<STRONG>"I recall seeing somewhere that when setting threshold in urban areas you should set threshold to a low hum and then with the coil raised at least two feet, slowly rotate in a 360 degree circle to locate any sources of radio frequency interference. Is this valid and should you make the adjustment facing the direction of the highest detectable RFI?"</STRONG>.... Let me correct just one or two things from your post or add some comments, making the changes in red:
I recall seeing somewhere that when setting threshold in urban areas you <STRONG><FONT COLOR="#ff0000">could/can</FONT></STRONG> set threshold to a low hum (<FONT COLOR="#ff0000">where it ought to be set in the All Metal mode anyway</FONT>) and then with the coil raised at least two feet (<FONT COLOR="#ff0000">any lift from the ground, and the higher the more prone to RF interference than near the ground</FONT>), slowly rotate in a 360 degree circle to locate any sources of radio frequency interference. Is this valid (<FONT COLOR="#ff0000">sort of, but ...</FONT>) and should you make the adjustment facing the direction of the highest detectable RFI?(<FONT COLOR="#ff0000">No</FONT>)
Also does threshold affect operation in disc mode?... <FONT COLOR="#ff0000">Yes</FONT>.
I recall reading that forum discussion/comment. There are a few very simple facts to consider when dealing with outside electrical interference.
#1.. It's going to happen most often in an urban setting because that's where all the many different sources are. <img src="/metal/html/sad.gif" border=0 width=15 height=15 alt=":sad">
#2.. Anytime you raise a coil into the air you are going to be more susceptible to outside RF interference, so the key here is keep the coil in an operating height position and don't raise it in the air! <img src="/metal/html/shocked.gif" border=0 width=15 height=15 alt=":shock">
#3.. The interfering power source(s) might be the type that are <EM>always</EM> going to cause you fits, and there are some that will be intermittent. That is, there are times you will be able to hunt close to them and times you can't. If you can't, don't. Go somewhere else. If you can, then hunt. Pretty simple. <img src="/metal/html/glasses.gif" border=0 width=15 height=15 alt="
">
#4.. The offending power could be from a federal, state or municipal power or transmition source. It could be from a commercial operation of some stationary or mobil power generating source. It might even be from a private source. One thing for sure. You can try to figure out what or who is causing the annoyance, but it isn't going to do you any good to ask them to 'turn it off so you can do some detecting.' <img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol">
Why waste your time trying to determine who or what is bugging you? Just deal with the fact that you are annoyed and consider that part of the reason could be your choice of search coil? Often, though not always, a larger searchcoil is going to be much more prone to outside interference and less stabile. Switch to a smaller coil. Often this will eliminate the problems or reduce it enough you can enjoy some hunting, and still have good performance. <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="
">
The only other two options, other than just hunting with all the troubling interference, is to just go away and find another place to detect, or hold off on that location if it is one where the offending source is intermittent.
Proper set-up of your detector calls for a slight audio hum in the All Metal mode. Your Discriminate mode does rely on a proper threshold setting for peak performance.
<EM><STRONG>Monte
