There are many different categories of "difficult". It
could be technically difficult (reflections/stray signals),
physically difficult (three hours hike into a wilderness
area for what would normally be an evening hunt in cars),
well hidden (the dummy brick or boulder trick), or more
socially difficult (like the previous post, the guy hiding
in a tree who invited a bunch of frat guys to picnic with
a keg of beer underneath it on the condition they keep
hunters away, the drunken bum on skid row, the madhouse
community Halloween party where police cordoned off the
area, or putting it too close to a "sensitive area" where
hunters arouse the "curosity" of the security services.)
Probably the most difficult one I have hidden was inside
an upside-down teacup on the table at a hamfest. The
transmitter put out occasional unmodulated pulses and
drifted past a busy frequency. This one took a while
to find, but others in a piece of pipe or down a gopher
hole didn't last long, even though hunters were only using
HTs with body shielding.
One of the others was to put the transmitter in a black
nylon bag and run it up a flagpole. At night. It was
totally invisible in the dark, and most people don't
expect to look UP.
And then there was the hunt at Dayton that included a
couple of transmitters inside logs (that had been split,
hollowed out, and screwd back together), tucked inside
a buried tire (it was great to watch folks walk back
and forth over a spot of bare dirt, wondering where it
could be hidden), and inside the access port of a
streetlight.
But the hardest ones are often unexpected: I've often
put out transmitters that I expect people to find
fairly easily only to hear horror stories later about
how bad the reflections were.
One of the guidelines for the international sport-type
of transmitter hunting is that it is a competition
among the hunters, not between the hunters and the hider.
Transmitters are marked by a orange and white flag, and
should be visible from 10 to 30 feet away: the point is
to get close enough to see it, rather than spending a
lot of time trying to sniff out one that is well hidden.
I try to use the same principle for other hunts
as well - the hiding needs to be appropriate to the
experience/abilities of the hunters. With inexperienced
hunters not having specialized equipment, finding a car
with a 2m antenna on it may be enough of a challange,
while experienced hunters with a good sniffer shouldn't
have any trouble telling you which strand of barbed wire
on the fence is the actual radiator. If the hunts are
too hard, people stop coming, then you don't get to hunt
as often. It's counterproductive.
That said, one of the most impressive hunts that I
remember was when someone borrowed one of my handheld
80m DF receivers for a competition and made sure he
changed the battery beforehand. Unfortunatly he put in
an old battery, and it didn't take long before the
receiver would work for less than a minute at a time.
So he would take a quick bearing then turn the receiver
off for 5 minutes until the transmitter was due to come
on again, by when the battery had recovered enough to
give him another 20 seconds of operation. He managed
to find all 4 transmitters that day in spite of it.
But the most difficult hunt I remember was when someone
hid a transmitter in the woods "behind a log somewhere
by some ferns" and failed to turn it on. We finally
found it 3 hours later as it was getting dark. I was
almost to to the point of building a DF receiver to
try to hear the crystal oscillator in the controller
module and tracking it down that way!