artifacts its usually coins that start appearing in a tight radius or a certain pattern that these coins are being dragged up after each pass of the plough you then have a very good indication that a hoard is in that location,you would tend to work the are with a detector with a stock coil 1st usally a VLF machine as this would indicate any possibility of iron.I usually then add a larger coil in the 15'' range and then peel the next layer of finds off and then use a 18'' coil to try and locate the main bulk of the hoard.Coils over a certain size on both VLF and Pulse machines start loosing sensitivity to single coins after a certain coil size but the will locate larger items much deeper.
For every day use a larger coil is a waste of time,masking or nulling of signal due to multiple targets under the coil at the same time ie iron and say a gold ring and the ring could be nulled out as it picks up on the iron then discriminates it out.So hence the reason you try to clear out the junk 1st before using the larger coil.
If i want to go deeper still then i can and do ue the big fella the Pulse with the 18'' coil on but that mainly for locating larger targets,totally forget about small coins with a large coil as its a waste of time,of course if one requires to go down and locate bigger targets then you can and i use a 2 box detector but once again only on site specific locations.
Do large coils have uses,of course they do but not for every day use,if you are in Aussie land after deep ultra large nuggets then you would tend to use the right tool for the job this usually is the mighty GPX with a big coil on,if you do relic hunting for CW artifacts then once again you would use a large coil on a detector or in my case hoard hunting.
Its all about the reward to effort ration in your favour and i guess thats when experiance starts to count on when and when not to use a larger coil,you would not use this type of combination in a tots lot.
- Jim